Mammary growth and plasma progesterone level during pregnancy in the house musk shrew, Suncus murinus Linnaeus.

The process of growth of the mammary gland and change in the plasma concentration of progesterone were investigated throughout the course of pregnancy (30 days) in the home musk shrew, Suncus murinus L. Development of the mammary gland of the musk shrew was limited during the first half of pregnancy. Extensive branching of ducts and conspicuous alveolar formation and a marked increase in the DNA content of the gland started between day 15 and 20 of pregnancy, and continued until term. Milk synthesis indicated by accumulation of the secondary fluid in the alveolar lumen and a sudden rise in the RNA/DNA ratio of the gland tissue seemed to be initiated 1 or 2 days before parturition. No lactose was detected in either the mammary tissue or milk of the house musk shrew. Plasma concentration of progesterone was very low until mid-pregnancy and began to rise after day 15, reaching a peak of 10--13 ng/ml around day 25. The steroid level started to fall shortly before parturition and returned to the basal level in post-parturient animals. Ovariectomy interrupted pregnancy in some animals, but not in others. When pregnancy was maintained, the mammary development and the plasma level of progesterone were normal.