The purpose of this study has been to obtain information on the dcvclopment of the envclop of horny cells that resists the action of keratinolytic agents. Toward this end the epidcrmis, oral mucosa, and tongue epithclium of various vcrtcbratcs, as well as the isolated envelopes of horny cclls, were examined by electron microscopy. It was found that small cytoplasmic granulcs (l,000 to 5,000 A) that devclop within differentiating epithelial cclls move toward the cell periphery, and after fusion with the plasma membrane, empty their contents into the intercellular spaces. The content of the granules spreads over the cell surfaces, and subsequently a thickened and coated cell envelope is formed that resists the action of keratinolytic agent. The membrane-coating granule is regarded as a specific differentiation product of the keratinizing epithelium. It contains numerous inner membranes and is assumed to engage in synthetic activities such as, perhaps, the formation of polysaccharides.
The epidermis of adult Rana pipiens has been studied by electron microscopy and histological and histochemical methods. It was found that the epidermis is engaged in the production of both keratin and mucus. The basal cells are mainly filled with tonofilaments, whereas the cells located in the mid-portion of the epidermis contain both tonofilaments and mucous granules. Golgi vesicles and endoplasmic reticulum are found in relative abundance in the mucus-producing cells and seem to be involved in the production of mucous granules. The mucus seen was partly retained within the cells and partly secreted into the intercellular spaces. The outermost keratinized cells contain mainly filaments and a few remnants of cell constituents.
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