Fine structure of the developing epidermis in the embryo of the American alligator (Alligator mississippiensis, Crocodilia, Reptilia)

The morphological transition from the simple epidermis that contacts the am niotic fluid of embryonic crocodilians to the adult epidermis required in a terrestrial environment has never been described. We used light and electr on microscopy to study the development, differentiation and keratinisation of the epidermis of the American alligator, Alligator. mississippiensis, be tween early and late stages of embryonic skin formation. In early embryonic development, the epidermis consists of a flat bilayer. As it develops, the bilayered epidermis comes to lie beneath the peridermis. Glycogen is almos t absent from the bilayered epidermis but increases in basal and suprabasal cells when scales form. Glycogen disappears from suprabasal cells that acc umulate keratin. The peridermis and 1 or 2 subperidermal layers form an emb ryonic epidermis that is partially or totally lost before hatching. These c ells accumulate coarse filaments and form reticulate bodies. Mucous and lam ellate granules are produced in the Golgi apparatus and are partly secreted extracellularly. The embryonic cells darken with the formation of larger r eticulate bodies that aggregate with intermediate filaments and other cell organelles, as their nuclear chromatin condenses. Thin beta -cells resembli ng those of scutate scales of birds develop beneath the embryonic epidermis and form a stratified beta -layer that varies in thickness in different bo dy regions. The epidermis differentiates first in the back, tail and belly. At the beginning of beta -cell differentiation, the cytoplasm contains spa rse bundles of alpha -keratin filaments, glycogen and lipid droplets or vac uoles apparently derived from the endoplasmic reticulum and Golgi apparatus . These organelles disappear rapidly as irregular bundles of electron-dense beta -keratin filaments accumulate and form larger bundles. The larger bun dles consist of 3 nm thick electron-pale keratin microfibrils and are deriv ed from the assemblage of beta -keratin molecules produced by ribosomes. Wh ile in mammals the epidermal barrier is formed by alpha -keratinocytes, in the alligator the barrier is formed by alpha -keratin cells. The beta -laye r is reduced or absent from the small hinge region between scales. In the l atter areas the barrier is made of or a mixture of alpha/beta keratinocytes . Thus alligators resemble birds where the beta -keratin molecules are depo sited directly over alpha -keratin scaffold, rather than an initial product ion of beta -keratin packets which then merge with alpha -keratin, as occur s in the Chelonia and Lepidosauria. The pigmentation of the epidermis of em bryos is mostly derived from epidermal melanocytes.