Metabolic adaptations in mouse mammary gland during a normal lactation cycle and in extended lactation.

The development and involution of mouse mammary tissue was measured in terms of changes in the cell population and the synthetic capacity of cells during the course of pregnancy, lactation and natural weaning, and after lactation was extended by introduction of foster litters of younger pups. Synthetic capacity was assessed by measuring the activities of several key enzymes closely concerned with the synthesis of milk constituents together with other constitutive enzymes. The capacity of the tissue to synthesize lactose and casein was also measured using short-term cultures of freshly prepared mammary explants. Results showed that the mammary cell population began to increase and differentiate around the time of parturition. Cell numbers increased up to day 5 of lactation, whereas cellular differentiation continued through lactation until day 15, around the time of peak milk yield. After day 15 there was a sharp drop, followed by a gradual decline in synthetic capacity and cell population, as pups were weaned. Substitution of younger pups for the natural litter at day 15 prevented the normal fall in milk secretion. In this case, mammary cell number remained at day 15 levels; cellular differentiation was also maintained to a considerable degree, although several markers of differentiation decreased during extended lactation.