The development of the interstitial tissue and of the steroidogenic capability of the infantile rat ovary were examined by light microscopy, by histochemistry and by in vitro incubation. Ovaries from 5, 7, 10, 12 and 14-day-old normal rats and from rats that had received 100μg testosterone propionate or sesame oil at 5 days of age were utilized. The structural development of the infantile rat ovary was examined in 0.5 μ sections of Epon-embedded tissues which hadbeen fixed with glutaraldehyde and osmium. The lipid content of interstitial tissue in untreated animals was minimal at 5 days of age and then increased. The greatest change occurred between postnatal days 5 and 7. 3β-Hydroxysteroid dehydrogenase activity also became prominent at this time. The interfollicular tissue of animals treated with testosterone propionate accumulated very little lipid between postnatal days 5 and 7 and exhibited very little 3β-hydroxysteroid dehydrogenase activity. Fibroblasts rather than interstitial cells were the predominant cell type in these ovaries. In incubations of ovaries of untreated rats with 73H-progesterone, little change was seen in the yield of tritiated estrone between 5 and 7 days of age (0.21 0.48%) and only a slight decrease at 14 days. In contrast, the conversion to estradiol changed markedly with age. On day 5, it was comparable to that of estrone (0.21 and 0.59%), increased about 3-fold by day 7 and reached a peak (1.83 and 1.79%) by day 10. After day 10 the amounts of labeled estradiol formed decreased and by day 14 were comparable to those found on day 5. Neonatal testosterone treatment reduced by about 90% the production of both tritiated estradiol and estrone by ovaries of 7 and 10-day-old rats. The amounts of both steroids increased slightly thereafter so that by day 14 the values were similar to those of untreated rats. The neonatal testosterone treatment eliminated the peak of labeled estradiol production found in normal rats at 10 days of age. The findings in rats injected at 5 days of age with sesame oil differed from those of untreated rats at two ages. At 7 days the yields of both labeled estradiol and estrone were increased more than 2-fold while at 12 days tritiated estrone production was reduced by about 50%.
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