Stage-Specific Initiation of Milk Synthesis in the Ovariectomized Pregnant Mouse.

Stage-specific initiation of milk synthesis was investigated in mice ovariectomized on days 7, 9, 11 and 13 of pregnancy, and the lactogenic response of the mammary gland was monitored by using the synthesis of casein, lactose content and PRL binding as parameters. Pregnant mice ovariectomized on day 7 or 9 showed no increase in these parameters 24 h after operation. After day 11 of pregnancy, a clear response was induced by ovariectomy, characterized by a low level of casein synthesis, small amounts of lactose and PRL binding, which increased linearly in a time-dependent manner thereafter. These results indicate that pregnant mice after 11 days are able to initiate synthesis of casein and lactose in parallel. Mammary glands were cultured with insulin, cortisol and PRL. The mammary explants of pregnant mice at 7 and 9 days showed active synthesis of casein after a lag period of 2 days, suggesting that the mammary gland in early pregnancy is less sensitive to PRL, probably due to the absence of an increase in PRL binding after ovariectomy.