Do oestrogens regulate placental growth in the mouse?

Some effects of litter size, ovariectomy and oestradiol (E2) on placental growth were determined. In order to increase variation in litter size, some mice had one oviduct removed and the remainder were unilaterally ovariectomized 3-4 weeks prior to mating. Thus after mating mice became unilaterally pregnant. Growth was examined qualitatively by measuring changes in placental DNA, protein, RNA : DNA ratios and in vivo incorporation of [5-3H]uridine into RNA. Spontaneous placenta hyperplasia due to low litter size occurred to an equal extent in intact mice and mice which were ovariectomized on day 10 of pregnancy and given 1.0 mg progesterone (P) and 10 ng E2 daily. Ovariectomy on day 10 increased the incorporation of [5-3H]uridine into placenta RNA on day 16 in mice with six or more foetuses. When mice were ovariectomized on day 5 and given 1.0 mg P and 10 or 60 ng E2 daily to maintain gestation, rates of placental growth remained similar to the rate in control, intact mice. The results suggest that, unlike the case in the rat, oestrogens do not regulate placental growth in the mouse.