Quantitative participation of placental mammotropic hormones in mammary development during pregnancy of mice.

In order to elucidate the importance of the biological role of placental mammotropic hormones in the mammary development during pregnancy, the correlations between the number and weight of placentae and some indices of mammary development were examined using nulliparous C3H/He pregnant mice. On day 8 of pregnancy, the number of placentae was adjusted to 1-12 each by removing surgically the placentae with fetuses and all the mice were killed on day 19 of pregnancy. DNA and RNA contents and RNA/DNA ratio in bilateral inguinal mammary glands and lobuloalveolar rating of the right third thoracic gland were employed as the indices of mammary development. Positive and statistically highly significant correlations were obtained between both the number and weight of placentae and each index of mammary development, and the number and weight of placentae had higher correlations with RNA and RNA/DNA than with DNA. On the other hand, pituitary levels of prolactin and growth hormone had no correlations with any index of mammary development. These results provide evidence that placental mammotropic hormones contribute quantitatively to the mammary development during pregnancy in mice and suggest that the lactogenic effect of placental mammotropic hormones would be more prominent than their mammogenic effect at late pregnancy.