MAMMARY CELL PROLIFERATION IN MICE DURING PREGNANCY AND LACTATION IN RELATION TO MILK YIELD

Abstract

Measurements have been made of body weight, mammary weight and total mammary DNA content (DNAt) during the second half of gestation and throughout lactation in mice. When corrected for conceptus weight and mammary weight, body weight was not significantly changed with advancing gestation but did increase during lactation. Mammary weight increased progressively until day 10 of lactation, then decreased until the end of lactation. On day 4 of involution mammary weight was not significantly different from that on day 12 of gestation. Mammary DNAt increased exponentially from day 12 of gestation to day 5 of lactation, with a mean doubling time of 6 d. No further increase was apparent at day 10, thereafter it fell in a similar manner to mammary weight. The incorporation of [3H]thymidine into mammary tissue was studied in vitro. [3H]Thymidine incorporation per unit weight of tissue was highest at mid‐gestation, fell during late gestation and then rose to a peak value 48 h post partum. For the remainder of lactation incorporation was very low. Incorporation calculated relative to DNAt showed similar, although less marked, changes. Milk yield of lactating mice has been estimated by daily weighing of the litter. Three phases were apparent: (i) a period from parturition to day 6 when milk yield was rising; (ii) a peak plateau from day 6 to day 10; (iii) a lower plateau from day 11 to day 15. Total daily litter weight gain was not significantly different between litters of nine and eighteen pups from day 7 to day 13 of lactation. Virgin and lactating mice were treated daily for seven days with 0·6 mg progesterone plus 0·2 µg oestradiol benzoate. This treatment significantly stimulated [3H]thymidine incorporation in the mammary glands of virgin mice and of mice in late lactation (days 9–15) but not in those of mice in early lactation (days 2–8). Milk yield was significantly reduced in the early‐lactation group, but not in the late‐lactation group.