CHANGES IN THE MAMMARY GLANDS OF RATS AND MICE DURING PREGNANCY, LACTATION AND INVOLUTION

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Abstract
SUMMARY: The changes in five measurements and four derived estimates of histological structure of the mammary gland were followed in rats and mice killed at 3-day intervals throughout pregnancy and lactation, or pregnancy, lactation and involution, respectively. A polynomial regression equation was fitted to the data for each variable. In the rat and to a lesser degree in the mouse, there was a striking fall in the number of alveoli per unit area at the time of parturition and a corresponding rise in alveolar diameter. The number of nuclei in the average alveolar cross-section was relatively constant during the latter part of pregnancy and in lactation, but increased approximately twofold about the time of parturition. There was little change in the estimate of total alveoli after parturition whereas volume of glandular tissue (estimated by the product of proportion of glandular tissue and area of histological section) increased steadily from early pregnancy to a maximum value in the middle or latter part of lactation. The curve fitted to the estimate of total nuclei was almost a perfect parabola in the mouse, with a maximum value at about day 7 of lactation, but was more nearly linear in the rat.