Protein secretion from rat mammary epithelial cells during pregnancy

Aspects of the apparent origin, maturation, and secretion of proteins in mammary epithelial cells of the pregnant rat were investigated using standard transmission electron microscopy. Epithelial cells from prelactating animals, sacrificed at intervals up to 1 day ante partum, contained protein-filled vesicles which appeared to arise from two separate intracellular sources, (1) Golgi apparatus dictyosomes, and (2) dilated cisternae of rough endoplasmic reticulum. Vesicles which appeared to originate in the Golgi apparatus were small (means = 0.28 micron diameter), contained a single protein particle, and appeared to release their content exocytotically into the lumen. Vesicles which appeared to originate in the endoplasmic reticulum were larger (means = 3.2 micron diameter), contained numerous protein particles, and appeared to be released from cells with their outer membrane. Post-exocytotic endoplasmic reticulum-derived vesicles in alveolar lumina appeared to be the 'nuclei' of structures previously described as colostrum cells or corpuscles of Donn é. Results from quantitative ultrastructural analysis of mammary tissue from pregnant animals suggested that the intracellular disposal mechanism for turnover of secretory protein may not be of major importance.