Mitotic Activity in Hypophysis of Pregnant Rat after Injections of Estrogen

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Abstract
The mitotic activity in the hypophysis of 3-mo.-old pregnant rats receiving two, 25 [image], injns. of estradiol benzoate 72 and 48 hrs. before death is compared with the activity in ovariectomized animals receiving the same amt. of estrogen. Animals pregnant 6-8 days when killed have an avg. of 17.3 mitoses/mm2 of section, which is not significantly different from the avg. of 22 mitoses/mm2 for the ovariectomized animals. Those pregnant 12-15 days have 4.4/mm2, indicating that the placenta produces a substance which inhibits the mitotic stimulating effect of the estrogen.