Comparison of α and β keratin in reptiles
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The different patterns of keratin formation that have evolved in the class Reptilia are all variations of a common process. In Squamata (snakes and lizards), a sequence of layers composed of α or β keratin is formed periodically, after which the old epidermal generation is shed. In Chelonia (turtles and tortoises), the epidermis of the shell is composed of only β keratin, whereas the skin of the neck and leg is composed exclusively of α keratin. Molting in toto does not occur and shedding is a continuous process comparable to that in avian and mammalian epidermis. In Crocodilia (crocodiles, caimans, alligators) there is only a single layer of cornified cells, but the composition of the layer varies in different parts of the scale. The hinge regions have many of the morphological characteristics of α and β keratin whereas the center resembles β keratin. The living cells beneath contain accumulations of keratohyalin.
There are four ultrastructural characteristics of a keratinized α layer: 1) cellular outlines remain distinct, 2) a thickened plasma membrane forms during keratinization, 3) 80 Å filaments embedded in an amorphous matrix can be seen, and 4) PAS-positive material accumulates in extracellular spaces between the desmosomes.
The β layer exhibits none of these features. Instead the cells more or less (depending on species) coalesce into a compact layer which becomes attenuated in the hinge regions. A 30 Å filament pattern can be seen.
The mesos layer of squamates resembles the hinge region of crocodilians, exhibiting a combination of the characteristics of both α and β keratin.
Key-WordsReptiles Skin Keratin Electron microscopy Evolution
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