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

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Abstract
SUMMARY: The changes in the level of deoxyribonucleic acid (DNA) and in the activities of alkaline and acid phosphatases were followed in the mammary glands of rats and mice killed at intervals of 3 days during pregnancy and lactation or pregnancy, lactation and involution, respectively. Polynomial regression equations were fitted to the data after they had been transformed to a logarithmic scale. During pregnancy and lactation, the general form of the curves described by the equations were similar for DNA and alkaline phosphatase, however expressed, and for acid phosphatase, other than when expressed as a concentration per mg. of DNA. All equations had a large linear term with a smaller significant negative and/or cubic term. During most of pregnancy and in early lactation the curves showed a steep, almost linear rise. During early pregnancy the slope was somewhat lower, while in the latter part of lactation the rate of increase was small or became negative. In the mouse, when the values for animals killed during involution were included in the analyses, the equations tended to be dominated by the quadratic term, a reflexion of the steep fall in the levels of all constituents after weaning. If the assumption was accepted that the average DNA content of cells did not change during the lactational cycle, the following conclusions could be drawn. In the rat, the marked similarity of the equation for total content of DNA and for concentration of DNA (per unit weight of tissue corrected for 'milk' content) indicated that the increase, and subsequent decrease, in cell numbers was mirrored almost exactly by inverse changes in average cell mass. The equations describing changes in total content of DNA (expressed in logarithms) implied that, for both species, the mitotic rate in the mammary gland was constant, or nearly constant, for most of pregnancy and the first part of lactation. In the rat and in the mouse, the increase in total alkaline phosphatase activity during pregnancy and throughout most of lactation was in part due to an increase in the enzyme activity per cell. On the other hand, the changes in total activity of acid phosphatase seemed to be due entirely to changes in the number of cells.