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Scanning electron microscopy of the scabies burrow and its contents, with special reference to the Sarcoptes scabiei egg
Scanning electron microscopy of the scabies burrow and its contents, with special reference to the Sarcoptes scabiei egg Walter B. Shelley, M . D . , Ph.D.,* and E. Dorinda Shelley, M . D . *
Peoria, IL An entire burrow of a female Sarcoptes scabiei (var. hominis) mite was examined by scanning electron microscopy during serial transverse sectioning. The elevated roof and the wall were composed of compacted corneocytes. The floor had a smooth surface except at the anterior end. Here it was made up of nucleated stratum granulosum cells, etched in appearance, presumably having been chewed by the mite. The female mite, the larvae, scybala, and eggs were also examined. The ultrastructure of the eggshell surface revealed it to have a distinctive geometric patterning. Cross-sectioning showed this shell or chorion to be composed of an outer layer of minute, closely packed penta- and hexahedrons resting on a thin inner homogenous layer. The potyhedric surface pattern was exactly replicated in the cement substance which attached the egg to the burrow floor. Observations were made on the embryonic positioning which permitted the larvae to emerge from the eggshells in a uniform manner. (J AM ACAD Dt~R~,fA'roe9:673-679, 1983.)
As clinicians, we look for the serpiginous burrow o f scabies and under the microscope may peer through a cleared specimen to see its contents of female mite, scybala, eggs. and larvae. I But such a view is largely shadow and outline. In this report, we present a close-up, highly magnified view of inside the scabietic burrow. METHOD A 5-ram sinuous black burrow and the surrounding area were subsected'-' in their entirety from the elbow of a healthy young woman who had had untreated scabies for 4 months. After preliminary examination in xylene by light microscopy, the entire specimen was critical point-dried, and, under 7-power magnification, serial
From the Depamnenl of Dermatology. University of Illinois College of Medicine at Peoria. Accepted tot publication Feb. 2, 1983. Reprint requests to: Dr. Walter B. Shelley, Medical College of Ohio, Toledo. OH 43699. *Present address: Medical College of Ohio, Toledo, OH 43699.
transverse sections were made with a series of Gillette Super Blue Blades. Each sectioning was followed in vacuo by gold coating and serial scanning electron microscopic observations using a JEOL-35 unit, and employing a 45" tilt. RESULTS T h e burrow The burrow was a black, tortuous, threadlike, slightly raised marking in the skin. It measured 5 mm in length and about 0.3 mm in width (Fig. 1). A narrow, shallow trough in the epidermal surface led to the opening. Just inside the burrow were eight empty, adherent eggshells. At the first bend were three larvae, one in the process of molting. Beyond them and around the first bend o f the tunnel were six more empty eggshells to the side o f the passageway. Next was an egg in the process o f hatching, and after a 5 0 0 - ~ m long space filled only with debris and fecal pellets (Fig. 2) were three larvae. Four unhatched eggs were next seen just before reaching the adult female Sarcoptes 673
674
Journal o|" the American Academy of Dermatology
Shelley and Shelley
Fig. 1. Scabietic burrow segment in the stratum corneum of human skin which has been partially unroofed by sectioning parallel to the skin surface. The burrow is 300/xm in width, three times wider than it appears here. Note compaction of comeocytes along the wall of the burrow. The loose corneocytes on the adjacent surface indicate the burrow is elevated, running as a ridge along the skin surface. The contents of the burrow are brightly stained. On the extreme right of the burrow is an oval egg amid debris. In the center, a larval mite is seen with its anterior half hidden under the ledge. Its characteristic hind leg bristles can be seen, the one on the right in contact with two fecal pellets. A short distance to the left of the mite, there is a cast-off egg casing, identifiable by its longitudinal dark V slit, through which the ]~va had recently emerged. (Magnification, x440.)
scabiei, w h o was found with her head wedged into the very end of the closed burrow. The r o o f of the b u r r o w showed no openings anywhere in its course. As it was serially removed, the burrow s h o w e d some undulation in its depth but always remained in the stratum corneum until reaching the blind end. Here, the head of the mite dipped into the n u c l e a t e d cellular layer of the stratum granulosum (Fig. 3).
The female mite See " D i s c u s s i o n " and Figs. 4-6. T h e egg a n d the h a t c h i n g p r o c e s s See "'Discussion" and Figs. 7-13. DISCUSSION The scanning electron microscope provides a rnarvelous entrance into the Lilliputian world of
Volume 9 Number 5 November, 1983
Electron microscopy of scabies burrow
675
Fig. 2. Fecal pellets of varying size indent the fissured floor of the burrow. The lighter irregular material on the lower border is stratum comeum. (Magnification, x 1,950.)
,,:.:
.......
. . . . . . . .e'z
I
Fig. 3. The surface of the floor at the anterior end of the burrow where the mite has advanced into the stratum granulosum for nourishment. The torn-open nucleated cells have an etched appearance revealing desmosomes as well as bundles of keratinous tonofilaments. Some cell membranes show microvilli. (Magnification, x5,000.) that tantalizingly small pathogenic mite, Sarcoptes scabiei. Wavering between the visible and invisible to the unaided eye, the mite and its life become very real on the electron screen. This series of electron micrographs brings new knowledge of the mite's habitat, habits, and reproductive biology. The burrow The very existence of a scabies burrow reflects a delicate balance between the mite's construction
Fig. 4. Capitulum of female Sarcoptes scabiei. This ventral view of the head of the mite shows bright J-shaped hairs on either side of a tongue-shaped upper lip which meets a slightly protruding lower lip in the form of a groove. The " b u l l " neck with its fimbriae blends into the thorax. Across the "forehead" are barely discernible mandibles with serrated medial surfaces. There are no eyes. Onty two of the six hairs extending from the dorsum of the head are seen. The first right foreleg with its characteristic long bristle can be seen. (Magnification, x 3,000.) of a temporary shelter for her eggs and the host's continuous shedding o f the stratum corneum in which her burrow is dug. There, eggs being deposited at the rate of one or two a d a y need 3 to 4 days for hatching, and the sexual maturation of the ensuing larval and nymph forms an additional 6 ? Racing against this is the desquamation of the entire stratum corneum, which normally occurs every 2 weeks. Thus the mite hastens to create the new even as the old is flaked off. Although the burrow is essentially a tunnel within the stratum corneum, it is important to recognize that the female mite burrows down to the live stratum granulosum for nourishment and fluids (Fig. 3). 4''~ Bun'ow f o r m a t i o n is indeed a dynamic process as we observe the floor changes.
676
Journal of the American Academy of Dermatology
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Fig. 5. Plicated anogenital area of the mite shown in Fig. 6. Para-anal bristles are seen extending from moundlike protuberances. The single white protuberance just above the orifice is the copulatory papilla. Both genital and anal orifices apparently share this common external opening. (Magnification, x3,000.) As the mite moves on, parakeratotic corneocytes cover over her fecding site, providing a new thin stratum corneum base for the burrow. The compact walls of the burrow (Fig. 1), as well as the smooth floor surface, support the view that the mite physically forces her way in between the corneocytes, rather than chewing a passage in termite fashion. She is propelled by two sets of powerful hind legs, and forward progress is aided by a cutting edge on each "knee joint" of the anterior pair of legs ~ (Fig. 1 1). Interestingly, the burrow winds not only from side to side in the horizontal plane, but also up and down to a limited degree in the vertical plane as the mite variably moves and burrows against the outward flow of the stratum corneum. Even as the mite sits still, new corneocytes are forming under her to propel her outward. Burrow housecleaning is minimal so the chamber becomes cluttered with abandoned eggshells, debris, and large amounts o f excrement. Most of the fecal pellets (scybala) fell out on sectioning, but some remained in niche-like depressions in the burrow floor and in the empty eggshells. These were firm and did not disintegrate in xylene. The larval mites are only about a third the width of the mother, and hence they can make their way be-
Fig. 6. Mid section of the ventral surface of a female mite showing transverse linear opening, i.e., the tocostoma, going across the abdomen, which is ridged, plated, and hirsute. The lips of the tocostoma open and widen as an egg is extruded from here once or twice each day. (Magnification, x 1,350.) tween the scybala and the discarded eggshells neatly aligned on the side. The female mite never turns back but continues to extend her home, working most actively in the night warmth. The present burrow is over ten times her length and probably represents about the best the mite can do in her race with desquamation. The female mite Although the Sarcoptes scabiei isolated from man (var. hominis) cannot be distinguished from that of pigs (var. suis), horses (var. equi), cows (var. bovis), or dogs (vat'. canis), it is distinguished from other mites by the fact that its skin hangs in folds. 7 The female mite is most easily recognized by the four long bristles trailing behind her, whereas the male has only two. The larval and nymph forms ,are distinguished by having only three pairs of legs, instead of the adult complement of four.
Volume 9 Number 5 November, 1983
Fig. 7. High-power view of the surface of a Sarcoptes scabiei egg studded with pentagonal and hexagonal
protuberances. In the upper left, the imprint of the egg on the burrow floor is faintly seen. (Magnification, x 7,500.)
Electron microscopy of scabies burrow
677
Fig, 9. Bursting of the eggshell (chorion) prior to emergence of the larva. The vitelline membrane seen in the center covers the embryo. (Magnification, x 1t ,000. )
Ftg. 8. Imprint of polyhedric surface markings of the egg in adhesive material on the floor of the burrow. (Magnification, x7,500.)
Fig. 10. Eggshell showing the two layers of the choroid membrane. In the lower left, the vitelline membrane covers the embryo. In the upper right is seen the adhesive material which attaches the egg to the floor of the burrow. (Magnification, x 21,500.)
On the ultrastructural level, only the female mite has been studied. Two complete sets of illustrations have appeared, a:' as well as some casual prints, t~ The capitulum (head) is the serviceable tool which permits bmTowing (Fig. 4). Instead of eyes, it has two sensory setae on the face. Six hairs on the dorsum of the head provide additional sensory input. The chelicerae (biting parts), used in opening the cells of the stratum gran-
ulosum, are rather inconspicuous jaws located outside, rather than inside, the mouth. The overhanging V-shaped lip gives her the face of a porpoise. The reproductive anatomy is less well understood. For years, a stublike papilla just above the anal orifice (Fig. 5) was considered to be the copulatory organ, :~ and evidence was presented that the sperm entered this and passed into a sac
678
Shelley and Shelh, v
Fig. 11. Birth of a larval mite. Eggshell has becn opened by the larva, revealing its stubby right foreleg (thigh), long lower leg (ambulacrum), and foot (ambulacral disc). A hooklike structure at the joint between this foreleg and the ambulacrum is considered to be the too[ used in cutting through the embryonic membranes. A second leg with bristles is also visible. The head is in the hidden upper field, still draped in the embryonic membrane. (Magnification, x4,000.)
Journal or" the American Academy of Dermatology
Fig. 13. Cross-section of postpartum eggshell showing how the choroid membrane coils inward to form a rolled edge for the exit of the larva. (Magnification, x6,000.) papilla, and it would appear more likely to be a sensory organ analogous to a clitoris. External inspection of the terminal orifice suggests it has a dual genital and anal role. However, since the male mite has no penis, the mechanism of sperm introduction remains unclear, although some evidence suggests the male uses his capitulum in this maneuver. ~:~ The fully developed fertilized egg passes out through the transverse cleft (tocostoma) on the ventral surface of the mite (Fig. 6).
The egg and the hatching process
Fig. 12. Postpartum eggshell within the burrow. This oval casing consistently has a boatlike shape with smooth membrane edges along the slit where the larva crawled out. Within the casing are seen a debris of vitelline membranes and fecal pellets. (Magnification, • 1,8003 which emptied into the ovary on each side just below. More recently, with scanning electron microscopy, the view is held that the posterior or anal orifice is actually the opening for both the genital tract and the rectum. '~ Our studies show no orifice in the copulatory
The mite begins its independent life within an egg which, under electron microscopy, is a structural marvel. Fig. 7 reveals the sculptured surface of the eggshell, and Fig. 8, the embossed replica of this surface in the subjacent cement substance. It is this cement which glues the egg in permanent position on the burrow floor. The cement is secreted by the "glue glands" along the oviduct, j:~ We surmise that the secretion is a glycoprotein analogous to fibronectin, not soluble in organic solvents such as the xylene used by us. ~4 The papillary surface of the Sarcoptes scabiei egg (phylum Arthropoda, subphylum Chelicerata, class Arachni&t, order Acarina) does not match any of the hundreds of scanning electron micrographs of insect eggs (phylum Arthropoda, subphylum Mandibulata, class lnsecta).~r' Nor does it resemble the eggs of the parasitic flatworms
Volume 9 Number 5 November, 1983
(phylum Platyhelminthes, classes Cestoda [tapeworms] and Trematoda [flukes]) or intestinal and filarial roundworms (phylum Nemathelminthes, class Nematoda). ~ H o w e v e r , whether or not this egg surface is unique for Sarcoptes scabiei is a question unanswerable at this time, since the mite group includes some 20,000 other species. On fracture, the eggshell (choroid membrane) proves to have two layers (Figs. 9-1 1). The outer layer is c o m p o s e d of closely packed five- and six-sided flat-topped papillary structures, 0 . 9 / , t m in height. The inner layer, resembling a basement membrane, is a h o m o g e n o u s sheath approximately 0,5 /xm in thickness. Although Barriere et al ~7 pointed out in 1975 that the Sarcoptes scabiei egg had a cuticle of polyhedric structures 0.8 ~ m in diameter, they published no photographs. In 1981, Van Neste et al TM failed to confirm this. Their illustrations showed the freshly laid egg surface as smooth, but roughening with age. Possibly, their use of B o u i n ' s fixative obliterated the exquisite patterning we h a v e observed and recorded. As has been stressed, egg hatching is a complex multistage event, involving not only mechanical tearing and cutting (Fig. 1 1), ~8 but also larval enzymatic digestive processes. ~:~ Interestingly, the embryo develops with its back to the part of the shell wall glued to the burrow floor. Thus, the legs are free to push and cut their way out of the envelopes a b o v e them (Fig. 11). Always the exit passage is a longitudinal opening with smooth edges (Fig. 12). This results from a curling in of the free edges of the torn choroid m e m b r a n e , as shown in a cross-section of the e m p t y egg casing (Fig. 13). In conclusion, it is hoped that these ultrastructural observations will bring new form and substance to the shadows and outlines seen under the light microscope by the clinician. We thank Veronika Burmeister tot" her invaluable technical assistance.
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REFERENCES 1. Martin WE, WheelEr CE: Diagnosis of human scabies by epidermal shave biopsy. J AM ACAD DERMATOL1:335337, 1979. 2. Shelley WB: Razor blade surgery, in Epstein EH, Epstein EH Jr, editors: Skin surgery, ed. 5, Springfield, IL, 1982, Charles C Thomas, Publisher, pp. 320-330. 3. Friedman R: The story of scabies. New York, 1947, Froben Press, Inc. 4. Heilesen B: Studies on Acartls scabiei and scabies. Acta Derm Venereol (Stockh) 26(suppl. 14):1-370, 1946. 5. AckErman AB: Histopathology of human scabies, in Orkin M, et al. editors: Scabies and pedicu]osis. Philadelphia, 1977, J. B. Lippincott Co., pp. 88-95. 6. Mellanby K: Biology of the parasite, in Orkin M, et al, editors: Scabies and pediculosis. Philadelphia, 1977, J. B. Lippincott Co., pp. 8-16. 7. Cheng TC: General parasitology. New York, 1973, Academic Press, Inc. 8. Juhlin L, Brunk U, Fredriksson BA: Scanning electron microscopy of Acarus scabiei. Acta Derna Venereol (Stockh) 55:35-37, 1975. 9. Pascual AM, Asensio A, Vazquez R: Morphologie du Sarcoptes scabiei (varirt6 hominis) au microscope e~lectronique a balayage. Ann Dcrmatol Venereol (Paris) 104:719-723, 1977. 10. Wolff HH, Selzle D: Pedicul,s vestimentorum, Sarcoptes scabiei, Pule.~ irritans. Rasterelektronen-microskopie. Hautarzt 29:224-225, 1978. 11. Van Neste D, LaChapelle JM: Host-parasite relationships in hyperkeratotic (Norwegian) scabies: Pathological and immunological findings. Br J Derrnatol 105:667-678, 1981. 12. Van Neste D: Immunologic studies in scabies. Int J Dennatol 20:264-269, 1981. 13. Belding DL: Textbook of parasitology, ed. 3. New York, 1965, Appleton-Century-Crofts. 14. Fawcett DW: The cell, ed. 2. Philadelphia, 1981, W. B. Saunders Co. 15. Hinton HE: Biology of insect eggs. New York, 1981, Pergamon Press Inc. 16. Race GW, Martin JH, Moore DV. Larsh JE: Scanning and transmission electrtmmicroscopy of Schistosonta mansrmi eggs, cercariae, and adults. Am J Trop Meal Hyg 20:914-924, 1971. 17. Barriere H, et al: Examen du sarcopte de la gS,le humaine en microscopie 61ectronique a balayage. Bull Soc Fr Dermatol Syph 82: 189-190, 1975. 18. Van Neste D, et al: Life cycle of scabies mite, A study in elcctron scanning microscopy. Ann Demlatol Venereol 108:355-361, 1981. 19. Holmes SD, Fairweather I: Hymenolepsis dimimtta: The mechanism of egg hatching. Parasitology 85:237-250, 1982.
Peoria, IL An entire burrow of a female Sarcoptes scabiei (var. hominis) mite was examined by scanning electron microscopy during serial transverse sectioning. The elevated roof and the wall were composed of compacted corneocytes. The floor had a smooth surface except at the anterior end. Here it was made up of nucleated stratum granulosum cells, etched in appearance, presumably having been chewed by the mite. The female mite, the larvae, scybala, and eggs were also examined. The ultrastructure of the eggshell surface revealed it to have a distinctive geometric patterning. Cross-sectioning showed this shell or chorion to be composed of an outer layer of minute, closely packed penta- and hexahedrons resting on a thin inner homogenous layer. The potyhedric surface pattern was exactly replicated in the cement substance which attached the egg to the burrow floor. Observations were made on the embryonic positioning which permitted the larvae to emerge from the eggshells in a uniform manner. (J AM ACAD Dt~R~,fA'roe9:673-679, 1983.)
As clinicians, we look for the serpiginous burrow o f scabies and under the microscope may peer through a cleared specimen to see its contents of female mite, scybala, eggs. and larvae. I But such a view is largely shadow and outline. In this report, we present a close-up, highly magnified view of inside the scabietic burrow. METHOD A 5-ram sinuous black burrow and the surrounding area were subsected'-' in their entirety from the elbow of a healthy young woman who had had untreated scabies for 4 months. After preliminary examination in xylene by light microscopy, the entire specimen was critical point-dried, and, under 7-power magnification, serial
From the Depamnenl of Dermatology. University of Illinois College of Medicine at Peoria. Accepted tot publication Feb. 2, 1983. Reprint requests to: Dr. Walter B. Shelley, Medical College of Ohio, Toledo. OH 43699. *Present address: Medical College of Ohio, Toledo, OH 43699.
transverse sections were made with a series of Gillette Super Blue Blades. Each sectioning was followed in vacuo by gold coating and serial scanning electron microscopic observations using a JEOL-35 unit, and employing a 45" tilt. RESULTS T h e burrow The burrow was a black, tortuous, threadlike, slightly raised marking in the skin. It measured 5 mm in length and about 0.3 mm in width (Fig. 1). A narrow, shallow trough in the epidermal surface led to the opening. Just inside the burrow were eight empty, adherent eggshells. At the first bend were three larvae, one in the process of molting. Beyond them and around the first bend o f the tunnel were six more empty eggshells to the side o f the passageway. Next was an egg in the process o f hatching, and after a 5 0 0 - ~ m long space filled only with debris and fecal pellets (Fig. 2) were three larvae. Four unhatched eggs were next seen just before reaching the adult female Sarcoptes 673
674
Journal o|" the American Academy of Dermatology
Shelley and Shelley
Fig. 1. Scabietic burrow segment in the stratum corneum of human skin which has been partially unroofed by sectioning parallel to the skin surface. The burrow is 300/xm in width, three times wider than it appears here. Note compaction of comeocytes along the wall of the burrow. The loose corneocytes on the adjacent surface indicate the burrow is elevated, running as a ridge along the skin surface. The contents of the burrow are brightly stained. On the extreme right of the burrow is an oval egg amid debris. In the center, a larval mite is seen with its anterior half hidden under the ledge. Its characteristic hind leg bristles can be seen, the one on the right in contact with two fecal pellets. A short distance to the left of the mite, there is a cast-off egg casing, identifiable by its longitudinal dark V slit, through which the ]~va had recently emerged. (Magnification, x440.)
scabiei, w h o was found with her head wedged into the very end of the closed burrow. The r o o f of the b u r r o w showed no openings anywhere in its course. As it was serially removed, the burrow s h o w e d some undulation in its depth but always remained in the stratum corneum until reaching the blind end. Here, the head of the mite dipped into the n u c l e a t e d cellular layer of the stratum granulosum (Fig. 3).
The female mite See " D i s c u s s i o n " and Figs. 4-6. T h e egg a n d the h a t c h i n g p r o c e s s See "'Discussion" and Figs. 7-13. DISCUSSION The scanning electron microscope provides a rnarvelous entrance into the Lilliputian world of
Volume 9 Number 5 November, 1983
Electron microscopy of scabies burrow
675
Fig. 2. Fecal pellets of varying size indent the fissured floor of the burrow. The lighter irregular material on the lower border is stratum comeum. (Magnification, x 1,950.)
,,:.:
.......
. . . . . . . .e'z
I
Fig. 3. The surface of the floor at the anterior end of the burrow where the mite has advanced into the stratum granulosum for nourishment. The torn-open nucleated cells have an etched appearance revealing desmosomes as well as bundles of keratinous tonofilaments. Some cell membranes show microvilli. (Magnification, x5,000.) that tantalizingly small pathogenic mite, Sarcoptes scabiei. Wavering between the visible and invisible to the unaided eye, the mite and its life become very real on the electron screen. This series of electron micrographs brings new knowledge of the mite's habitat, habits, and reproductive biology. The burrow The very existence of a scabies burrow reflects a delicate balance between the mite's construction
Fig. 4. Capitulum of female Sarcoptes scabiei. This ventral view of the head of the mite shows bright J-shaped hairs on either side of a tongue-shaped upper lip which meets a slightly protruding lower lip in the form of a groove. The " b u l l " neck with its fimbriae blends into the thorax. Across the "forehead" are barely discernible mandibles with serrated medial surfaces. There are no eyes. Onty two of the six hairs extending from the dorsum of the head are seen. The first right foreleg with its characteristic long bristle can be seen. (Magnification, x 3,000.) of a temporary shelter for her eggs and the host's continuous shedding o f the stratum corneum in which her burrow is dug. There, eggs being deposited at the rate of one or two a d a y need 3 to 4 days for hatching, and the sexual maturation of the ensuing larval and nymph forms an additional 6 ? Racing against this is the desquamation of the entire stratum corneum, which normally occurs every 2 weeks. Thus the mite hastens to create the new even as the old is flaked off. Although the burrow is essentially a tunnel within the stratum corneum, it is important to recognize that the female mite burrows down to the live stratum granulosum for nourishment and fluids (Fig. 3). 4''~ Bun'ow f o r m a t i o n is indeed a dynamic process as we observe the floor changes.
676
Journal of the American Academy of Dermatology
Shelley and Shelley
Fig. 5. Plicated anogenital area of the mite shown in Fig. 6. Para-anal bristles are seen extending from moundlike protuberances. The single white protuberance just above the orifice is the copulatory papilla. Both genital and anal orifices apparently share this common external opening. (Magnification, x3,000.) As the mite moves on, parakeratotic corneocytes cover over her fecding site, providing a new thin stratum corneum base for the burrow. The compact walls of the burrow (Fig. 1), as well as the smooth floor surface, support the view that the mite physically forces her way in between the corneocytes, rather than chewing a passage in termite fashion. She is propelled by two sets of powerful hind legs, and forward progress is aided by a cutting edge on each "knee joint" of the anterior pair of legs ~ (Fig. 1 1). Interestingly, the burrow winds not only from side to side in the horizontal plane, but also up and down to a limited degree in the vertical plane as the mite variably moves and burrows against the outward flow of the stratum corneum. Even as the mite sits still, new corneocytes are forming under her to propel her outward. Burrow housecleaning is minimal so the chamber becomes cluttered with abandoned eggshells, debris, and large amounts o f excrement. Most of the fecal pellets (scybala) fell out on sectioning, but some remained in niche-like depressions in the burrow floor and in the empty eggshells. These were firm and did not disintegrate in xylene. The larval mites are only about a third the width of the mother, and hence they can make their way be-
Fig. 6. Mid section of the ventral surface of a female mite showing transverse linear opening, i.e., the tocostoma, going across the abdomen, which is ridged, plated, and hirsute. The lips of the tocostoma open and widen as an egg is extruded from here once or twice each day. (Magnification, x 1,350.) tween the scybala and the discarded eggshells neatly aligned on the side. The female mite never turns back but continues to extend her home, working most actively in the night warmth. The present burrow is over ten times her length and probably represents about the best the mite can do in her race with desquamation. The female mite Although the Sarcoptes scabiei isolated from man (var. hominis) cannot be distinguished from that of pigs (var. suis), horses (var. equi), cows (var. bovis), or dogs (vat'. canis), it is distinguished from other mites by the fact that its skin hangs in folds. 7 The female mite is most easily recognized by the four long bristles trailing behind her, whereas the male has only two. The larval and nymph forms ,are distinguished by having only three pairs of legs, instead of the adult complement of four.
Volume 9 Number 5 November, 1983
Fig. 7. High-power view of the surface of a Sarcoptes scabiei egg studded with pentagonal and hexagonal
protuberances. In the upper left, the imprint of the egg on the burrow floor is faintly seen. (Magnification, x 7,500.)
Electron microscopy of scabies burrow
677
Fig, 9. Bursting of the eggshell (chorion) prior to emergence of the larva. The vitelline membrane seen in the center covers the embryo. (Magnification, x 1t ,000. )
Ftg. 8. Imprint of polyhedric surface markings of the egg in adhesive material on the floor of the burrow. (Magnification, x7,500.)
Fig. 10. Eggshell showing the two layers of the choroid membrane. In the lower left, the vitelline membrane covers the embryo. In the upper right is seen the adhesive material which attaches the egg to the floor of the burrow. (Magnification, x 21,500.)
On the ultrastructural level, only the female mite has been studied. Two complete sets of illustrations have appeared, a:' as well as some casual prints, t~ The capitulum (head) is the serviceable tool which permits bmTowing (Fig. 4). Instead of eyes, it has two sensory setae on the face. Six hairs on the dorsum of the head provide additional sensory input. The chelicerae (biting parts), used in opening the cells of the stratum gran-
ulosum, are rather inconspicuous jaws located outside, rather than inside, the mouth. The overhanging V-shaped lip gives her the face of a porpoise. The reproductive anatomy is less well understood. For years, a stublike papilla just above the anal orifice (Fig. 5) was considered to be the copulatory organ, :~ and evidence was presented that the sperm entered this and passed into a sac
678
Shelley and Shelh, v
Fig. 11. Birth of a larval mite. Eggshell has becn opened by the larva, revealing its stubby right foreleg (thigh), long lower leg (ambulacrum), and foot (ambulacral disc). A hooklike structure at the joint between this foreleg and the ambulacrum is considered to be the too[ used in cutting through the embryonic membranes. A second leg with bristles is also visible. The head is in the hidden upper field, still draped in the embryonic membrane. (Magnification, x4,000.)
Journal or" the American Academy of Dermatology
Fig. 13. Cross-section of postpartum eggshell showing how the choroid membrane coils inward to form a rolled edge for the exit of the larva. (Magnification, x6,000.) papilla, and it would appear more likely to be a sensory organ analogous to a clitoris. External inspection of the terminal orifice suggests it has a dual genital and anal role. However, since the male mite has no penis, the mechanism of sperm introduction remains unclear, although some evidence suggests the male uses his capitulum in this maneuver. ~:~ The fully developed fertilized egg passes out through the transverse cleft (tocostoma) on the ventral surface of the mite (Fig. 6).
The egg and the hatching process
Fig. 12. Postpartum eggshell within the burrow. This oval casing consistently has a boatlike shape with smooth membrane edges along the slit where the larva crawled out. Within the casing are seen a debris of vitelline membranes and fecal pellets. (Magnification, • 1,8003 which emptied into the ovary on each side just below. More recently, with scanning electron microscopy, the view is held that the posterior or anal orifice is actually the opening for both the genital tract and the rectum. '~ Our studies show no orifice in the copulatory
The mite begins its independent life within an egg which, under electron microscopy, is a structural marvel. Fig. 7 reveals the sculptured surface of the eggshell, and Fig. 8, the embossed replica of this surface in the subjacent cement substance. It is this cement which glues the egg in permanent position on the burrow floor. The cement is secreted by the "glue glands" along the oviduct, j:~ We surmise that the secretion is a glycoprotein analogous to fibronectin, not soluble in organic solvents such as the xylene used by us. ~4 The papillary surface of the Sarcoptes scabiei egg (phylum Arthropoda, subphylum Chelicerata, class Arachni&t, order Acarina) does not match any of the hundreds of scanning electron micrographs of insect eggs (phylum Arthropoda, subphylum Mandibulata, class lnsecta).~r' Nor does it resemble the eggs of the parasitic flatworms
Volume 9 Number 5 November, 1983
(phylum Platyhelminthes, classes Cestoda [tapeworms] and Trematoda [flukes]) or intestinal and filarial roundworms (phylum Nemathelminthes, class Nematoda). ~ H o w e v e r , whether or not this egg surface is unique for Sarcoptes scabiei is a question unanswerable at this time, since the mite group includes some 20,000 other species. On fracture, the eggshell (choroid membrane) proves to have two layers (Figs. 9-1 1). The outer layer is c o m p o s e d of closely packed five- and six-sided flat-topped papillary structures, 0 . 9 / , t m in height. The inner layer, resembling a basement membrane, is a h o m o g e n o u s sheath approximately 0,5 /xm in thickness. Although Barriere et al ~7 pointed out in 1975 that the Sarcoptes scabiei egg had a cuticle of polyhedric structures 0.8 ~ m in diameter, they published no photographs. In 1981, Van Neste et al TM failed to confirm this. Their illustrations showed the freshly laid egg surface as smooth, but roughening with age. Possibly, their use of B o u i n ' s fixative obliterated the exquisite patterning we h a v e observed and recorded. As has been stressed, egg hatching is a complex multistage event, involving not only mechanical tearing and cutting (Fig. 1 1), ~8 but also larval enzymatic digestive processes. ~:~ Interestingly, the embryo develops with its back to the part of the shell wall glued to the burrow floor. Thus, the legs are free to push and cut their way out of the envelopes a b o v e them (Fig. 11). Always the exit passage is a longitudinal opening with smooth edges (Fig. 12). This results from a curling in of the free edges of the torn choroid m e m b r a n e , as shown in a cross-section of the e m p t y egg casing (Fig. 13). In conclusion, it is hoped that these ultrastructural observations will bring new form and substance to the shadows and outlines seen under the light microscope by the clinician. We thank Veronika Burmeister tot" her invaluable technical assistance.
Electron microsct)py oj" scabies burrow
679
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