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New understandings of the pathogenesis of acne
DEVELOPED FOR THE CLEVELAND CLINIC FOUNDATION-SPONSORED PROGRAM “NEW PERSPECTIVES ON ACNE” UNDER AN UNRESTRICTED EDUCATIONAL GRANT FROM THE DERMATOLOGICAL DIVISION, ORTHO PHARMACEUTICAL CORPORATION
New understandings of the pathogenesis of acne James J. Leyden, MD Philadelphia,
Pennsylvania
Acne vulgaris is the most common skin disease, affectingnearly 80%of youngadultsaged11to 30.’ Not only can it causedisfiguration and permanent scarring, but it can have an adverseeffect on psychologicdevelopment,resultingin profoundemotional scarring,which may lead to social phobias, withdrawal from society,and clinical depression2 Researchduring the past threedecadeshasconfirmed the multifactorial originsof the disease.Even the most difficult casesmay now be controlledby usingrational therapiesbasedon currentconceptsof pathogenesis. This article reviewsthe pathophysiologicfactors that influencethe developmentof acne,3most notably ( 1) excessivesebum production,(2) abnormal desquamation of sebaceousfollicle epithelium (comedogenesis), and (3) proliferationof Propionibacterium acne+ resultingin inflammation. SEBACEOUS GLANDS
of Dermatology, Philadelphia.
the University
of Pennsylvania
Reprint requests: James J. Leyden, MD, Hospital of the University Pennsylvania, Department of Dermatology, 2 Maloney Bldg., Spruce St., Philadelphia, PA 19104.4285. J AM ACAD DERMATOL 1995;32:S15-25. Copyright @ 1995 by the American Academy 0190-9622/95
$3.00 + 0
squamae
Sebaceous duct
AND SEBUM
The pilosebaceousunit consists of sebaceous glandsand hair follicles. Sebaceousglandsand hair follicles are physically connectedand usually vary inverselyin size.Theseunits are locatedin all skin areasexceptthe palms and soles.There areapproximately 900glandspersquarecentimeteronthe face (particularly at midface), upper neck, and chest. The restof the bodyaveragesfewerthan 100glands per square centimeter. “Free” sebaceousglands (thosenot attachedto a hair follicle) that opento the surface of the skin are presentin the meibomian glandsof the eyelid, the areolaeof the nipples,and
From the Department School of Medicine,
Canal
16/O/64083
of Dermatology,
of 440
Inc.
Sebaceous gland
Fig. 1. Sebaceous follicle.
the glabrousvermilion bordersof the lip and in the mouth. Pilosebaceousunits, involving follicles that producesmall hairs but associatedwith largesebaceousglands,are called “sebaceousfollicles” (Fig. 1). When sebaceousfollicles are affected, acne occurs. Sebaceousglandsare composedof undifferentiated,differentiated,and mature cells.Undifferentiated cells are located at the peripheryof a gland, adjacentto the basementmembrane.As they are pushedtoward the sebaceousduct, theseundifferentiated cells developinto differentiatedlipid-containing cells.The increasedconcentrationof lipids in mature cellsleadsto cell disintegrationandthe formation of sebum,a complex lipid mixture that is eventuallyexcretedonto the surfaceof skin. s15
Journal of the American Academy of Dermatology May 1995
S16 Leyden Table I. Sebum composition Isolated gland (‘35)
Triglycerides Free fatty acids Wax esters Squalene Cholesterol esters Cholesterol *After
hydrolysis
57 0 25 15 2 1
Surface* (%I
42 15 25 15 2 1
of triglycerides.
One study indicated that sebaceousgland secretions may be partly the result of holocrineproduction of sebum,involvingthe contributionof suchhydrolytic enzymesas acid phosphatesand esterases.5 It nowappearsthat sebaceous glandsarepermeable to certain molecules,suggestingthat sebumproduction may be more complexthan simple autolysisof sebocytes.Thus the final lipid film excretedontothe skin surface may have incorporated substances directly from the dermis,aswell assecretions(without lysis) from active cells within the sebaceous gland.6 Composition of sebum Sebum is primarily composedof squalene,wax esters, triglycerides,
and small amounts of sterols
and sterolesters(Table I). Bacteriallipasesresultin theformation of monoglyceridesanddiglycerides,as well asfree fatty acids,within the sebaceous follicle duct. The lipid film found on skin surfacesis the result of the admixture of pure sebum with epidermally derived lipids. As the compositionof sebum varies by species,7it is assumedthat the function varies. In mammals, the principal function of sebum
is to maintain the hydration of the skin and hair in the stratum corneum. The hydrolysisof sebaceous glandtriglyceridesby P. acYteSin surface lipids yields free fatty acid$; thesefatty acidsaccountfor the principal difference betweenisolatedhuman sebaceousglandlipids and the skin-surfacelipid film. The amount of free cholesterol in surface lipids is not sufficient to account for the productionof thosefree fatty acidsfrom ste-
rol esters;further, the chainsof fatty acidsfrom wax estersarelongerthan thoseof skin surfacelipid-film free fatty acids. The complexity of the human skin surface lipid film is largely
a function
of the diversity
and
uniquenessof its fatty acid components.Only 37%
of “biologically valuable acids,” including linoleic andoleicacids,normally found in internaltissue,are alsofoundin surfacelipid fatty acids.In comparison, the many other fatty acidsthat havebeenidentified in human skin surface lipid film are not normally found in internal tissues.9 Another distinguishingcharacteristicof skin-surface free fatty acids is the wide variety of chain lengths;randomatom lengthsrangefrom 4 to 30.A varietyof carbonnumberskeletaltypeswith straightchain structures,odd and evencarbonnumbers,iso and ante-is0chain structures,and branchedchain structureshas beenidentified.9The pattern of unsaturationof skin surfacefatty acids,in which double bondsare found betweenthe sixth and seventh carbonatomsin monoenoicfatty acids,differsfrom the pattern found in circulating lipids, in which the double bond is frequently betweenthe ninth and tenth carbon atoms.9Thus the fatty acid composition of wax esters and triglycerides cannot be acquiredfrom circulating lipids. Sebum differences in personswith acne Acne patientshavea significantly lower concentration of linoleic acidin their skin surfacelipidsthan do personswithout acne.lo-l4The higher sebumsecretion rates of personswith acne accountsfor a portion of that difference,becausethereis aninverse correlationbetweenthe rate of sebumsecretionand the linoleatecontentof surfacewax esters.l2 In other words,as the amount of sebumsecretionincreases, there is a correspondingdecreasein linoleic acid contentof sebaceous gland wax esters,triglycerides, and free fatty acidsof skin surfacelipids.13 How and why the concentrationof linoleate in sebumdecreases may beof critical importancein the pathogenesisof acne. The current hypothesisregarding the role of linoleic acid in the development of acneoriginatesfrom studieson acnepatientsin which lipids recoveredfrom comedones(abnormal follicles)of acnewereexamined.Identificationof an unanticipateddeficiencyof linoleateled researchers to considerthat an essential-fatty-aciddeficiencyin the follicular epithelium of sebaceousfollicles was involved in comedogenesis. l4 The current explanation of the pathogenesisof acneis based on these findings. When the peripheral undifferentiatedsebaceousgland cells differentiate, the cells haveaccessto circulating lipids, including linoleic acid.However,nomore circulatinglipids are admitted after sebum synthesisbegins. Thus the
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
more sebumsynthesizedper cell, the greaterthe dilution of the initial linoleate.Linoleateis originally in the phospholipidsof cell membranes and is releasedwith the final cell rupture. It is then incorporatedinto variouslipids in proportionto the relative rates at which thoselipids were being synthesized at the time of disintegrationof the sebocyte. Linoleate is found, in decreasingamounts, in cholesterolesters,triglycerides, and wax esters.‘O Follicular epithelium differentiation may be influencedby the lower concentrationof linoleatein the sebumof personswith acne.The essentialfatty acids of the follicular epithelium may easily be obtained from the generalcirculation through diffusionfrom the dermis.Additional fatty acidsmay be obtainedfrom sebaceouslipids that diffuse into the epithelial wall from the follicular lumen. In persons with acne,sebum secretionis high and the concentration of linoleateis consequentlylow; thusthe lipid mixture surroundingthe follicular epithelium is deficient in essentialfatty acids. The relation betweenacne and the overproduction of sebaceous glandlipids haslongbeenacknowledged, as has been the correlation between the severityof acne and the amount of sebum excretion.15,I6 The 1983study by Harris et al-l7 helped quantify the differencein sustainableratesof sebum production betweenpersonswith acne and those without. It was found that sebumdeliveredto the skin surfaceafter depletionof the ductal reservoir with bentoniteis an accuratemeasureof sebaceous gland activity. Hormonal
influences
Androgenic hormonescontrol the secretoryactivity of the sebaceous gland.Testosterone(the main circulating androgen)and androstenediolare synthesizedin the testes.The adrenalglands are the main sourceof dehydroepiandrosterone, andits sulfated form, which is the most abundantcirculating adrenalandrogen;they alsoproduceandrostenedione and androstenediol.Although the ovariescan produce testosterone,androstenedione,and dehydroepiandrosterone, the adrenalgland is an important sourceof circulating androgensin women. In addition,themetabolismof dehydroepiandrosterone can producetestosterone. It is currently assumedthat testosteroneis convertedto dihydrotestosterone, which then bindsto a high-afhnity specific cytoplasmic receptor protein that is transportedto the cell nucleus.At this point,
Leyden
SIT
the dihydrotestosterone-proteincomplex initiates DNA-controlled events(Fig. 2). Although we know that sebaceousgland activity is dependenton androgenichormones,we do not yet understandwhy sebum is overproduced.Further, the resultsof studiesexamining the concentrations of circulating androgensin personswith acneand thosewithout acneoftenconflict. Onesuggestedapproachwould beto measuretheseconcentrations(at severaldifferent points of the menstrual cycle in women),while determiningthe end-organresponse (i.e., sebaceousgland activity). Adequate controls and additional information regardingthe existence of other signsindicativeof excessiveandrogenproduction (e.g.,hirsutism,male patternalopecia,gross menstrual abnormalities) are necessaryfor adequate interpretation.Unfortunately, much of the researchto datelackssufficientinformation for such interpretation.In fact, many studieshave reported abnormalitiesin hormoneconcentrations1s-29; most notably, women were far more likely to have had abnormally high concentrationsof hormonesthan were men. One interpretation that has been offered,29> 3obut not consistentlyconfirmed,31is that thereis an abnormaltarget-tissuemetabolismasthe causeof the disparity. Despitedisagreementsregardingthe resultsand interpretationsof thesestudies,it is possibleto set forth some basic guidelinesand conclusionsconcerning the hormonal influenceson acne.A small proportionof acnepatientswill exhibit signsof excessiveandrogenproduction (hirsutism or significant menstrual dysfunction);thesepatientsshould receivea completeendocrineand gynecologicevaluation.The majority of personswith acne,however, do not exhibit these clinical abnormalities and, therefore,do not warrant such investigations. Lastly, althoughit is not yet known exactly how androgenichormonesare causally related to the overproductionof sebum, it is apparentthat hormonal andnonhormonaltherapiesthat significantly reducethe productionof sebummay be beneficial, COMEDOGENESIS
Comedogenesis, an abnormality in the processof desquamationof follicular corneocytesin the sebaceousfollicle ducts,is centralto the pathogenesisof acne,Comedogenesisis so named becauseit producesa microcomedo,or a microscopic,clinically inapparent,precursorlesion (Fig. 3). The microcomedo evolvesinto either a clinically apparent
S18
Journal of the American Academy of Dermatology May 1995
Leyden
HO-.W Dehydroepiandrosterone
Ii
0 !:-.:-1:l ’
Androstenedione vIf OII
OW Testosterone Sa-reductase
,+ ,“I
Dihydrotestosterone k,‘~ ‘\, ‘4
Scu-Androstane&,17/%diol
Scr-Androstane3/3,17P-diol
Fig. 2. Androgen metabolism in skin: 17@OH pathway.
lesion (an open comedo, or “blackhead”) (Fig. 4), a closed comedo (“whitehead”) (Fig. 5), or an inflammatory lesion, if P. acnes proliferate and generate inflammatory mediators.32 Comedogenesis occurs specifically in sebaceous noninflammatory
follicles, not in terminal or vellus follicles. Histologically, sebaceous follicles have a widely dilated follicular canal lined by stratified squamous epithelium; a fine, vellus-like hair usually in the resting (telogen) phase; and multiacinar sebaceous glands.
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
S19
Leyden
Sebaceous gland
\
Fig. 4. Opencomedo.
Fig. 3. Microcomedo.
The glandsare connectedto the main follicular canal through short sebaceousducts.Within the follicular canal are loose,keratinoussquames(scales) that are impregnatedwith lipids. The classichistologicstudy by Straussand Kligman,32in which biopsyspecimenswere taken from skin that appearedclinically normal, found early microscopicchangesthat are precursorsof clinical lesions.A slight dilatation of the sebaceousfollicle canal, aswell as a slightly hyperplasticepithelium, are the earliestchangesevidentunder light microscopy(Fig. 3). The study alsofound a slight increase in the amount of keratinousmaterial within the canal that forms a microscopic lesion of coherent squames-the microcomedo. The follicular
Sebaceous gland
\
Fig. 5. Closed comedo.
canal in acne
The follicular canal is composedof two portions, the more distal acroinfundibulum,contiguouswith the surfaceepithelium, and the infrainfundibulum, or the region betweenthe epithelium of the sebaceousduct and the follicular epithelium (Fig. 6). In personswithout acne,the acroinfundibulum appearsmorphologically similar to the epidermis. The basallayersof the follicular epithelium produce cells that are organized into a stratified epithe-
lium. Within the surface epithelium are tonofilaments, desmosomes,keratohyalin granules, and melanocytes. The patternof keratinizationundergoesa gradual transition from the infrainfundibulum to the acroinfundibulum, so that wherethe two portionsjoin, the granularlayer, in additionto the well-organized stratum corneum, have each decreasedor disappeared.There are fewer tonofilamentsand desmo-
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Journal of the American Academy of Dermatology May 1995
Leyden
Fig. 6. Infundibulum
in normal follicle.
somesthan in the acroinfundibulum; additionally, the keratohyalin granules are smaller and fewer. There are more lamellar granules(Odland bodies) in the infrainfundibulum. Those intracellular organellesare extrudedinto the extracellularspaceof the stratum corneum,wheretheir lipid constituents form a lamellar structurethat is involvedin corneocyte cohesion.Unlike the acroinfundibulum, the thinner stratum corneum of the infrainfundibulum doesnot form a coherentlayer with the respective partially fragmentedcorneocytes. Keratinization
Comedogenesisbegins with dilatation of sebaceousfollicles as a resultof the accumulationof abnormally desquamatedcorneocytes.Under an electron microscopeit is possibleto seethe alteredpat-
Fig. 7. Infundibulum during comedo formation.
tern of keratinizationof the infrainfundibulum.33a 34 Above a thickenedgranular layer containingmany larger keratohyalin granules is a firm, coherent stratum corneum(Fig. 7). In a personwith acne,intrainfundibulum cells contain numerous tonofilaments and desmosomes,but fewer lamellar granules. Large, intracellular inclusionsthat are lowelectron-dense,homogeneousand probably lipid, are easily visible. These inclusions are contiguous with lamellar structuresthat have a configuration suggestiveof polar lipid bilayers.35In follicles that areprecursorsto comedones(clinical noninflammatory lesions),there appearto be alternatelayersof corneocyteswith andwithout lipid inclusions.In the centralportionof the “coccoon”of coherentcorneocytes,large lacunae-containinglipids, bacteria,and cell fragments can be found. Lastly, the cellular
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
mitotic activity of the follicular epithelium is increased.36 A numberof theorieshavebeenofferedto explain comedogenesis. Immunohistochemicalstudieshave generallydemonstratedthat comedogenesisis not the result of abnormalkeratinization,exceptfor the presenceof abnormallipid inclusions.37, 38This possible deviation from normal protein formation is suggestiveof either a flaw in cell differentiation or the result of passivediffusion from lipids within the lumen of sebaceousfollicles. Further, the decreased number of lamellar granulesis indicative of a critical defectthat explainsthe unusualcohesiveness of desquamatedfollicular corneocytes. Experimentsusingthe albinorabbit-earmodel(in which it is possibleto induce comedonesclinically andhistologicallysimilar to thoseof human beings) haveshownthat sebaceous lipids (and,in particular, squaleneand free fatty acids39)havecomedogenic properties.A recent theory proposesthe oxide of squaleneto be a possiblefactor in comedogenesis.40 Additional studieshave suggestedthat ultraviolet irradiation enhancesthe comedogenicproperty of squaleneby increasingthe amountof squaleneperoxidesas well asby forming fatty acid peroxides.41 The finding discussedaboveregarding the decreasedconcentrationof linoleic acid in sebum of acnepatientsoffersoneof the most promisingareas for comedogenesisresearch.1°-12 The essentialrole playedby linoleic acid in maintaining the integrity and function of the epidermal stratum corneum is also probably true for the follicular epithelium. Other researchon children with early acne has provedthat the presenceof bacteria, especiallyP. ucytes,is not essentialfor the initiation of comedogenesis. The microcomedonesandclinically apparentcomedonesin thesechildrenare devoidof P. acnes.Conversely,once P. acnes colonizationoccurs, thoseorganismsliberate free fatty acids that arecomedogenicin the rabbit ear model and,probably, in human skin.42 The role of cosmeticsas an exogenousfactor in comedogenesishas been examined for more than two decades.43According to a recent consensus panel of the American Academy of Dermatology, the ingredientsused in current cosmeticsare predominantly not comedogenic,44 unlike thoseuseda generation ago. Topical corticosteroids,however, have been found to be comedogenicin human beings.45
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S21
AND INFLAMMATION
The potential for postlesionalphysical and emotional scarringis probablyproportionatelyrelatedto the developmentof inflammatory pustules,papules, and nodulocysticlesions.According to Straussand Kligman,32the inflammatory phaseof acneoriginateswith the developmentof intercellular and intracellular edema(spongiosis)of the follicular epithelium as a result of a perivascularinfiltrate that exudesinto the follicular epithelium.The ageof the lesiondetermineswhetherthe predominantcellsare lymphocytes(particularly T-helper cells) or polymorphonuclearleukocytesin the infiltrate.46If the lesionprogresses,the integrity of the follicular epithelium may break or rupture, enabling lymphocytes, neutrophils,and foreign-body-typeor activated macrophages(e.g., those seen in allergic granulomas)to infiltrate the dermis.47It has recentlybeensuggestedthat lossof integrity of the sebaceousfollicle epithelium is not a prerequisitefor smaller inflammatory lesions.46Marked disruption of pilosebaceousunits occurs in late-stageinflammation, especiallywith larger papulesand nodules. Cornified cells,lipids, hair, and bacteria are deposited into the dermis, producinga nonimmune-type inflammation. Bacteria
in the pathogenesis
of acne
It was oncebelievedthat acnewas an infectious processcausedby the bacteria “acne bacillus.“48 The role of bacteriain the pathogenesisof acnewas castinto doubtafter it wasfoundthat this organism, now calledP. acnes, could be recoveredfrom acnefree control subjects.49Conversely,the possibleassociationbetweenbacteriaand the pathogenesisof acne was strengthenedby the well-documented findings of clinical improvement in acne patients treatedwith systemicantibiotic agents.50 Thereis little qualitativedifferencebetweenacne patientsand acne-freecontrol subjectsin microbiologic surveys.The cutaneousflora in both groupsis essentiallya triad of aerobiccocci (predominantly Staphylococcus epidermidis), the yeasts Pityrosporum ovale and Pityrosporum orbiculare, and the anaerobicdiphtheroidPropionibacterium acnes (formerly Corynebacterium acnes).51 There is currently no evidencelinking S. epidermidis, which resides primarily on the skin surfacesrather than within the follicles, with the pathogenesisof acne. Indeed,S. epidermidis countsremain at pretreat-
Journal of the American Academy of Dermatology May 1995
S22 Leyden
Skin
Fig. 8. Chemotactic factors from P. acnes in inflammatory
acne. PMN,
polymorphonuclear
leukocyte.
(Modifiedfrom ShalitaAR, Lee WL. DermatolClin 1983;1:361-4.)
ment levels,52after an initial drop, during the course of successfulantibiotic acnetherapy.Antibacterial therapy doesnot affect either P. orbiculare or P. ovale, aseachresidesin the upperportion of the acrofundibulum of the sebaceousfollicle. The anaerobicdiphtheroidP. acrzes, in contrast, appearsto play a centralrole in the developmentof inflammation in acne.Perhapsthe most compelling evidenceis the demonstrationthat antibiotic therapy that resultsin significantsuppressionof P. acnes is accompaniedby a reduction in the number of inflammatory lesions,whereasother agents,suchas penicillin, which do not suppressP. acne,s in vivo, have been found to be clinically ineffective in the treatment of acne.53 Inflammation
in acne
Researchhas providedat least a partial explanation of the pathogenesis of inflammation in acne,and the role of P. acnes. One finding is that inflammation in acne is apparently a two-stage process. Initially, lymphocytesand polymorphonuclearleukocytesare recruitedinto the follicular epithelium; if the epithelium is disrupted,intrafollicular material is extrudedinto the dermis, resultingin a variety of inflammatory processes. The “free fatty acid hypothesis”was one of the first explanationsofferedfor the inflammation that accompaniesacne.It wasbasedon two findings:(1) thereis a decreasein the percentageof free fatty acids in skin surfacelipids with successfulantibiotic therapy,and(2) a largeproportionof thosefreefatty
acids resulted from hydrolysis of sebaceousgland triglycerides by P. acne,s.50~54 This theory was strengthenedby the finding that intradermal injection of free fatty acids into human volunteers resultedin intenseinflammation.55This hypothesis assumesthat free fatty acidscansufficiently irritate thefollicular epitheliumto resultin its breakage,and can therebyenablefatty acidsto penetratethe dermis and causeinflammation. Recentresearchhasdemonstratedthat P. acnes can generateinflammation by means other than productionof freefatty acids.In fact, it is now questionableif free fatty acidscan be producedin quantities great enoughto causeinflammation.56 Light microscopyof biopsy specimensfrom inflamed acne lesions was used in early studiesto demonstrateinfiltration of polymorphonuclearleukocytesandlymphocyteswithin intact follicular epithelium.38Integrating thesefindings with current theoriesregardingdirectedmigration of leukocytes would anticipatethe existenceof chemotacticfactors within sebaceousfollicles. This was recently confirmedwhen it wasdemonstrated,in severallaboratories,57, 58that P. acnes producesa dialyzable, low-molecular-weightsubstancethat is chemotactic for polymorphonuclearleukocytesand that does not requireserumor complement.It is believedthat the small sizeof the moleculesenablesthem to diffusethroughintact follicular epithelium and attract polymorphonuclearleukocytes,which then insinuate themselvesinto the lumen of the sebaceous follicle. Thereis a significantdifferenceof P. acnes counts onthe skin of teenagerswith acne,ascomparedwith the skin of age-matchedcontrols;P. acnex countson the former are often severaldegreesof magnitude higher.59It is believedthat P. acnes are ingestedby polymorphonuclearleukocyteswithin the sebaceous follicle lumen. Neutrophil ingestionof P. acne,s has beenshownto result in the releaseof intracellular hydrolytic enzymes,but doesnot kill P. acnes.60 It has already beendemonstratedthat there are specific antibodiesto P. acnes in microcomedone@$ the interaction betweenthose antibodiesand P. acnes results in the releaseof hydrolytic proteasesthat might be of importance in follicular epithelium damage,eventuallycausingthe extrusionof follicular contentsinto the dermis. As hasbeendiscussed,with the disruption of the follicular epithelium,sebaceous lipids,hair, P. acnes, and cornified epithelial cells are extrudedinto the
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
dermis, causing inflammation. Lipids, hairs, and cornified cells generatenonimmune foreign bodytype reactionsassociatedat first with mononuclear cells,and later with macrophagesandgiant cells.P. acne,rgeneratesa seriesof immune and nonimmune inflammatory reactions.The ingestionof P. awes by polymorphonuclearleukocytesresultsin the release of hydrolases,which consequentlycausesfurther tissuedestruction.P. acnesactivatesboth the classic andthe alternativepathwaysof complementto produceCS-derived(C5a) neutrophilchemotacticfactor, which in turn causesthe ingressof polymorphonuclearleukocytesand further inflammation, while simultaneouslyenhancingthe releaseof hydrolytic enzymesfrom neutrophils(Fig. 8).60t62~ 63 The severityof inflammation of acneis paralleled by antibody titers to P. acnes.64, 65This finding is significant becauseantibody is requiredto activate the classicpathway of complement. Immunofluorescencestudiesreveal immunoglobulin and C3 in inflammatory acnelesions,@suggestingevidencefor the role of complement.The significanceof the bacterium wasstrengthenedby the finding that patients with severeinflammatory acnehaveelevatedindices of lymphocytetransformationto P. acnes antigens.67 These findings, in conjunction with evidencethat intradermal injection of P. acnes producesintense inflammation in,the skin of acnepatientsbut only transientinflammation in controlsubjects,68 strongly imply the significanceof P. acnes in inflammation after the follicle has ruptured. Several extracellular enzymes produced by P. ucyles,suchashyaluronidaseandprotease,may also be of importance in inflammation associatedwith acne.69,7oFurther, free fatty acids extractedfrom comedoneshave been reported to be cytotoxic.71 Thus it is possiblethat extrusionof free fatty acids may result in cytotoxicity, thereby facilitating inflammation. The recentfinding of abnormalitiesin neutrophil chemotaxisand phagocytosisof P. acnes in patients with severeacnemay offer insight into the chronic, haphazard nature of inflammation in these patients.72Similarly, the chronicity and intensity of inflammation in patients with highly destructive acneconglobatamight now be explainedby the description of macrophagesin the dermal infiltrate of thesepatientsashavingmorphologiccharacteristics of activatedmacrophagessimilar to thosefound in allergic granulomas.48 In most instances, inflammation is gradually
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down-regulated,and papulesand pustuleslast 3 to 14days.Understandably,largerlesionspersistlonger andhavea greaterprobability of causingpermanent damageor scarring. DISCUSSION We now have a greater understandingof the pathogenesisof acneand are directing the rational therapyapproachesagainstthe three abnormalities discussedherein:(1) the overproductionof sebum; (2) the abnormaldesquamationof sebaceous follicle epithelium; and (3) the proliferation of P. acnes, which generatesinflammation. A great deal remains unknown. The role of androgenreceptorsis unclear;the understandingof comedogenesis at the molecularlevelis incomplete. Although the acuteaspectsof inflammation in acne is well researched,knowledgeregardingits chronic aspectsis neverthelessprimitive. It is not yet known why somepatientsscar,while othersdo not. Nor is it understoodwhat determineswhether a scar becomeshypertrophicor atrophic.Lastly, althoughit is known that remission is not associatedwith changesin sebumor a reductionin P. acnes, it is as yet unknownwhy acne,in most patients,will spontaneouslyimprove and evendisappear. REFERENCES 1. Kraning KK, Odland GF (eds.). Prevalence, morbidity, and cost of dermatological diseases. J Invest Dermatol 1979; 73(5, pt II):395513. 2. Koo JY, Smith LL. Psychological aspects of acne. Pediatr Dermatol 1991;8(3):185-8. 3. Cunliffe W. Acne. London, England: Martin Dunitz, 1989. 4. Montagna W. The sebaceous glands in man. In: Montagna W, Ellis RA, Silver AF, eds. Advances in biology of skin: the sebaceous glands, IV. Oxford, England: Pergamon Press, 1963:19-31. 5. Brandes D, Bertini F, Smith EW. Role of lysosomes in cellular lytic process, II: cell death during holocrine secretion in sebaceous glands. Exp Mol Path01 1965;4:245-65. 6. Jackson D, McQueen L, Jenkinson DM, Nimmo MC, Elder HY, Montgomery I. Passage of lanthanum through the intercellular spaces of the sebaceous gland. Res Vet Sci 1986;40:48-53. 7. Nikkari T. Comparative chemistry of sebum. J Invest Dermatol 1974;62:257-67. 8. Shalita AR. Genesis of free fatty acids. J Invest Dermatol 1974;62:332-5. 9. Nicolaides N. Skin lipids: their biochemical uniqueness. Science 1974;186:19-26. IO. Downing DT, Stewart ME, Wertz PW, Strauss JS. Essential fatty acids and acne. J AM ACAD DERMATOL 1986; 14221-5.
11. Morello AM, Downing DT, Strauss JS. Octadecadienoic acids in the skin surface lipids of acne patients and normal subjects. J Invest Dermatol 1976;66:319-23. 12. Stewart ME, Wertz PW, Grahek MO, Downing DT. Re-
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lationship between sebum secretionrates and the concentration of linoleate in sebum and epidermal lipids [Abstract]. Clin Res 1985;33:684A. 13. Stewart ME, Greenwood R, Cunliffe WJ, Strauss JS, Downing DT. Effect of cyproterone acetateethinyl estradiol treatment on the proportions of linoleic and sebaleic acids in various skin surfacelipid classes.Arch Dermatol Res 1986;278:481-5. 14. Wertz PW, Miethke MC, Long SA, et al. The composition of the ceramidesfrom human stratum corneum and from comedones.J Invest Dermatol 1985;84:410-2. 15. PochiPE, StraussJS. Endocrinologiccontrol of the development and activity of the human sebaceousgland. J Invest Dermatol 1974;62:191-201. 16. StraussJS, Pochi PE, Downing DT. Acne perspectives.J Invest Dermatol 1974;62:321-5. 17. Harris HH, Downing DT, Stewart ME, Strauss JS. Sustainablerates of sebum secretionin acne patients and matched normal control subjects.J AM ACADDERMATOL 1983;8(2):200-3. 18. Jarrett A. The effectsof progesteroneand testosteroneon surface sebum and acne vulgaris. Br J Dermatol 1969; 71:102-16. 19. Pochi PE, Strauss JS. Effect of cyclic administration of conjugated equine estrogens on sebum production in women. J Invest Dermatol 1966;47:582-5. 20. StraussJS, PochiPE. The human sebaceousgland: its regulation by steroidal hormonesand its use as an end organ for assayingandrogenicity in vivo. Recent Prog Horm Res 1963;19:385-444. 21. PochiPE, StraussJS,Mescon H. Sebum secretionand urinary fractional 17-ketosteroidand total 17-hydroxycorticoid excretion in male castrates.J Invest Dermatol 1962; 39:475-83. 22. StraussJS, K&man AM. The effect of progesteroneand progesterone-like compounds on the human sebaceous gland. J Invest Dermatol 1961;36:309-19. 23. Zeligman I, Hubener LF. Experimental production of acne by progesterone.Arch Dermatol 1957;76:652-8. 24. SimpsonNB, Bowden PE, Forster RA, Cunliffe WJ. The effect of topically applied progesteroneon sebumexcretion rate. Br J Dermatol 1979;100:687-92. 25. Strauss JS, Pochi PE. Effect of cyclic progcstin-estrogen therapy on sebum and acne in women. JAMA 1964; 190:815-9. 26. PochiPE, StraussJS.Effect of sequentialmestranol-chlormadinone on sebum production. Arch Dermatol 1967; 95:47-9. 27. Pye RJ, Meyrick G, Pye MJ, Burton JL. Effect of oral contraceptiveson sebum excretion rate. Br Med J 1977; 2:1581-2. 28. Miller JA, Wojnarowska FT, Dowd PM, et al. Antiandrogen treatment in women with acne:a controlledtrial. Br J Dermatol 1986;114:705-16. 29. SansoneG, ReisnerRM. Differential rates of conversionof testosteroneto dihydrotestosteronein acne and in normal human skin: a possiblepathogenic factor in acne. J Invest Dermatol 1979;56:366-72. 30. Lookingbill DP, Horton R, Demers LM, Egan N, Marks JG Jr, Santen RJ. Tissue production of androgens in women with acne. J AM ACAD DERMATOL 1985;12: 481-7. 31. Pochi PE, Longcope C. Sa-Androstane-3cr,17P-diolglucuronide blood levelsand sebumsecretionin acneand normal subjects.In: Marks R, PlewigG, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:67-9.
Journal of the American Academy of Dermatology May 1995
32. StraussJS,Kligman AM. The pathologicdynamicsof acne vulgaris. Arch Dermatol 1960;82:779-90. 33. Knutson DD. Ultrastructural observationsin acnevulgaris: the normal sebaceousfollicle and acne lesions.J Invest Dermatol 1974;62:288-307. 34. WoltI HH, Plewig G, Braun-Falco 0. Ultrastructure of human sebaceousfolliclesand comedonesfollowing treatment with vitamin A acid. Acta Derm Venereol (Stockh) 1975;55(suppl74):99-110. 35. Lavker RM, Leyden JJ. Lamellar inclusionsin follicular horny cells:a new aspectof abnormal keratinization. J Ultrastruct Res 1979;69:362-70. 36. Plewig G, Fulton JE, Kligman AM. Cellular dynamicsof comedo formation in acnevulgaris.Arch Dermatol Forsch 1971;242:12-29. 37. Kurokawa I, Mayer-da-Silva A, Gollnick H, Orfanos CF. Occurrenceand distribution of cytokeratinsand filaggrin in the human pilosebaceousunit: an immunocytochemical study. In: Marks R, Plewig G, eds.Acne and related disorders. London, England: Martin Dunitz, 1989:19-22. 38. Kluznik AR, Wood EJ, CunlifIe WJ. Keratin characterization in the pilosebaceousducts of acne patients. In: Marks R, Plewig G, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:113-5. 39. Kligman AM, Katz AG. Pathogen&s of acne vulgaris: comedogenicproperties of human sebum in external ear canal of the rabbit. Arch Dermatol 1968;98:53-7. 40. Saint-Leger D, Bague A, Cohen E, Chivot M. A possible rolefor squalenein the pathogenesisof acne,I: in vitro study of squaleneoxidation. Br J Dermatol 1986;114:535-42. 41. Motoyoshi K. Enhanced comedo formation in rabbit ear skin by squaleneand oleic acid peroxides.Br J Dermatol 1983;109:191-8. 42. Lavker RM, Leyden JJ, McGinley KJ. The relationship betweenbacteria and the abnormal follicular keratinization in acne vulgaris. J Invest Dermatol 1981;77:325-30. 43. Kligman AM, Mills OH Jr. Acne cosmetica.Arch Dermatol 1972;106:843-50. 44. StraussJS, JacksonEM. Academy of Dermatology Invitational Symposium on Comedogenicity. J AM ACAD DERMATOL1989;20:272-7. 45. Kaidbey KH, Kligman AM. The pathogenesisof topical steroid acne. J Invest Dermatol 1974;62:3l-6. 46. Norris JFB, Cunliffe WJ. A histological and immunocytochemical study of early acne lesions. Br J Dermatol 1988;118:651-9. 47. Lavker RM, Leyden JJ, Kligman AM. The anti-inflammatory activity of isotretinoin is a major factor in the clearing of acne conglobata. In: Marks R, Plewig G, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:207-17. 48. Gilchrist TC. The etiology of acne vulgaris. J Cutan Dis Syph 1903;21:107-20. 49. EvansCA, Smith WM, Johnston EA, Giblett ER. Bacte rial flora of the normal human skin. J Invest Dermatol 1950;15:305-24. 50. Freinkel RK, StraussJS, Yip SY, PochiPE. Effect of tetracyclineon the compositionof sebum in acnevulgaris. N Engl J Med 1965;273:850-4. 5 1. Marples RR, Leyden JJ, Stewart RN, Mills OH Jr, Kligman AM. The skin microflora in acne vulgaris. J Invest Dermatol 1974;62:37-41. 52. Marples RR, Kligman AM. Ecologicaleffects of oral antibiotics on the microflora of human skin. Arch Dermatol 1971;103:148-53. 53. Ad Hoc Committee on the Useof Antibiotics in Dermatol-
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54. 55. 56. 57. 58.
59. 60. 61. 62.
ogy. Systemic antibiotics for treatment of acne vulgaris: efficacy and safety. Arch Dermatol 1975; 111: 1630-6. Marples RR, Downing DT, Kligrnan AM. Control of free fatty acids in human surface lipids by Corynebacterium acnes. J Invest Dermatol 1971;56:127-3 1. Strauss JS, Pochi PE. Intracutaneous injection of sebum and comedones: histological observations. Arch Dermatol 1965;92:443-56. Puhvel SM, Sakamoto M. An in vivo evaluation of the inflammatory effect of purified comedonal components in human skin. J Invest Dermatol 1977;69:401-6. Puhvel SM, Sakamoto M. The chemoattractant properties of comedonal components. J Invest Dermatol 1978; 71:324-9. Webster GF, Leyden JJ. Characterization of serum-inde pendent polymorphonuclear leukocyte chemotactic factors produced by Propionibacterium acnes. Inflammation 1980; 4:261-9. Leyden JJ, McGinley KJ, Mills OH Jr, Kligman AM. Propionibacterium levels in patients with and without acne vulgaris. J Invest Dermatol 1975;65:382-4. Webster GF, Tsai C-C, Leyden JJ. Neutrophil lysosomal release in response to Propionibacterium acnes [Abstract]. J Invest Dermatol 1979;72:209. Webster GF, Kligman AM. A method for the assay of inflammatory mediators in follicular casts. J Invest Dermato1 1979;73:266-8. Webster GF, Leyden JJ, Nilsson UR. Complement activation in acne vulgaris: consumption of complement by comedones. Infect Immun 1979;26:183-6.
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63. Webster GF, Leyden JJ, Norman ME, Nilsson UR. Complement activation in acne vulgaris: in vitro studies with Propionibacterium acnes and Propionibacterium granulosum. Infect Immun 1978;22:523-9. 64. Puhvel SM, Barfatani M, Warnick M, Sternberg TH. Study of antibody levels to Corynebacterium acnes. Arch Dermatol 1964;90:421-7. 65. Puhvel SM, Hoffman LK, Sternberg TH. Presence of complement fixing antibodies to Corynebacterium acnes in the sera of acne patients with acne vulgaris. Arch Dermato1 1966;93:364-6. 66. Dahl MGC, McGibbon DH. Complement in inflammatory acne vulgaris [Letter]. Br Med J 1976;2:1383. 67. Puhvel SM, Amircan D, Weintraub J, Reisner RM. Lymphocyte transformation in subjects with nodulo-cystic acne. Br J Dermatol 1977;97:205-11. 68. Puhvel SM, Hoffman IK, Reisner RM, Sternberg TH. Dermal hypersensitivity of patients with acne vulgaris to Corynebacterium acnes. J Invest Dermatol 1967;49: 154-8. 69. Puhvel SM, Reisner RM. The production of hyaluronidase (hyaluronate lyase) by Corynebacterium acnes. J Invest Dermatol 1972;58:66-70. 70. Voss JG. Differentiation of two groups of Corynebacterium acnes. J Bacterial 1970;101:392-7. 7 1. Tucker SB, Rogers RS, Winkelmann RK, Privett OS, Jordan RE. Inflammation in acne vulgaris: leukocyte attraction and cytotoxicity by comedonal material. J Invest Dermatol 1980;74:21-5. 72. Lee WL, Shalita AR. Leukocyte abnormalities in acne conglobata [Abstract]. J Invest Dermatol 1980;74:258.
New understandings of the pathogenesis of acne James J. Leyden, MD Philadelphia,
Pennsylvania
Acne vulgaris is the most common skin disease, affectingnearly 80%of youngadultsaged11to 30.’ Not only can it causedisfiguration and permanent scarring, but it can have an adverseeffect on psychologicdevelopment,resultingin profoundemotional scarring,which may lead to social phobias, withdrawal from society,and clinical depression2 Researchduring the past threedecadeshasconfirmed the multifactorial originsof the disease.Even the most difficult casesmay now be controlledby usingrational therapiesbasedon currentconceptsof pathogenesis. This article reviewsthe pathophysiologicfactors that influencethe developmentof acne,3most notably ( 1) excessivesebum production,(2) abnormal desquamation of sebaceousfollicle epithelium (comedogenesis), and (3) proliferationof Propionibacterium acne+ resultingin inflammation. SEBACEOUS GLANDS
of Dermatology, Philadelphia.
the University
of Pennsylvania
Reprint requests: James J. Leyden, MD, Hospital of the University Pennsylvania, Department of Dermatology, 2 Maloney Bldg., Spruce St., Philadelphia, PA 19104.4285. J AM ACAD DERMATOL 1995;32:S15-25. Copyright @ 1995 by the American Academy 0190-9622/95
$3.00 + 0
squamae
Sebaceous duct
AND SEBUM
The pilosebaceousunit consists of sebaceous glandsand hair follicles. Sebaceousglandsand hair follicles are physically connectedand usually vary inverselyin size.Theseunits are locatedin all skin areasexceptthe palms and soles.There areapproximately 900glandspersquarecentimeteronthe face (particularly at midface), upper neck, and chest. The restof the bodyaveragesfewerthan 100glands per square centimeter. “Free” sebaceousglands (thosenot attachedto a hair follicle) that opento the surface of the skin are presentin the meibomian glandsof the eyelid, the areolaeof the nipples,and
From the Department School of Medicine,
Canal
16/O/64083
of Dermatology,
of 440
Inc.
Sebaceous gland
Fig. 1. Sebaceous follicle.
the glabrousvermilion bordersof the lip and in the mouth. Pilosebaceousunits, involving follicles that producesmall hairs but associatedwith largesebaceousglands,are called “sebaceousfollicles” (Fig. 1). When sebaceousfollicles are affected, acne occurs. Sebaceousglandsare composedof undifferentiated,differentiated,and mature cells.Undifferentiated cells are located at the peripheryof a gland, adjacentto the basementmembrane.As they are pushedtoward the sebaceousduct, theseundifferentiated cells developinto differentiatedlipid-containing cells.The increasedconcentrationof lipids in mature cellsleadsto cell disintegrationandthe formation of sebum,a complex lipid mixture that is eventuallyexcretedonto the surfaceof skin. s15
Journal of the American Academy of Dermatology May 1995
S16 Leyden Table I. Sebum composition Isolated gland (‘35)
Triglycerides Free fatty acids Wax esters Squalene Cholesterol esters Cholesterol *After
hydrolysis
57 0 25 15 2 1
Surface* (%I
42 15 25 15 2 1
of triglycerides.
One study indicated that sebaceousgland secretions may be partly the result of holocrineproduction of sebum,involvingthe contributionof suchhydrolytic enzymesas acid phosphatesand esterases.5 It nowappearsthat sebaceous glandsarepermeable to certain molecules,suggestingthat sebumproduction may be more complexthan simple autolysisof sebocytes.Thus the final lipid film excretedontothe skin surface may have incorporated substances directly from the dermis,aswell assecretions(without lysis) from active cells within the sebaceous gland.6 Composition of sebum Sebum is primarily composedof squalene,wax esters, triglycerides,
and small amounts of sterols
and sterolesters(Table I). Bacteriallipasesresultin theformation of monoglyceridesanddiglycerides,as well asfree fatty acids,within the sebaceous follicle duct. The lipid film found on skin surfacesis the result of the admixture of pure sebum with epidermally derived lipids. As the compositionof sebum varies by species,7it is assumedthat the function varies. In mammals, the principal function of sebum
is to maintain the hydration of the skin and hair in the stratum corneum. The hydrolysisof sebaceous glandtriglyceridesby P. acYteSin surface lipids yields free fatty acid$; thesefatty acidsaccountfor the principal difference betweenisolatedhuman sebaceousglandlipids and the skin-surfacelipid film. The amount of free cholesterol in surface lipids is not sufficient to account for the productionof thosefree fatty acidsfrom ste-
rol esters;further, the chainsof fatty acidsfrom wax estersarelongerthan thoseof skin surfacelipid-film free fatty acids. The complexity of the human skin surface lipid film is largely
a function
of the diversity
and
uniquenessof its fatty acid components.Only 37%
of “biologically valuable acids,” including linoleic andoleicacids,normally found in internaltissue,are alsofoundin surfacelipid fatty acids.In comparison, the many other fatty acidsthat havebeenidentified in human skin surface lipid film are not normally found in internal tissues.9 Another distinguishingcharacteristicof skin-surface free fatty acids is the wide variety of chain lengths;randomatom lengthsrangefrom 4 to 30.A varietyof carbonnumberskeletaltypeswith straightchain structures,odd and evencarbonnumbers,iso and ante-is0chain structures,and branchedchain structureshas beenidentified.9The pattern of unsaturationof skin surfacefatty acids,in which double bondsare found betweenthe sixth and seventh carbonatomsin monoenoicfatty acids,differsfrom the pattern found in circulating lipids, in which the double bond is frequently betweenthe ninth and tenth carbon atoms.9Thus the fatty acid composition of wax esters and triglycerides cannot be acquiredfrom circulating lipids. Sebum differences in personswith acne Acne patientshavea significantly lower concentration of linoleic acidin their skin surfacelipidsthan do personswithout acne.lo-l4The higher sebumsecretion rates of personswith acne accountsfor a portion of that difference,becausethereis aninverse correlationbetweenthe rate of sebumsecretionand the linoleatecontentof surfacewax esters.l2 In other words,as the amount of sebumsecretionincreases, there is a correspondingdecreasein linoleic acid contentof sebaceous gland wax esters,triglycerides, and free fatty acidsof skin surfacelipids.13 How and why the concentrationof linoleate in sebumdecreases may beof critical importancein the pathogenesisof acne. The current hypothesisregarding the role of linoleic acid in the development of acneoriginatesfrom studieson acnepatientsin which lipids recoveredfrom comedones(abnormal follicles)of acnewereexamined.Identificationof an unanticipateddeficiencyof linoleateled researchers to considerthat an essential-fatty-aciddeficiencyin the follicular epithelium of sebaceousfollicles was involved in comedogenesis. l4 The current explanation of the pathogenesisof acneis based on these findings. When the peripheral undifferentiatedsebaceousgland cells differentiate, the cells haveaccessto circulating lipids, including linoleic acid.However,nomore circulatinglipids are admitted after sebum synthesisbegins. Thus the
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
more sebumsynthesizedper cell, the greaterthe dilution of the initial linoleate.Linoleateis originally in the phospholipidsof cell membranes and is releasedwith the final cell rupture. It is then incorporatedinto variouslipids in proportionto the relative rates at which thoselipids were being synthesized at the time of disintegrationof the sebocyte. Linoleate is found, in decreasingamounts, in cholesterolesters,triglycerides, and wax esters.‘O Follicular epithelium differentiation may be influencedby the lower concentrationof linoleatein the sebumof personswith acne.The essentialfatty acids of the follicular epithelium may easily be obtained from the generalcirculation through diffusionfrom the dermis.Additional fatty acidsmay be obtainedfrom sebaceouslipids that diffuse into the epithelial wall from the follicular lumen. In persons with acne,sebum secretionis high and the concentration of linoleateis consequentlylow; thusthe lipid mixture surroundingthe follicular epithelium is deficient in essentialfatty acids. The relation betweenacne and the overproduction of sebaceous glandlipids haslongbeenacknowledged, as has been the correlation between the severityof acne and the amount of sebum excretion.15,I6 The 1983study by Harris et al-l7 helped quantify the differencein sustainableratesof sebum production betweenpersonswith acne and those without. It was found that sebumdeliveredto the skin surfaceafter depletionof the ductal reservoir with bentoniteis an accuratemeasureof sebaceous gland activity. Hormonal
influences
Androgenic hormonescontrol the secretoryactivity of the sebaceous gland.Testosterone(the main circulating androgen)and androstenediolare synthesizedin the testes.The adrenalglands are the main sourceof dehydroepiandrosterone, andits sulfated form, which is the most abundantcirculating adrenalandrogen;they alsoproduceandrostenedione and androstenediol.Although the ovariescan produce testosterone,androstenedione,and dehydroepiandrosterone, the adrenalgland is an important sourceof circulating androgensin women. In addition,themetabolismof dehydroepiandrosterone can producetestosterone. It is currently assumedthat testosteroneis convertedto dihydrotestosterone, which then bindsto a high-afhnity specific cytoplasmic receptor protein that is transportedto the cell nucleus.At this point,
Leyden
SIT
the dihydrotestosterone-proteincomplex initiates DNA-controlled events(Fig. 2). Although we know that sebaceousgland activity is dependenton androgenichormones,we do not yet understandwhy sebum is overproduced.Further, the resultsof studiesexamining the concentrations of circulating androgensin personswith acneand thosewithout acneoftenconflict. Onesuggestedapproachwould beto measuretheseconcentrations(at severaldifferent points of the menstrual cycle in women),while determiningthe end-organresponse (i.e., sebaceousgland activity). Adequate controls and additional information regardingthe existence of other signsindicativeof excessiveandrogenproduction (e.g.,hirsutism,male patternalopecia,gross menstrual abnormalities) are necessaryfor adequate interpretation.Unfortunately, much of the researchto datelackssufficientinformation for such interpretation.In fact, many studieshave reported abnormalitiesin hormoneconcentrations1s-29; most notably, women were far more likely to have had abnormally high concentrationsof hormonesthan were men. One interpretation that has been offered,29> 3obut not consistentlyconfirmed,31is that thereis an abnormaltarget-tissuemetabolismasthe causeof the disparity. Despitedisagreementsregardingthe resultsand interpretationsof thesestudies,it is possibleto set forth some basic guidelinesand conclusionsconcerning the hormonal influenceson acne.A small proportionof acnepatientswill exhibit signsof excessiveandrogenproduction (hirsutism or significant menstrual dysfunction);thesepatientsshould receivea completeendocrineand gynecologicevaluation.The majority of personswith acne,however, do not exhibit these clinical abnormalities and, therefore,do not warrant such investigations. Lastly, althoughit is not yet known exactly how androgenichormonesare causally related to the overproductionof sebum, it is apparentthat hormonal andnonhormonaltherapiesthat significantly reducethe productionof sebummay be beneficial, COMEDOGENESIS
Comedogenesis, an abnormality in the processof desquamationof follicular corneocytesin the sebaceousfollicle ducts,is centralto the pathogenesisof acne,Comedogenesisis so named becauseit producesa microcomedo,or a microscopic,clinically inapparent,precursorlesion (Fig. 3). The microcomedo evolvesinto either a clinically apparent
S18
Journal of the American Academy of Dermatology May 1995
Leyden
HO-.W Dehydroepiandrosterone
Ii
0 !:-.:-1:l ’
Androstenedione vIf OII
OW Testosterone Sa-reductase
,+ ,“I
Dihydrotestosterone k,‘~ ‘\, ‘4
Scu-Androstane&,17/%diol
Scr-Androstane3/3,17P-diol
Fig. 2. Androgen metabolism in skin: 17@OH pathway.
lesion (an open comedo, or “blackhead”) (Fig. 4), a closed comedo (“whitehead”) (Fig. 5), or an inflammatory lesion, if P. acnes proliferate and generate inflammatory mediators.32 Comedogenesis occurs specifically in sebaceous noninflammatory
follicles, not in terminal or vellus follicles. Histologically, sebaceous follicles have a widely dilated follicular canal lined by stratified squamous epithelium; a fine, vellus-like hair usually in the resting (telogen) phase; and multiacinar sebaceous glands.
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
S19
Leyden
Sebaceous gland
\
Fig. 4. Opencomedo.
Fig. 3. Microcomedo.
The glandsare connectedto the main follicular canal through short sebaceousducts.Within the follicular canal are loose,keratinoussquames(scales) that are impregnatedwith lipids. The classichistologicstudy by Straussand Kligman,32in which biopsyspecimenswere taken from skin that appearedclinically normal, found early microscopicchangesthat are precursorsof clinical lesions.A slight dilatation of the sebaceousfollicle canal, aswell as a slightly hyperplasticepithelium, are the earliestchangesevidentunder light microscopy(Fig. 3). The study alsofound a slight increase in the amount of keratinousmaterial within the canal that forms a microscopic lesion of coherent squames-the microcomedo. The follicular
Sebaceous gland
\
Fig. 5. Closed comedo.
canal in acne
The follicular canal is composedof two portions, the more distal acroinfundibulum,contiguouswith the surfaceepithelium, and the infrainfundibulum, or the region betweenthe epithelium of the sebaceousduct and the follicular epithelium (Fig. 6). In personswithout acne,the acroinfundibulum appearsmorphologically similar to the epidermis. The basallayersof the follicular epithelium produce cells that are organized into a stratified epithe-
lium. Within the surface epithelium are tonofilaments, desmosomes,keratohyalin granules, and melanocytes. The patternof keratinizationundergoesa gradual transition from the infrainfundibulum to the acroinfundibulum, so that wherethe two portionsjoin, the granularlayer, in additionto the well-organized stratum corneum, have each decreasedor disappeared.There are fewer tonofilamentsand desmo-
S20
Journal of the American Academy of Dermatology May 1995
Leyden
Fig. 6. Infundibulum
in normal follicle.
somesthan in the acroinfundibulum; additionally, the keratohyalin granules are smaller and fewer. There are more lamellar granules(Odland bodies) in the infrainfundibulum. Those intracellular organellesare extrudedinto the extracellularspaceof the stratum corneum,wheretheir lipid constituents form a lamellar structurethat is involvedin corneocyte cohesion.Unlike the acroinfundibulum, the thinner stratum corneum of the infrainfundibulum doesnot form a coherentlayer with the respective partially fragmentedcorneocytes. Keratinization
Comedogenesisbegins with dilatation of sebaceousfollicles as a resultof the accumulationof abnormally desquamatedcorneocytes.Under an electron microscopeit is possibleto seethe alteredpat-
Fig. 7. Infundibulum during comedo formation.
tern of keratinizationof the infrainfundibulum.33a 34 Above a thickenedgranular layer containingmany larger keratohyalin granules is a firm, coherent stratum corneum(Fig. 7). In a personwith acne,intrainfundibulum cells contain numerous tonofilaments and desmosomes,but fewer lamellar granules. Large, intracellular inclusionsthat are lowelectron-dense,homogeneousand probably lipid, are easily visible. These inclusions are contiguous with lamellar structuresthat have a configuration suggestiveof polar lipid bilayers.35In follicles that areprecursorsto comedones(clinical noninflammatory lesions),there appearto be alternatelayersof corneocyteswith andwithout lipid inclusions.In the centralportionof the “coccoon”of coherentcorneocytes,large lacunae-containinglipids, bacteria,and cell fragments can be found. Lastly, the cellular
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
mitotic activity of the follicular epithelium is increased.36 A numberof theorieshavebeenofferedto explain comedogenesis. Immunohistochemicalstudieshave generallydemonstratedthat comedogenesisis not the result of abnormalkeratinization,exceptfor the presenceof abnormallipid inclusions.37, 38This possible deviation from normal protein formation is suggestiveof either a flaw in cell differentiation or the result of passivediffusion from lipids within the lumen of sebaceousfollicles. Further, the decreased number of lamellar granulesis indicative of a critical defectthat explainsthe unusualcohesiveness of desquamatedfollicular corneocytes. Experimentsusingthe albinorabbit-earmodel(in which it is possibleto induce comedonesclinically andhistologicallysimilar to thoseof human beings) haveshownthat sebaceous lipids (and,in particular, squaleneand free fatty acids39)havecomedogenic properties.A recent theory proposesthe oxide of squaleneto be a possiblefactor in comedogenesis.40 Additional studieshave suggestedthat ultraviolet irradiation enhancesthe comedogenicproperty of squaleneby increasingthe amountof squaleneperoxidesas well asby forming fatty acid peroxides.41 The finding discussedaboveregarding the decreasedconcentrationof linoleic acid in sebum of acnepatientsoffersoneof the most promisingareas for comedogenesisresearch.1°-12 The essentialrole playedby linoleic acid in maintaining the integrity and function of the epidermal stratum corneum is also probably true for the follicular epithelium. Other researchon children with early acne has provedthat the presenceof bacteria, especiallyP. ucytes,is not essentialfor the initiation of comedogenesis. The microcomedonesandclinically apparentcomedonesin thesechildrenare devoidof P. acnes.Conversely,once P. acnes colonizationoccurs, thoseorganismsliberate free fatty acids that arecomedogenicin the rabbit ear model and,probably, in human skin.42 The role of cosmeticsas an exogenousfactor in comedogenesishas been examined for more than two decades.43According to a recent consensus panel of the American Academy of Dermatology, the ingredientsused in current cosmeticsare predominantly not comedogenic,44 unlike thoseuseda generation ago. Topical corticosteroids,however, have been found to be comedogenicin human beings.45
Leyden PROLIFERATION
S21
AND INFLAMMATION
The potential for postlesionalphysical and emotional scarringis probablyproportionatelyrelatedto the developmentof inflammatory pustules,papules, and nodulocysticlesions.According to Straussand Kligman,32the inflammatory phaseof acneoriginateswith the developmentof intercellular and intracellular edema(spongiosis)of the follicular epithelium as a result of a perivascularinfiltrate that exudesinto the follicular epithelium.The ageof the lesiondetermineswhetherthe predominantcellsare lymphocytes(particularly T-helper cells) or polymorphonuclearleukocytesin the infiltrate.46If the lesionprogresses,the integrity of the follicular epithelium may break or rupture, enabling lymphocytes, neutrophils,and foreign-body-typeor activated macrophages(e.g., those seen in allergic granulomas)to infiltrate the dermis.47It has recentlybeensuggestedthat lossof integrity of the sebaceousfollicle epithelium is not a prerequisitefor smaller inflammatory lesions.46Marked disruption of pilosebaceousunits occurs in late-stageinflammation, especiallywith larger papulesand nodules. Cornified cells,lipids, hair, and bacteria are deposited into the dermis, producinga nonimmune-type inflammation. Bacteria
in the pathogenesis
of acne
It was oncebelievedthat acnewas an infectious processcausedby the bacteria “acne bacillus.“48 The role of bacteriain the pathogenesisof acnewas castinto doubtafter it wasfoundthat this organism, now calledP. acnes, could be recoveredfrom acnefree control subjects.49Conversely,the possibleassociationbetweenbacteriaand the pathogenesisof acne was strengthenedby the well-documented findings of clinical improvement in acne patients treatedwith systemicantibiotic agents.50 Thereis little qualitativedifferencebetweenacne patientsand acne-freecontrol subjectsin microbiologic surveys.The cutaneousflora in both groupsis essentiallya triad of aerobiccocci (predominantly Staphylococcus epidermidis), the yeasts Pityrosporum ovale and Pityrosporum orbiculare, and the anaerobicdiphtheroidPropionibacterium acnes (formerly Corynebacterium acnes).51 There is currently no evidencelinking S. epidermidis, which resides primarily on the skin surfacesrather than within the follicles, with the pathogenesisof acne. Indeed,S. epidermidis countsremain at pretreat-
Journal of the American Academy of Dermatology May 1995
S22 Leyden
Skin
Fig. 8. Chemotactic factors from P. acnes in inflammatory
acne. PMN,
polymorphonuclear
leukocyte.
(Modifiedfrom ShalitaAR, Lee WL. DermatolClin 1983;1:361-4.)
ment levels,52after an initial drop, during the course of successfulantibiotic acnetherapy.Antibacterial therapy doesnot affect either P. orbiculare or P. ovale, aseachresidesin the upperportion of the acrofundibulum of the sebaceousfollicle. The anaerobicdiphtheroidP. acrzes, in contrast, appearsto play a centralrole in the developmentof inflammation in acne.Perhapsthe most compelling evidenceis the demonstrationthat antibiotic therapy that resultsin significantsuppressionof P. acnes is accompaniedby a reduction in the number of inflammatory lesions,whereasother agents,suchas penicillin, which do not suppressP. acne,s in vivo, have been found to be clinically ineffective in the treatment of acne.53 Inflammation
in acne
Researchhas providedat least a partial explanation of the pathogenesis of inflammation in acne,and the role of P. acnes. One finding is that inflammation in acne is apparently a two-stage process. Initially, lymphocytesand polymorphonuclearleukocytesare recruitedinto the follicular epithelium; if the epithelium is disrupted,intrafollicular material is extrudedinto the dermis, resultingin a variety of inflammatory processes. The “free fatty acid hypothesis”was one of the first explanationsofferedfor the inflammation that accompaniesacne.It wasbasedon two findings:(1) thereis a decreasein the percentageof free fatty acids in skin surfacelipids with successfulantibiotic therapy,and(2) a largeproportionof thosefreefatty
acids resulted from hydrolysis of sebaceousgland triglycerides by P. acne,s.50~54 This theory was strengthenedby the finding that intradermal injection of free fatty acids into human volunteers resultedin intenseinflammation.55This hypothesis assumesthat free fatty acidscansufficiently irritate thefollicular epitheliumto resultin its breakage,and can therebyenablefatty acidsto penetratethe dermis and causeinflammation. Recentresearchhasdemonstratedthat P. acnes can generateinflammation by means other than productionof freefatty acids.In fact, it is now questionableif free fatty acidscan be producedin quantities great enoughto causeinflammation.56 Light microscopyof biopsy specimensfrom inflamed acne lesions was used in early studiesto demonstrateinfiltration of polymorphonuclearleukocytesandlymphocyteswithin intact follicular epithelium.38Integrating thesefindings with current theoriesregardingdirectedmigration of leukocytes would anticipatethe existenceof chemotacticfactors within sebaceousfollicles. This was recently confirmedwhen it wasdemonstrated,in severallaboratories,57, 58that P. acnes producesa dialyzable, low-molecular-weightsubstancethat is chemotactic for polymorphonuclearleukocytesand that does not requireserumor complement.It is believedthat the small sizeof the moleculesenablesthem to diffusethroughintact follicular epithelium and attract polymorphonuclearleukocytes,which then insinuate themselvesinto the lumen of the sebaceous follicle. Thereis a significantdifferenceof P. acnes counts onthe skin of teenagerswith acne,ascomparedwith the skin of age-matchedcontrols;P. acnex countson the former are often severaldegreesof magnitude higher.59It is believedthat P. acnes are ingestedby polymorphonuclearleukocyteswithin the sebaceous follicle lumen. Neutrophil ingestionof P. acne,s has beenshownto result in the releaseof intracellular hydrolytic enzymes,but doesnot kill P. acnes.60 It has already beendemonstratedthat there are specific antibodiesto P. acnes in microcomedone@$ the interaction betweenthose antibodiesand P. acnes results in the releaseof hydrolytic proteasesthat might be of importance in follicular epithelium damage,eventuallycausingthe extrusionof follicular contentsinto the dermis. As hasbeendiscussed,with the disruption of the follicular epithelium,sebaceous lipids,hair, P. acnes, and cornified epithelial cells are extrudedinto the
Journal of the American Academy of Dermatology Volume 32, Number 5, Part 3
dermis, causing inflammation. Lipids, hairs, and cornified cells generatenonimmune foreign bodytype reactionsassociatedat first with mononuclear cells,and later with macrophagesandgiant cells.P. acne,rgeneratesa seriesof immune and nonimmune inflammatory reactions.The ingestionof P. awes by polymorphonuclearleukocytesresultsin the release of hydrolases,which consequentlycausesfurther tissuedestruction.P. acnesactivatesboth the classic andthe alternativepathwaysof complementto produceCS-derived(C5a) neutrophilchemotacticfactor, which in turn causesthe ingressof polymorphonuclearleukocytesand further inflammation, while simultaneouslyenhancingthe releaseof hydrolytic enzymesfrom neutrophils(Fig. 8).60t62~ 63 The severityof inflammation of acneis paralleled by antibody titers to P. acnes.64, 65This finding is significant becauseantibody is requiredto activate the classicpathway of complement. Immunofluorescencestudiesreveal immunoglobulin and C3 in inflammatory acnelesions,@suggestingevidencefor the role of complement.The significanceof the bacterium wasstrengthenedby the finding that patients with severeinflammatory acnehaveelevatedindices of lymphocytetransformationto P. acnes antigens.67 These findings, in conjunction with evidencethat intradermal injection of P. acnes producesintense inflammation in,the skin of acnepatientsbut only transientinflammation in controlsubjects,68 strongly imply the significanceof P. acnes in inflammation after the follicle has ruptured. Several extracellular enzymes produced by P. ucyles,suchashyaluronidaseandprotease,may also be of importance in inflammation associatedwith acne.69,7oFurther, free fatty acids extractedfrom comedoneshave been reported to be cytotoxic.71 Thus it is possiblethat extrusionof free fatty acids may result in cytotoxicity, thereby facilitating inflammation. The recentfinding of abnormalitiesin neutrophil chemotaxisand phagocytosisof P. acnes in patients with severeacnemay offer insight into the chronic, haphazard nature of inflammation in these patients.72Similarly, the chronicity and intensity of inflammation in patients with highly destructive acneconglobatamight now be explainedby the description of macrophagesin the dermal infiltrate of thesepatientsashavingmorphologiccharacteristics of activatedmacrophagessimilar to thosefound in allergic granulomas.48 In most instances, inflammation is gradually
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down-regulated,and papulesand pustuleslast 3 to 14days.Understandably,largerlesionspersistlonger andhavea greaterprobability of causingpermanent damageor scarring. DISCUSSION We now have a greater understandingof the pathogenesisof acneand are directing the rational therapyapproachesagainstthe three abnormalities discussedherein:(1) the overproductionof sebum; (2) the abnormaldesquamationof sebaceous follicle epithelium; and (3) the proliferation of P. acnes, which generatesinflammation. A great deal remains unknown. The role of androgenreceptorsis unclear;the understandingof comedogenesis at the molecularlevelis incomplete. Although the acuteaspectsof inflammation in acne is well researched,knowledgeregardingits chronic aspectsis neverthelessprimitive. It is not yet known why somepatientsscar,while othersdo not. Nor is it understoodwhat determineswhether a scar becomeshypertrophicor atrophic.Lastly, althoughit is known that remission is not associatedwith changesin sebumor a reductionin P. acnes, it is as yet unknownwhy acne,in most patients,will spontaneouslyimprove and evendisappear. REFERENCES 1. Kraning KK, Odland GF (eds.). Prevalence, morbidity, and cost of dermatological diseases. J Invest Dermatol 1979; 73(5, pt II):395513. 2. Koo JY, Smith LL. Psychological aspects of acne. Pediatr Dermatol 1991;8(3):185-8. 3. Cunliffe W. Acne. London, England: Martin Dunitz, 1989. 4. Montagna W. The sebaceous glands in man. In: Montagna W, Ellis RA, Silver AF, eds. Advances in biology of skin: the sebaceous glands, IV. Oxford, England: Pergamon Press, 1963:19-31. 5. Brandes D, Bertini F, Smith EW. Role of lysosomes in cellular lytic process, II: cell death during holocrine secretion in sebaceous glands. Exp Mol Path01 1965;4:245-65. 6. Jackson D, McQueen L, Jenkinson DM, Nimmo MC, Elder HY, Montgomery I. Passage of lanthanum through the intercellular spaces of the sebaceous gland. Res Vet Sci 1986;40:48-53. 7. Nikkari T. Comparative chemistry of sebum. J Invest Dermatol 1974;62:257-67. 8. Shalita AR. Genesis of free fatty acids. J Invest Dermatol 1974;62:332-5. 9. Nicolaides N. Skin lipids: their biochemical uniqueness. Science 1974;186:19-26. IO. Downing DT, Stewart ME, Wertz PW, Strauss JS. Essential fatty acids and acne. J AM ACAD DERMATOL 1986; 14221-5.
11. Morello AM, Downing DT, Strauss JS. Octadecadienoic acids in the skin surface lipids of acne patients and normal subjects. J Invest Dermatol 1976;66:319-23. 12. Stewart ME, Wertz PW, Grahek MO, Downing DT. Re-
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lationship between sebum secretionrates and the concentration of linoleate in sebum and epidermal lipids [Abstract]. Clin Res 1985;33:684A. 13. Stewart ME, Greenwood R, Cunliffe WJ, Strauss JS, Downing DT. Effect of cyproterone acetateethinyl estradiol treatment on the proportions of linoleic and sebaleic acids in various skin surfacelipid classes.Arch Dermatol Res 1986;278:481-5. 14. Wertz PW, Miethke MC, Long SA, et al. The composition of the ceramidesfrom human stratum corneum and from comedones.J Invest Dermatol 1985;84:410-2. 15. PochiPE, StraussJS. Endocrinologiccontrol of the development and activity of the human sebaceousgland. J Invest Dermatol 1974;62:191-201. 16. StraussJS, Pochi PE, Downing DT. Acne perspectives.J Invest Dermatol 1974;62:321-5. 17. Harris HH, Downing DT, Stewart ME, Strauss JS. Sustainablerates of sebum secretionin acne patients and matched normal control subjects.J AM ACADDERMATOL 1983;8(2):200-3. 18. Jarrett A. The effectsof progesteroneand testosteroneon surface sebum and acne vulgaris. Br J Dermatol 1969; 71:102-16. 19. Pochi PE, Strauss JS. Effect of cyclic administration of conjugated equine estrogens on sebum production in women. J Invest Dermatol 1966;47:582-5. 20. StraussJS, PochiPE. The human sebaceousgland: its regulation by steroidal hormonesand its use as an end organ for assayingandrogenicity in vivo. Recent Prog Horm Res 1963;19:385-444. 21. PochiPE, StraussJS,Mescon H. Sebum secretionand urinary fractional 17-ketosteroidand total 17-hydroxycorticoid excretion in male castrates.J Invest Dermatol 1962; 39:475-83. 22. StraussJS, K&man AM. The effect of progesteroneand progesterone-like compounds on the human sebaceous gland. J Invest Dermatol 1961;36:309-19. 23. Zeligman I, Hubener LF. Experimental production of acne by progesterone.Arch Dermatol 1957;76:652-8. 24. SimpsonNB, Bowden PE, Forster RA, Cunliffe WJ. The effect of topically applied progesteroneon sebumexcretion rate. Br J Dermatol 1979;100:687-92. 25. Strauss JS, Pochi PE. Effect of cyclic progcstin-estrogen therapy on sebum and acne in women. JAMA 1964; 190:815-9. 26. PochiPE, StraussJS.Effect of sequentialmestranol-chlormadinone on sebum production. Arch Dermatol 1967; 95:47-9. 27. Pye RJ, Meyrick G, Pye MJ, Burton JL. Effect of oral contraceptiveson sebum excretion rate. Br Med J 1977; 2:1581-2. 28. Miller JA, Wojnarowska FT, Dowd PM, et al. Antiandrogen treatment in women with acne:a controlledtrial. Br J Dermatol 1986;114:705-16. 29. SansoneG, ReisnerRM. Differential rates of conversionof testosteroneto dihydrotestosteronein acne and in normal human skin: a possiblepathogenic factor in acne. J Invest Dermatol 1979;56:366-72. 30. Lookingbill DP, Horton R, Demers LM, Egan N, Marks JG Jr, Santen RJ. Tissue production of androgens in women with acne. J AM ACAD DERMATOL 1985;12: 481-7. 31. Pochi PE, Longcope C. Sa-Androstane-3cr,17P-diolglucuronide blood levelsand sebumsecretionin acneand normal subjects.In: Marks R, PlewigG, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:67-9.
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32. StraussJS,Kligman AM. The pathologicdynamicsof acne vulgaris. Arch Dermatol 1960;82:779-90. 33. Knutson DD. Ultrastructural observationsin acnevulgaris: the normal sebaceousfollicle and acne lesions.J Invest Dermatol 1974;62:288-307. 34. WoltI HH, Plewig G, Braun-Falco 0. Ultrastructure of human sebaceousfolliclesand comedonesfollowing treatment with vitamin A acid. Acta Derm Venereol (Stockh) 1975;55(suppl74):99-110. 35. Lavker RM, Leyden JJ. Lamellar inclusionsin follicular horny cells:a new aspectof abnormal keratinization. J Ultrastruct Res 1979;69:362-70. 36. Plewig G, Fulton JE, Kligman AM. Cellular dynamicsof comedo formation in acnevulgaris.Arch Dermatol Forsch 1971;242:12-29. 37. Kurokawa I, Mayer-da-Silva A, Gollnick H, Orfanos CF. Occurrenceand distribution of cytokeratinsand filaggrin in the human pilosebaceousunit: an immunocytochemical study. In: Marks R, Plewig G, eds.Acne and related disorders. London, England: Martin Dunitz, 1989:19-22. 38. Kluznik AR, Wood EJ, CunlifIe WJ. Keratin characterization in the pilosebaceousducts of acne patients. In: Marks R, Plewig G, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:113-5. 39. Kligman AM, Katz AG. Pathogen&s of acne vulgaris: comedogenicproperties of human sebum in external ear canal of the rabbit. Arch Dermatol 1968;98:53-7. 40. Saint-Leger D, Bague A, Cohen E, Chivot M. A possible rolefor squalenein the pathogenesisof acne,I: in vitro study of squaleneoxidation. Br J Dermatol 1986;114:535-42. 41. Motoyoshi K. Enhanced comedo formation in rabbit ear skin by squaleneand oleic acid peroxides.Br J Dermatol 1983;109:191-8. 42. Lavker RM, Leyden JJ, McGinley KJ. The relationship betweenbacteria and the abnormal follicular keratinization in acne vulgaris. J Invest Dermatol 1981;77:325-30. 43. Kligman AM, Mills OH Jr. Acne cosmetica.Arch Dermatol 1972;106:843-50. 44. StraussJS, JacksonEM. Academy of Dermatology Invitational Symposium on Comedogenicity. J AM ACAD DERMATOL1989;20:272-7. 45. Kaidbey KH, Kligman AM. The pathogenesisof topical steroid acne. J Invest Dermatol 1974;62:3l-6. 46. Norris JFB, Cunliffe WJ. A histological and immunocytochemical study of early acne lesions. Br J Dermatol 1988;118:651-9. 47. Lavker RM, Leyden JJ, Kligman AM. The anti-inflammatory activity of isotretinoin is a major factor in the clearing of acne conglobata. In: Marks R, Plewig G, eds.Acne and related disorders.London, England: Martin Dunitz, 1989:207-17. 48. Gilchrist TC. The etiology of acne vulgaris. J Cutan Dis Syph 1903;21:107-20. 49. EvansCA, Smith WM, Johnston EA, Giblett ER. Bacte rial flora of the normal human skin. J Invest Dermatol 1950;15:305-24. 50. Freinkel RK, StraussJS, Yip SY, PochiPE. Effect of tetracyclineon the compositionof sebum in acnevulgaris. N Engl J Med 1965;273:850-4. 5 1. Marples RR, Leyden JJ, Stewart RN, Mills OH Jr, Kligman AM. The skin microflora in acne vulgaris. J Invest Dermatol 1974;62:37-41. 52. Marples RR, Kligman AM. Ecologicaleffects of oral antibiotics on the microflora of human skin. Arch Dermatol 1971;103:148-53. 53. Ad Hoc Committee on the Useof Antibiotics in Dermatol-
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54. 55. 56. 57. 58.
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ogy. Systemic antibiotics for treatment of acne vulgaris: efficacy and safety. Arch Dermatol 1975; 111: 1630-6. Marples RR, Downing DT, Kligrnan AM. Control of free fatty acids in human surface lipids by Corynebacterium acnes. J Invest Dermatol 1971;56:127-3 1. Strauss JS, Pochi PE. Intracutaneous injection of sebum and comedones: histological observations. Arch Dermatol 1965;92:443-56. Puhvel SM, Sakamoto M. An in vivo evaluation of the inflammatory effect of purified comedonal components in human skin. J Invest Dermatol 1977;69:401-6. Puhvel SM, Sakamoto M. The chemoattractant properties of comedonal components. J Invest Dermatol 1978; 71:324-9. Webster GF, Leyden JJ. Characterization of serum-inde pendent polymorphonuclear leukocyte chemotactic factors produced by Propionibacterium acnes. Inflammation 1980; 4:261-9. Leyden JJ, McGinley KJ, Mills OH Jr, Kligman AM. Propionibacterium levels in patients with and without acne vulgaris. J Invest Dermatol 1975;65:382-4. Webster GF, Tsai C-C, Leyden JJ. Neutrophil lysosomal release in response to Propionibacterium acnes [Abstract]. J Invest Dermatol 1979;72:209. Webster GF, Kligman AM. A method for the assay of inflammatory mediators in follicular casts. J Invest Dermato1 1979;73:266-8. Webster GF, Leyden JJ, Nilsson UR. Complement activation in acne vulgaris: consumption of complement by comedones. Infect Immun 1979;26:183-6.
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63. Webster GF, Leyden JJ, Norman ME, Nilsson UR. Complement activation in acne vulgaris: in vitro studies with Propionibacterium acnes and Propionibacterium granulosum. Infect Immun 1978;22:523-9. 64. Puhvel SM, Barfatani M, Warnick M, Sternberg TH. Study of antibody levels to Corynebacterium acnes. Arch Dermatol 1964;90:421-7. 65. Puhvel SM, Hoffman LK, Sternberg TH. Presence of complement fixing antibodies to Corynebacterium acnes in the sera of acne patients with acne vulgaris. Arch Dermato1 1966;93:364-6. 66. Dahl MGC, McGibbon DH. Complement in inflammatory acne vulgaris [Letter]. Br Med J 1976;2:1383. 67. Puhvel SM, Amircan D, Weintraub J, Reisner RM. Lymphocyte transformation in subjects with nodulo-cystic acne. Br J Dermatol 1977;97:205-11. 68. Puhvel SM, Hoffman IK, Reisner RM, Sternberg TH. Dermal hypersensitivity of patients with acne vulgaris to Corynebacterium acnes. J Invest Dermatol 1967;49: 154-8. 69. Puhvel SM, Reisner RM. The production of hyaluronidase (hyaluronate lyase) by Corynebacterium acnes. J Invest Dermatol 1972;58:66-70. 70. Voss JG. Differentiation of two groups of Corynebacterium acnes. J Bacterial 1970;101:392-7. 7 1. Tucker SB, Rogers RS, Winkelmann RK, Privett OS, Jordan RE. Inflammation in acne vulgaris: leukocyte attraction and cytotoxicity by comedonal material. J Invest Dermatol 1980;74:21-5. 72. Lee WL, Shalita AR. Leukocyte abnormalities in acne conglobata [Abstract]. J Invest Dermatol 1980;74:258.