Variation of DNA Synthesis with Age in Chick Embryo Skin

Abstract

THE embryonic epidermis maintains a two or three cell layer population of undifferentiated cells for several days before about the eleventh day when a sudden burst of proliferation occurs. Although these events immediately precede the initiation of differentiation, an interrelationship between proliferation and differentiation has not been demonstrated. Because of the stimulation, the chick embryo epidermis at days 11–14 is five-fold more active mitotically than the terminal embryonic or post-embryonic chick epidermis¹. The same stimulus or stimuli may also be related to the induction of DNA polymerase found during days 10–13 (ref. 2), an event which precedes the increase in DNA synthesis during this period. This increase was verified³ by in vitro experiments in which 2-¹⁴C-thymidine was incorporated into the DNA of cultured chick embryo skin. Both the epidermis and dermis of the day 14 embryonic skin show similar stimulation of DNA synthesis compared with synthesis in the day 19 embryonic skin. The study of DNA synthesis in the chick embryo skin is therefore of considerable interest because it encompasses both the stimulatory and inhibitory stages of synthesis. We here report the variation of DNA synthesis in the chick embryonic skin as a function of age (embryonic) as a way of clarifying these events before further investigation of the natural stimulants and inhibitors of DNA synthesis and their influences on the epidermal cell cycle.