 Reference Base  Epidermal structures in a rhinoceros (Ceratotherium simum) |
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Morphology |
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White Rhino |
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Ceratotherium simum. Most accounts of rhinoceros skin detail its more obvious characteristics, namely, its rough, wrinkled and mammillated exterior, its well-keratinized epidermis and its thick, weighty and inelastic dermis. Concerning the structure of such skin and the possible presence of epidermal derivatives (other than horns) information is curiously wanting in authoritative zoological treatises. It is frequently stated that body hairs, save those constituting the ear- and tail-fringes are lacking in the Rhinocerotidae, though Beddard recognized an unobtrusive and rather sparse hairy covering as a general familial character.
Neuville observed hairs around the base of the horn in the three Asiatic species, and Lydekker considered Didermocerus a form specially prone to hairiness, the so-called species 'Rhinoceros lasiotis' being based on nothing more than a particularly hirsute specimen. Bigalke et al. described, in an infant Ceratotherium simum, a very sparse hairy covering, becoming less obvious with advancing age and we have noted a discrete hair tuft upon the nuchal eminence of an immature animal of this species. Reliable records as to the hairiness of young specimens of Rhinoceros unicornis, R. sondaicus and Diceros bicornis appear to be lacking.
It is probable that in some rhinoceros species at least, as in the elephant, the neonatus manifests an extremely sparse hairy coat which disappears gradually either as the result of friction or the accumulation of subcutaneous fat.
It is not surprising, therefore, that recent microscopical examination of the cervical and abdominal skin of an immature specimen of Ceratotherium should reveal the presence of hair follicles (provided with sebaceous glands) containing greater or lesser portions of well-formed hair shafts. Most unexpected, however, was the finding of large apocrine sweat glands, characterized by an abundance of relatively large, ectodermally developed myoepithelial cells, an anatomical arrangement clearly subserving the rapid and copious discharge of sweat.
The skin examined was freshly procured and well fixed in the field; after paraffm blocking it was sectioned with difficulty, the best sections being 15m-20m in thickness. These stained well with haematoxylin and eosin, and with Mallory's triple stain.
Study of these skin sections reveals (a) a heavily keratinized and pigmented epidermis, 1 mm thick. showing the customary component layers save for a stratum lucidum, (b) an exceedingly thick (18-20 mm) and dense dermis, composed exclusively of pure collagen fibres disposed in every direction to the skin surface, and (c) hair follicles, hairs, sebaceous glands and apocrine sweat glands. The entire skin appears remarkably vascular. The ordinary small type of sweat gland is not observable.
The obtrusively large apocrine sweat glands are not particularly numerous, although estimation of their incidence is precluded by the thickness of the section. Each gland surrounds, in open basket fashion, the base of a hair follicle and appears to be supplied by an independent arteriole: its spiral duct is fairly capacious in its intra-dermal course, but narrows perceptibly in its intra-epidermal course. A striking feature of the glands and ducts is the association therewith of large and numerous ectodermally developed myoepithelial cells: these lie between the secretory cells and the basement membrane of the glands and are disposed helicoidally around the ducts.
These distinctive apocrine sweat glands seem to have escaped previous notice in Ceratotherium and it is not known whether they occur in other rhinoceros species. Histologically the skin of Ceratotherium demonstrated to be of typically mammalian constitution and to lack none of the customary epidermal derivatives. It is specialized in respect of the degree of keratinization of its epidermis, the thickness and pirely collagenous nature of its dermis and the size and structure of its peculiar sweat glands.
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