Daily administration of estrogen to immature female chicks results in marked oviduct growth and appearance of characteristic tubular gland cells which contain lysozyme. Although a rapid increase in total DNA and RNA content begins within 24 hr, cell specific protein, lysozyme, is first detectable after 3 days of estrogen. Progesterone administered concomitantly with estrogen antagonizes the estrogen-induced tissue growth as well as appearance of tubular gland cells and their specific products, lysozyme and ovalbumin. When the initiation of progesterone administration is delayed for progressively longer periods (days) during estrogen treatment, proportionally greater growth occurs with more lysozyme and tubular gland cells after 5 days of total treatment. Progesterone does not inhibit the estrogen-stimulated increase in uptake of α-aminoisobutyric acid and water by oviduct occurring within 24 hr or the estrogen-induced increase in total lipid, phospholipid, and phosphoprotein content of serum. The above results of progesterone antagonism can best be explained by the hypothesis that progesterone inhibits the initial proliferation of cells which become tubular gland cells but does not antagonize the subsequent cytodifferentiation leading to the synthesis of lysozyme and ovalbumin once such cell proliferation has occurred.

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