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

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Abstract
The morphological transition from the simple epidermis that contacts the amniotic fluid of embryonic crocodilians to the adult epidermis required in a terrestrial environment has never been described. We used light and electron microscopy to study the development, differentiation and keratinisation of the epidermis of the American alligator, Alligator mississippiensis, between 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 almost absent from the bilayered epidermis but increases in basal and suprabasal cells when scales form. Glycogen disappears from suprabasal cells that accumulate keratin. The peridermis and 1 or 2 subperidermal layers form an embryonic epidermis that is partially or totally lost before hatching. These cells accumulate coarse filaments and form reticulate bodies. Mucous and lamellate granules are produced in the Golgi apparatus and are partly secreted extracellularly. The embryonic cells darken with the formation of larger reticulate bodies that aggregate with intermediate filaments and other cell organelles, as their nuclear chromatin condenses. Thin beta-cells resembling those of scutate scales of birds develop beneath the embryonic epidermis and form a stratified beta-layer that varies in thickness in different body regions. The epidermis differentiates first in the back, tail and belly. At the beginning of beta-cell differentiation, the cytoplasm contains sparse bundles of alpha-keratin filaments, glycogen and lipid droplets or vacuoles 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 bundles consist of 3 nm thick electron-pale keratin microfibrils and are derived from the assemblage of beta-keratin molecules produced by ribosomes. While in mammals the epidermal barrier is formed by alpha-keratinocytes, in the alligator the barrier is formed by beta-keratin cells. The beta-layer is reduced or absent from the small hinge region between scales. In the latter areas the barrier is made of alpha or a mixture of alpha/beta keratinocytes. Thus alligators resemble birds where the beta-keratin molecules are deposited directly over an alpha-keratin scaffold, rather than an initial production of beta-keratin packets which then merge with alpha-keratin, as occurs in the 'Chelonia and Lepidosauria. The pigmentation of the epidermis of embryos is mostly derived from epidermal melanocytes.