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Lipase expression in human skin
Journal of Dermatological Science, 1 (1990) 195-200 Elsevier
195
DESC 00023
Lipase expression in human skin F. Jimenez-Acosta,
Lilian Planas, and Neal S. Penneys
Department of Dermatology, University of Miami School of Medicine, Miami, Florida, V.S.A (Received 21 August 1989; accepted
16 January
Key words: Lipase; Immunohistochemistry;
1990)
Sebaceous
glands
Abstract We have used a monoclonal antibody against human pancreatic lipase to study the immunohistochemical expression of lipase in formalinfixed, paraffm-embedded biopsy specimens from normal and a variety of tumoral and inflammatory skin diseases. In normal skin, lipase is detected in the sebaceous glands and in the external root sheath of the hair follicle. Antibody to lipase might be a valuable reagent to help confirm sebaceous and follicular differentiation in adnexal tumors. Immunoreactivity to lipase is also detected within mononuclear phagocytes in situations where extracellular release of lipids occurs, such as in xanthomas or in inflammation of the fat with significant lysis of adipocytes. The antibody to lipase identifies a major protein in pilosebaceous homogenates of 42.7 kDa following immunoblotting.
Introduction Lipases are lipolytic enzymes indispensible for the biological turnover of lipids. Lipases are longchain fatty acid ester hydrolases that hydrolyze triglycerides to glycerol and fatty acids. Lipases function in the deposition and mobilization of fat and in the metabolism of intracellular lipids [ 11. The enzyme from the pancreas is the most completely described lipase. Human pancreatic lipase consists of a protein composed of a single polypeptide chain of 48 kDa [ 21. However, other lipases with obvious biological importance in human tissues and secretions have been described, such as hormone-sensitive lipase, lipoCorrespondence to: Neal S. Penneys, 016940, Miami, FL 33101, U.S.A. 0923-181 l/90/$03.50
M.D.,
Ph.D.,
Box
0 1990 Elsevier Science Publishers
protein lipase, lingual lipase or milk lipase, each of which has its own unique function in lipid metabolism and fat digestion [ 31. The immunohistochemical detection of lipase in normal human skin as well as in cutaneous disorders has not been previously described. This study reports preliminary mapping in formalinfixed, paraffin-embedded tissues of the immunohistochemical distribution of lipase in normal human skin as well as in a variety of inflammatory and tumoral skin diseases using a monoclonal antibody raised against human pancreatic lipase. Materials and Methods Biopsy Material An immunohistochemical
formed B.V. (Biomedical
on 6 pm sections Division)
study was perfrom formalin-fixed,
196
paraffin-embedded biopsy specimens from normal skin, cutaneous tumors and inflammatory skin diseases. The tumors represented a diversity of pilar and sebaceous neoplasms, basal cell carcinomas of the solid type and with follicular differentiation, as well as a variety of tumors of different histogenesis as listed in Table I. The inflammatory skin diseases that were chosen for the present study included erythema nodosum (5 cases studied), subcutaneous fat necrosis of the newborn (3 cases), xanthomas (10 cases), granuloma annulare (10 cases), sarcoid granuloma (3 cases) and granulation tissue (5 cases). Immunoperoxidase
staining
The immunohistochemical demonstration of lipase was performed with the use of standard techniques and the avidin-biotin-peroxidase comTABLE
I
Immunoblotting
Immunohistochemical tumors
expression
of lipase
Tumor (number studied in parentheses)
in cutaneous
Immunostaining
Pilar tumors:
Trichilemmoma (10) T. follicular infundibulum (5) Pilomatrixoma (10) Trichoepithelioma (10) Steatocystoma multiplex (5)
Sebaceous
plex system [4]. After deparafIinization with xylene and hydration of the tissues with ethanol, endogenous peroxidase activity was blocked with 0.6% hydrogen peroxide in methanol for 20 min. Each slide was treated sequentially with normal goat serum (Vector Laboratories, Inc., Burlingame, CA), monoclonal mouse antibody against human pancreatic lipase (Beckman Instruments, Inc., Carlsbad, CA) at a dilution of 1 : 100, biotinylated antimouse IgG (Vector Laboratories) at a 1 : 50 dilution, and avidin-biotin-peroxidase complex (Vector Laboratories) at a 1 : 220 dilution. The reaction was visualized by exposure to diaminobenzidine and hydrogen peroxide with the use of standard methods, then stained with hematoxylin and mounted. Appropriate antibody controls were included in each experiment. Sections of normal pancreas served as positive control.
+ in shadow cells + in cuticle and sebaceous gland
tumors:
Sebaceous Sebaceous
adenoma (5) epithelioma (5)
Basal cell carcinoma:
Solid or undifferentiated type (10) With follicular differentiation (10)
+ in focal areas
Mscellaneous:
Mammary and extramammary Paget (3) Kaposi’s sarcoma (5) Merkell cell carninoma (3) Malignant melanoma (5) Malignant fibrous histiocytoma + : positive immunostaining;
Sebaceous glands and hair follicles were isolated by direct microdissection from whole skin, homogenized using a motorized glass stirrer, and then the proteins were separated by electrophoresis on polyacrylamide-10% SDS gels [ 51. The gel lanes were subsequently transferred to nitrocellulose sheets according to the method of Towbin [ 61. The blot was incubated with a monoclonal mouse antibody against human pancreatic lipase at a dilution of 1 : 100 overnight at room temperature with shaking. After incubation with the lipase antibody, bound antibody was detected by incubation with biotinylated antimouse IgG followed by streptavidin-horseradish peroxidase with subsequent detection of bound enzyme using 4-chloro- 1-naphthol and hydrogen peroxide as substrates. Results Normal skin
(3)
- : negative immunostaining.
In normal skin specimens, immunoreactive lipase was uniformly detected in the pilosebaceous units, both in the sebaceous glands and in the hair follicle. Within the sebaceous glands, staining
was seen in the more differentiated and centrally located cells rather than in the basophilic cuboidal cells forming the peripheral layer (Fig. 1). In hair follicles, lipase immunoreactivity was detected in the external root sheath, sparing the inner root sheath, the cuticle, the cortex and the matrix of the hair. In some specimens variable expression was detected in sweat glands as well as in nerve fascicles. The remaining structures in skin, including normal adipocytes, did not show lipase immunoreactivity. Skin tumors In the group of pilar tumors, trichilemmoma and tumor of follicular infundibulum (Mehregan’s tumor) diffusely stained (Fig. 2). Pilomatrixoma
Fig. 2. Trichilemmoma
Fig. 1. Staining of sebaceous
glands by lipase antibody.
showing intense immunoreactivity lipase.
to
had strongly positive lipase staining in the shadow cells but not in the basophilic undifferentiated cells (Fig. 3). Trichoepithelioma, in contrast, showed no lipase expression. The homogenous, eosinophilic horny layer seen adjacent to the lumen in steatocystoma multiplex showed positive staining as did sebaceous glands normally present in the vicinity of this type of cyst. Basal cell carcinomas of the solid or undifferentiated cell type were negative, however, basal cell carcinomas with follicular differentiation contained lipase in areas of follicular differentiation. All sebaceous tumours contained lipase in well-differentiated sebaceous cells but not in the undifferentiated germinative cells at the periphery of the tumor (Fig. 4).
Fig. 3. Pilomatrixoma. Note strong immunoreactivity to lipase in the shadow cells but not in the basophilic undifferentiated cells.
Fig. 4. Sebaceous adenoma. Lipase immunoreactivity present in well-differentiated sebaceous cells.
Lipase was not found in other skin tumors of different histogenesis (see Table I).
Immunoblotting Immunoblotting experiments using the lipase antibody revealed a distinct band of apparent molecular weight of approximately 42.7 kDa (Fig. 6). This value was estimated by logarithmic calibration of the gel with respect to the molecular weight markers.
Inflammatory skin disorders In erythema nodosum, lipase staining was detected within scattered mononuclear phagocytes surrounding the adipocytes in areas with fat inflammation, but not in intact fat cells. A similar immunohistochemical pattern was observed in the subcutaneous fat necrosis of the newborn (Fig. 5). In xanthoma lesions, foamy cells contained lipase. Cells composing granuloma annulare and granulation tissue did not demonstrate immunoreactive lipase (Table II).
is
Discussion In this study we have examined by immunohistochemical procedures the distribution of lipase in normal skin and in a variety of skin tumors and inflammatory skin diseases. In normal skin, lipase was detected in sebace-
199
MOL
Fig. 5. Erythema nodosum. Lipase staining is present within mononuclear phagocytes surrounding adipocytes in areas with fat inflammation. TABLE
II
Immunohistochemical diseases
expression
of lipase in inflammatory
Disease
Immunostaining
Erythema nodosum Subcutaneous fat necrosis of newborn Xanthomas Granuloma annulare Granulation tissue
+ in mononuclear + in mononuclear
+ : positive immunostaining;
phagocytes phagocytes
+ in foamy cells _ - : negative immunostaining.
cells and in the cells forming the external root sheath of the pilar apparatus. Sebaceous glands contain variable amounts of lipids including tri-
ous
WT
Fig. 6. Western blot of homogenized pilosebaceous units incubated with the monoclonal antibody against human pancreatic lipase. Track A shows a prominent band about 42.7 kDa precipitated by the antibody to lipase. Track WT shows molecular weight markers (kDa): 97.4 is rabbit muscle phosphorylase b; 66.2 is bovine serum albumin; 42.7 is hen egg white ovalbumin; 31.0 is bovine carbonic anhydrase; 21.5 is soybean trypsin inhibitor and 14.0 is hen egg white lysozyme.
glycerides, wax esters, squalene, cholesterol and cholesterol esters [7]. Lipase released from sebaceous cells may hydrolyze the triglycerides of the sebum, contributing to the formation of free fatty acids identified in human surface lipids [ 81. A similar mechanism has been postulated through the action of lipase contained in microorganisms usually found in the follicular canal and sebaceous ducts such as P. acnes [9] or Demodex mites [ 101.
200
There are at least two types of lipase in normal adipose tissue: one is hormone-sensitive lipase, and the other is lipoprotein lipase. The former releases fatty acids, while the latter deposits fatty acids in adipose tissue. The absence of immunoreactivity in normal adipose tissue suggests that these enzymes do not share the epitope that is recognized by the monoclonal antibody used in our study against human pancreatic lipase. The antibody to lipase might well be of value in confirming sebaceous and follicular differentiation in some adnexal tumors. Tumors with sebaceous differentiation showed positive lipase staining. However, in the group of pilar tumors there was variability, apparently related to the state of differentiation of the tumor cell. Lipase was detected in pilar tumors of external root sheath origin such as trichilemmoma and tumor of the follicular infundibulum. In addition, the staining of the eosinophilic horny layer of steatocystoma multiplex by lipase antibody is in accord with the evidence which suggests that this type of cyst is related to the outer root sheath [ 111. The relationship between the follicular differentiation and lipase expression is further strengthened by the lipase immunoreactivity of the cornified ‘shadow’ cells of pilomatrixoma, cells which represent faulty attempts at formation of hair shafts [ 121. Among the inflammatory disorders, the patterns of lipase expression in erythema nodosum and in subcutaneous fat necrosis of newborn were very similar. In both diseases, lipase immunoreactivity was detected in mononuclear phagocytes adjacent to adipocytes in areas with evident fat inflammation. We hypothesize that the inflammation disrupts fat cells, releasing lipid which induces the expression of lipase in adjacent mononuclear phagocytes. The induction of lipase would facilitate digestion and clearance of the
lipid material released by the damaged adipocytes. In summary, although the lipase staining pattern is not absolutely specific, it is usually present in sebaceous glands, external root sheaths, as well as in tumors thought to be related to these structures. Lipase is also present in mononuclear phagocytes as a digestive enzyme in those cases where extracellular deposits of lipids are found. References 1 Mead JF, Alfin-Slater RB, Howton DR, Popjak G: Lipids: Chemistry, Biochemistry and Nutrition. Plenum Press, New York, 1986. 2 De Caro A, Figarella C, Amic J, Michel R, Guy 0: Human pancreatic lipase: a glycoprotein. Biochim Biophys Acta 490: 411-419, 1977. 3 Borgstrom B, Brockman HL: Lipases. Elsevier, Amsterdam, 1984. 4 Taylor CR: Immunoperoxidase techniques: Practical and theoretical aspects. Arch Path01 Lab Med 102: 113-121, 1978. 5 Laemmli UK: Cleavage of structural proteins during the assembly of the head of the bacteriophage T4. Nature 227: 680-685, 1970. 6 Towbin H, Staehelin T, Gordon J: Electrophoretic transfer of proteins from acrylamide gels to nitrocellulose sheets: procedure and applications. Proc Nat1 Acad Sci USA 76: 4350-4354, 1979. I Downing DT, Strauss JS, Pochi PE: Variability in the chemical composition of human skin surface lipids, J Invest Dermatol 53: 322-327, 1969. 8 Shalita AR: Genesis offree fatty acids. J Invest Dermatol 62: 332-335, 1974. 9 Weary P: Comedogenic potential of the lipid extract of Pityrosporum ovale. Arch Dermatol 102: 84-91, 1970. 10 Jimenez-Acosta F, Planas L, Penneys NS: Demodex mites contain immunoreactive lipase. Arch Dermatol 125: 1436-1437, 1988. 11 Lever WF, Schaumburg-Lever G: Histopathology of the skin (6th. edn) JB Lippincott Company, Philadelphia, 1983, pp. 486-487. 12 Jacobson M, Ackerman AB: ‘Shadow’ cells as clues to follicular differentiation. Am J Dermatopathol 9: 51-57, 1987.
195
DESC 00023
Lipase expression in human skin F. Jimenez-Acosta,
Lilian Planas, and Neal S. Penneys
Department of Dermatology, University of Miami School of Medicine, Miami, Florida, V.S.A (Received 21 August 1989; accepted
16 January
Key words: Lipase; Immunohistochemistry;
1990)
Sebaceous
glands
Abstract We have used a monoclonal antibody against human pancreatic lipase to study the immunohistochemical expression of lipase in formalinfixed, paraffm-embedded biopsy specimens from normal and a variety of tumoral and inflammatory skin diseases. In normal skin, lipase is detected in the sebaceous glands and in the external root sheath of the hair follicle. Antibody to lipase might be a valuable reagent to help confirm sebaceous and follicular differentiation in adnexal tumors. Immunoreactivity to lipase is also detected within mononuclear phagocytes in situations where extracellular release of lipids occurs, such as in xanthomas or in inflammation of the fat with significant lysis of adipocytes. The antibody to lipase identifies a major protein in pilosebaceous homogenates of 42.7 kDa following immunoblotting.
Introduction Lipases are lipolytic enzymes indispensible for the biological turnover of lipids. Lipases are longchain fatty acid ester hydrolases that hydrolyze triglycerides to glycerol and fatty acids. Lipases function in the deposition and mobilization of fat and in the metabolism of intracellular lipids [ 11. The enzyme from the pancreas is the most completely described lipase. Human pancreatic lipase consists of a protein composed of a single polypeptide chain of 48 kDa [ 21. However, other lipases with obvious biological importance in human tissues and secretions have been described, such as hormone-sensitive lipase, lipoCorrespondence to: Neal S. Penneys, 016940, Miami, FL 33101, U.S.A. 0923-181 l/90/$03.50
M.D.,
Ph.D.,
Box
0 1990 Elsevier Science Publishers
protein lipase, lingual lipase or milk lipase, each of which has its own unique function in lipid metabolism and fat digestion [ 31. The immunohistochemical detection of lipase in normal human skin as well as in cutaneous disorders has not been previously described. This study reports preliminary mapping in formalinfixed, paraffin-embedded tissues of the immunohistochemical distribution of lipase in normal human skin as well as in a variety of inflammatory and tumoral skin diseases using a monoclonal antibody raised against human pancreatic lipase. Materials and Methods Biopsy Material An immunohistochemical
formed B.V. (Biomedical
on 6 pm sections Division)
study was perfrom formalin-fixed,
196
paraffin-embedded biopsy specimens from normal skin, cutaneous tumors and inflammatory skin diseases. The tumors represented a diversity of pilar and sebaceous neoplasms, basal cell carcinomas of the solid type and with follicular differentiation, as well as a variety of tumors of different histogenesis as listed in Table I. The inflammatory skin diseases that were chosen for the present study included erythema nodosum (5 cases studied), subcutaneous fat necrosis of the newborn (3 cases), xanthomas (10 cases), granuloma annulare (10 cases), sarcoid granuloma (3 cases) and granulation tissue (5 cases). Immunoperoxidase
staining
The immunohistochemical demonstration of lipase was performed with the use of standard techniques and the avidin-biotin-peroxidase comTABLE
I
Immunoblotting
Immunohistochemical tumors
expression
of lipase
Tumor (number studied in parentheses)
in cutaneous
Immunostaining
Pilar tumors:
Trichilemmoma (10) T. follicular infundibulum (5) Pilomatrixoma (10) Trichoepithelioma (10) Steatocystoma multiplex (5)
Sebaceous
plex system [4]. After deparafIinization with xylene and hydration of the tissues with ethanol, endogenous peroxidase activity was blocked with 0.6% hydrogen peroxide in methanol for 20 min. Each slide was treated sequentially with normal goat serum (Vector Laboratories, Inc., Burlingame, CA), monoclonal mouse antibody against human pancreatic lipase (Beckman Instruments, Inc., Carlsbad, CA) at a dilution of 1 : 100, biotinylated antimouse IgG (Vector Laboratories) at a 1 : 50 dilution, and avidin-biotin-peroxidase complex (Vector Laboratories) at a 1 : 220 dilution. The reaction was visualized by exposure to diaminobenzidine and hydrogen peroxide with the use of standard methods, then stained with hematoxylin and mounted. Appropriate antibody controls were included in each experiment. Sections of normal pancreas served as positive control.
+ in shadow cells + in cuticle and sebaceous gland
tumors:
Sebaceous Sebaceous
adenoma (5) epithelioma (5)
Basal cell carcinoma:
Solid or undifferentiated type (10) With follicular differentiation (10)
+ in focal areas
Mscellaneous:
Mammary and extramammary Paget (3) Kaposi’s sarcoma (5) Merkell cell carninoma (3) Malignant melanoma (5) Malignant fibrous histiocytoma + : positive immunostaining;
Sebaceous glands and hair follicles were isolated by direct microdissection from whole skin, homogenized using a motorized glass stirrer, and then the proteins were separated by electrophoresis on polyacrylamide-10% SDS gels [ 51. The gel lanes were subsequently transferred to nitrocellulose sheets according to the method of Towbin [ 61. The blot was incubated with a monoclonal mouse antibody against human pancreatic lipase at a dilution of 1 : 100 overnight at room temperature with shaking. After incubation with the lipase antibody, bound antibody was detected by incubation with biotinylated antimouse IgG followed by streptavidin-horseradish peroxidase with subsequent detection of bound enzyme using 4-chloro- 1-naphthol and hydrogen peroxide as substrates. Results Normal skin
(3)
- : negative immunostaining.
In normal skin specimens, immunoreactive lipase was uniformly detected in the pilosebaceous units, both in the sebaceous glands and in the hair follicle. Within the sebaceous glands, staining
was seen in the more differentiated and centrally located cells rather than in the basophilic cuboidal cells forming the peripheral layer (Fig. 1). In hair follicles, lipase immunoreactivity was detected in the external root sheath, sparing the inner root sheath, the cuticle, the cortex and the matrix of the hair. In some specimens variable expression was detected in sweat glands as well as in nerve fascicles. The remaining structures in skin, including normal adipocytes, did not show lipase immunoreactivity. Skin tumors In the group of pilar tumors, trichilemmoma and tumor of follicular infundibulum (Mehregan’s tumor) diffusely stained (Fig. 2). Pilomatrixoma
Fig. 2. Trichilemmoma
Fig. 1. Staining of sebaceous
glands by lipase antibody.
showing intense immunoreactivity lipase.
to
had strongly positive lipase staining in the shadow cells but not in the basophilic undifferentiated cells (Fig. 3). Trichoepithelioma, in contrast, showed no lipase expression. The homogenous, eosinophilic horny layer seen adjacent to the lumen in steatocystoma multiplex showed positive staining as did sebaceous glands normally present in the vicinity of this type of cyst. Basal cell carcinomas of the solid or undifferentiated cell type were negative, however, basal cell carcinomas with follicular differentiation contained lipase in areas of follicular differentiation. All sebaceous tumours contained lipase in well-differentiated sebaceous cells but not in the undifferentiated germinative cells at the periphery of the tumor (Fig. 4).
Fig. 3. Pilomatrixoma. Note strong immunoreactivity to lipase in the shadow cells but not in the basophilic undifferentiated cells.
Fig. 4. Sebaceous adenoma. Lipase immunoreactivity present in well-differentiated sebaceous cells.
Lipase was not found in other skin tumors of different histogenesis (see Table I).
Immunoblotting Immunoblotting experiments using the lipase antibody revealed a distinct band of apparent molecular weight of approximately 42.7 kDa (Fig. 6). This value was estimated by logarithmic calibration of the gel with respect to the molecular weight markers.
Inflammatory skin disorders In erythema nodosum, lipase staining was detected within scattered mononuclear phagocytes surrounding the adipocytes in areas with fat inflammation, but not in intact fat cells. A similar immunohistochemical pattern was observed in the subcutaneous fat necrosis of the newborn (Fig. 5). In xanthoma lesions, foamy cells contained lipase. Cells composing granuloma annulare and granulation tissue did not demonstrate immunoreactive lipase (Table II).
is
Discussion In this study we have examined by immunohistochemical procedures the distribution of lipase in normal skin and in a variety of skin tumors and inflammatory skin diseases. In normal skin, lipase was detected in sebace-
199
MOL
Fig. 5. Erythema nodosum. Lipase staining is present within mononuclear phagocytes surrounding adipocytes in areas with fat inflammation. TABLE
II
Immunohistochemical diseases
expression
of lipase in inflammatory
Disease
Immunostaining
Erythema nodosum Subcutaneous fat necrosis of newborn Xanthomas Granuloma annulare Granulation tissue
+ in mononuclear + in mononuclear
+ : positive immunostaining;
phagocytes phagocytes
+ in foamy cells _ - : negative immunostaining.
cells and in the cells forming the external root sheath of the pilar apparatus. Sebaceous glands contain variable amounts of lipids including tri-
ous
WT
Fig. 6. Western blot of homogenized pilosebaceous units incubated with the monoclonal antibody against human pancreatic lipase. Track A shows a prominent band about 42.7 kDa precipitated by the antibody to lipase. Track WT shows molecular weight markers (kDa): 97.4 is rabbit muscle phosphorylase b; 66.2 is bovine serum albumin; 42.7 is hen egg white ovalbumin; 31.0 is bovine carbonic anhydrase; 21.5 is soybean trypsin inhibitor and 14.0 is hen egg white lysozyme.
glycerides, wax esters, squalene, cholesterol and cholesterol esters [7]. Lipase released from sebaceous cells may hydrolyze the triglycerides of the sebum, contributing to the formation of free fatty acids identified in human surface lipids [ 81. A similar mechanism has been postulated through the action of lipase contained in microorganisms usually found in the follicular canal and sebaceous ducts such as P. acnes [9] or Demodex mites [ 101.
200
There are at least two types of lipase in normal adipose tissue: one is hormone-sensitive lipase, and the other is lipoprotein lipase. The former releases fatty acids, while the latter deposits fatty acids in adipose tissue. The absence of immunoreactivity in normal adipose tissue suggests that these enzymes do not share the epitope that is recognized by the monoclonal antibody used in our study against human pancreatic lipase. The antibody to lipase might well be of value in confirming sebaceous and follicular differentiation in some adnexal tumors. Tumors with sebaceous differentiation showed positive lipase staining. However, in the group of pilar tumors there was variability, apparently related to the state of differentiation of the tumor cell. Lipase was detected in pilar tumors of external root sheath origin such as trichilemmoma and tumor of the follicular infundibulum. In addition, the staining of the eosinophilic horny layer of steatocystoma multiplex by lipase antibody is in accord with the evidence which suggests that this type of cyst is related to the outer root sheath [ 111. The relationship between the follicular differentiation and lipase expression is further strengthened by the lipase immunoreactivity of the cornified ‘shadow’ cells of pilomatrixoma, cells which represent faulty attempts at formation of hair shafts [ 121. Among the inflammatory disorders, the patterns of lipase expression in erythema nodosum and in subcutaneous fat necrosis of newborn were very similar. In both diseases, lipase immunoreactivity was detected in mononuclear phagocytes adjacent to adipocytes in areas with evident fat inflammation. We hypothesize that the inflammation disrupts fat cells, releasing lipid which induces the expression of lipase in adjacent mononuclear phagocytes. The induction of lipase would facilitate digestion and clearance of the
lipid material released by the damaged adipocytes. In summary, although the lipase staining pattern is not absolutely specific, it is usually present in sebaceous glands, external root sheaths, as well as in tumors thought to be related to these structures. Lipase is also present in mononuclear phagocytes as a digestive enzyme in those cases where extracellular deposits of lipids are found. References 1 Mead JF, Alfin-Slater RB, Howton DR, Popjak G: Lipids: Chemistry, Biochemistry and Nutrition. Plenum Press, New York, 1986. 2 De Caro A, Figarella C, Amic J, Michel R, Guy 0: Human pancreatic lipase: a glycoprotein. Biochim Biophys Acta 490: 411-419, 1977. 3 Borgstrom B, Brockman HL: Lipases. Elsevier, Amsterdam, 1984. 4 Taylor CR: Immunoperoxidase techniques: Practical and theoretical aspects. Arch Path01 Lab Med 102: 113-121, 1978. 5 Laemmli UK: Cleavage of structural proteins during the assembly of the head of the bacteriophage T4. Nature 227: 680-685, 1970. 6 Towbin H, Staehelin T, Gordon J: Electrophoretic transfer of proteins from acrylamide gels to nitrocellulose sheets: procedure and applications. Proc Nat1 Acad Sci USA 76: 4350-4354, 1979. I Downing DT, Strauss JS, Pochi PE: Variability in the chemical composition of human skin surface lipids, J Invest Dermatol 53: 322-327, 1969. 8 Shalita AR: Genesis offree fatty acids. J Invest Dermatol 62: 332-335, 1974. 9 Weary P: Comedogenic potential of the lipid extract of Pityrosporum ovale. Arch Dermatol 102: 84-91, 1970. 10 Jimenez-Acosta F, Planas L, Penneys NS: Demodex mites contain immunoreactive lipase. Arch Dermatol 125: 1436-1437, 1988. 11 Lever WF, Schaumburg-Lever G: Histopathology of the skin (6th. edn) JB Lippincott Company, Philadelphia, 1983, pp. 486-487. 12 Jacobson M, Ackerman AB: ‘Shadow’ cells as clues to follicular differentiation. Am J Dermatopathol 9: 51-57, 1987.