Abstract
AT the age of 9–10 days, the epidermis of the developing chick embryo is composed of two to three undifferentiated cell layers. In the next 10 days of embryonic life, proliferation and differentiation occur in the epidermis which results in the development of a layer of keratinized epidermis, six to ten cells deep. Interrelationship between proliferation and differentiation during the 10 day period has been reported1, but the extent of the interreaction is not clear. An interrelationship has been suggested2–4, in which differentiation (keratinization) only begins when cell division has begun to decline. Tonofilaments and tonofibrils, the first indicators of differentiation, are detected2 for the first time in the basal cells of the embryonic epidermis on the thirteenth day. Also on the thirteenth day, labelling experiments with thymidine indicate a reduction in the total number of labelled nuclei3 and these are present only in the basal cell layer after that day.
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