DNA synthesis in the absence of cell reproduction during functional differentiation of mouse mammary gland

Abstract

DNA synthesis, mitotic indices and DNA content of mouse mammary gland at different stages of pregnancy and lactation were determined. 3H-Thymidine specific activity of mid-pregnancy mammary gland DNA was 3 times higher than the lactating gland at its peak period of DNA replication. But, the frequency of mammary cells incorporating 3H-Thymidine was equally high in both the pregnant and lactating tissues. Consistent with its low specific activity of the DNA, the average number of silver grains per nucleus of the 6-day lactating gland was nearly 60% less than that of the pregnant mammary tissue. The nucleolus/chromatin grain ratio in the lactating tissue was significantly greater than in the mammary gland of pregnancy. In spite of the consistent rise of the labeling index between the 3rd and 6th days of lactation, the low mitotic index of the lactating gland virtually remained unaltered during this interval, whereas the high mitotic index of the pregnant tissue was proportionate to its labeling index. The total mammary tissue DNA increased significantly from the 15th day of pregnancy to the 6th day of lactation. A rise of nearly 30% per nuclear DNA content was also evident from 3 to 6 days of lactation. The results indicate that DNA synthesis in a high proportion of cells in the lactating gland may not represent premitotic DNA replication and suggest a partial replication of the cellular genome during early lactogenesis. The significance of these results with respect to “gene amplification” has been discussed.