Effects of ovarian hormones on foetal and placental growth in the mouse

The effects of ovariectomy and treatment with progesterone and oestradiol on foetal and placental growth were examined. The occurrence of cellular hypertrophy and hyperplasia in the placenta in response to treatments was determined by measuring protein, DNA and RNA:DNA ratios. In control mice, which had not been ovariectomized, the daily administration of 0.01-2.56 microgram oestradiol on days 10-15 of pregnancy caused only small decreases in the number of live foetuses and foetal and placental weights on day 16. When mice were ovariectomized on day 10, gestation was maintained by administering 5 mg progesterone daily, but terminated when only 1 mg progesterone was given daily. However, when ovariectomized mice receiving 1 mg progesterone were also given 0.01 microgram oestradiol on days 10-15, gestation was maintained and the number of live foetuses and foetal and placental weights on day 16 were normal. Increasing the daily dose of oestradiol up to 2.56 microgram in mice receiving 1 mg progesterone on days 10-15 had no effect on foetal and placental growth. In contrast, the daily administration of 0.64-2.56 microgram oestradiol to ovariectomized mice receiving 5 mg progesterone on days 10-15 terminated gestation in some mice and considerably reduced the number of live foetuses and foetal and placental weights in those mice which remained pregnant on day 16. None of the treatments increased protein DNA or the RNA:DNA ratio in the placenta to levels above those seen in control mice. The results are discussed in relation to ovariectomy-induced placental overgrowth in the rat.