TRANSFERRIN AND PROLACTIN TRANSCYTOSIS IN THE LACTATING MAMMARY EPITHELIAL-CELL

The mammary epithelial cell ferries constituents originating from bloo d and from stromal cells, into milk, by transcytosis. Morphological an alysis of a membrane marker of transcytosis in the lactating mammary e pithelial cell showed that very rapid endocytosis of surface membrane occurs from both the basolateral and the apical side of the cell. In b oth cases, membrane trafficking between endosomes and the Golgi comple x allows communication between the endocytic and the biosynthetic path ways. Transferrin and prolactin are internalized in mammary cells and transported through multivesicular bodies and Golgi stacks. They are r eleased into milk via different types of secretory vesicles, prolactin being carried in secretory vesicles containing casein micelles. Conse quences of the intracellular transport of these proteins and physiolog ical benefits for cell function are discussed.