Changes in mouse mammary epithelial cell size during mammary gland development.

The average volume and size distribution of mouse mammary epithelial cells (MMEC) at different stages of functional activity of the mammary gland have been estimated. During early pregnancy, the mean MMEC volume begins to enlarge, resulting in a 21-fold cell volume increase by late lactation (Day 18). Progressive increases in mean MMEC volume during late pregnancy and throughout lactation result from the appearance and progressive hypertrophy of secretory alveolar cells. Involution of the mammary tissue brings about a precipitous decrease both in the average MMED volume and in the distribution of cell sizes established during the later phases of lactation. The progressive hypertrophy of cells particularly throughout the lactational period appear to correlate with previously reported changes in the metabolic activities of the mammary gland.