Mammary development in mice: effects of hemihysterectomy in pregnancy and of litter size post partum.

1. Mice were hemihysterectomized on day 8 of pregnancy to reduce the number of feto-placental units.2. Fetal mortality was not affected by hemihysterectomy; mean single-pup weight at birth was increased when compared with sham-operated controls.3. Pregnant sham-operated and hemihysterectomized animals were killed on days 13 and 18 of gestation, and their mammary glands were analysed for total DNA (DNA(t)) and RNA (RNA(t)). Both were significantly lower in the hemihysterectomized group on day 18, but not on day 13.4 Milk yield was assessed, by daily weighing of the litter, in groups of sham-operated and hemihysterectomized lactating animals suckling nine pups each. There was no difference in yield between the two groups.5. One group of sham-operated mice suckling nine pups, one of hemihysterectomized mice suckling nine pups and one of hemihysterectomized mice suckling four pups were killed on day 5 of lactation for mammary gland analysis. There was no significant difference in mammary weight or DNA(t) between the sham-operated and hemihysterectomized animals suckling nine pups, although RNA(t) was still reduced in the latter. Mammary weight, DNA(t) and RNA(t) were all significantly lower in the hemihysterectomized group suckling four pups than in either of the other two groups.6. It is concluded that the less well developed mammary glands of mice which give birth to small litters are capable of compensatory growth during the first few days of lactation if a sufficiently strong suckling stimulus is given.7. It is suggested that control of mammary development by the fetus during gestation and by the suckling young during early lactation are both mechanisms designed to ensure that milk yield is appropriate to the needs of the young.