Rates of DNA synthesis were measured as an index of cellular proliferation during the pubertal development of the rat epididymis. A highly reproducible pattern of DNA synthesis was defined by (1) a prepubertal, testosterone-insensitive peak of DNA synthesis at 25 days; (2) a dramatic decrease in DNA synthesis with the onset of puberty; (3) a major burst of testosterone-dependent synthesis peaking at 40 days in the head of the epididymis and at 40–60 days in the tail; (4) a fall to low levels as the adult organ weight was attained. An organ culture system was defined and utilized to analyse further the hormonal dependence of DNA synthesis in the epididymis. Testosterone and dihydrotestosterone failed to activate DNA synthesis at any stage of development in vitro. DNA synthesis was stimulated 100–300% by insulin at supra-physiological concentrations and by protein serum factor(s) at physiological concentrations. The serum activity was stable to heat treatment at 60 °C, destroyed by heating at 70 °C, and was present in the sera of hypophysectomized animals. These results indicate a primarily 'permissive' role for the action of testosterone on DNA synthesis in the epididymis: testosterone acts to permit the expression of a developmental 'programme' of cell proliferation which is activated by specific protein(s) in serum.
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