Distribution of the lysosomal enzyme aryl sulfatase in murine mammary tissue through pregnancy, lactation, and involution.

Mammary tissue from 44 primiparous mice at various stages of pregnancy, lactation, and involution were stained for aryl sulfatase (lysosomal marker enzyme) activity and prepared for electron microscopy. Stereological techniques were used to determine the distribution of lysosomes per unit of cytoplasmic area in mammary epithelial cells. The number of primary lysosomes (containing only enzyme) was stable throughout the study, except for a temporary increase following parturition. Secondary lysosomes (containing substrate undergoing active digestion) designated as either telolysosomes or dense bodies were more frequent in mammary epithelial cells from animals in the late involution and early pregnancy stages than in lactating animals. However, telolysosomes did increase temporarily at the onset of lactation and casein micelles were identified within secondary lysosomes throughout the lactation stage. The frequencies for endoplasmic reticulum and Golgi apparatus containing aryl sulfatase reaction product were highest in secretory cells, when these structures were predominant. Shifts in lysosomal populations throughout the study suggests that lysosomes may have an active role within the mammary epithelial cells during differentiation and secretion as well as during involution.