Distribution of keratin and associated proteins in the epidermis of monotreme, marsupial, and placental mammals

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Abstract
The expression of acidic and basic keratins, and of some keratinization marker proteins such as filaggrin, loricrin, involucrin, and trichohyalin, is known for the epidermis of only a few eutherian species. Using light and high‐resolution immunocytochemistry, the presence of these proteins has been studied in two monotreme and five marsupial species and compared to that in eutherians. In both monotreme and marsupial epidermis lamellar bodies occur in the upper spinosus and granular layers. Development of the granular layer varies between species and regionally within species. There is great interspecific variation in the size (0.1–3.0 μm) of keratohyalin granules (KHGs) associated with production of orthokeratotic corneous tissues. Those skin regions lacking hairs (platypus web), or showing reduced pelage density (wombat) have, respectively, minute or indiscernible KHGs, associated with patchy, or total, parakeratosis. Ultrastructural analysis shows that monotreme and marsupial KHGs comprise irregular coarse filaments of 25–40 nm that contact keratin filaments. Except for parakeratotic tissues of platypus web, distribution of acidic and basic proteins in monotreme and marsupial epidermis as revealed by anti‐keratin antibodies AE1, AE2, and AE3 resembles that of eutherian epidermis. Antibodies against human or rat filaggrins have little or no cross‐reactivity with epidermal proteins of other mammals: only sparse areas of wombat and rabbit epidermis show a weak immunofluorescence in transitional cells and in the deepest corneous tissues. Of the available, eutherian‐derived antibodies, that against involucrin shows no cross‐reactivity with any monotreme and marsupial epidermal tissues and that against trichohyalin cross‐reacts only with cells in the inner root sheath and medulla of hairs. These results suggest that if involucrin and trichohyalin are present throughout noneutherian epidermis, they may have species‐specific molecular structures. By contrast, eutherian‐derived anti‐loricrin antibodies show a weak to intense cross‐reactivity to KHGs and corneous tissues of both orthokeratotic and parakeratotic epidermis in monotremes and marsupials. High‐resolution immunogold analysis of loricrin distribution in immature keratinocytes of platypus parakeratotic web epidermis identifies labeled areas of round or irregular, electron‐pale granules within the denser keratohyalin component and keratin network. In the deepest mature tissues, loricrin‐like labeling is diffuse throughout the cytoplasm, so that cells lack the preferential distribution of loricrin along the corneous envelope that characterizes mature eutherian keratinocytes. Thus, the irregular distribution of loricrin in platypus parakeratotic tissues more resembles that which has been described for reptilian and avian keratinocytes. These observations on the noneutherian epidermis show that a stratum granulosum is present to different degrees, even discontinuous within one tissue, so that parakeratotic and orthokeratotic areas may alternate: this might imply that parakeratotic monotreme epidermis reflects the primitive pattern of amniote α‐keratogenesis. Absent from anamniote epidermis and all sauropsid β‐keratogenic tissues, the ubiquitous presence of a loricrin‐like protein as a major component of other amniote corneous tissues suggests that this is a primitive feature of amniote α‐keratogenesis. The apparent lack of specific regionalization of loricin near the plasma membranes of monotreme keratinocytes could be an artifactual result of the immunofluorescence technique employed, or there may be masking of the antigenicity of loricrin‐like proteins once they are incorporated into the corneous envelope. Nevertheless, the mechanism of redistribution of such proteins during maturation of monotreme keratinocytes is different from, perhaps more primitive, or less specialized, than that in the epidermis of therian mammals. J. Morphol. 258:49–66, 2003.