Endocytosis and intracellular transport of transferrin across the lactating rabbit mammary epithelial cell.

To study the transcytosis and segregation of ligand in the mammary epithelial cell, endocytosis and intracellular transit of human blood transferrin were followed in lactating rabbit mammary epithelial cells. Human transferrin labeled with biotin added to an incubation medium was bound to the basal membrane of mammary epithelial cells and carried across the cell to the lumen of the acini within 5-60 min. At the same time, biotinylated human transferrin accumulated at the apex of the cell. After incubation with human transferrin labeled with colloidal gold, label was detected inside endosome-like structures, vesicles and saccules of the Golgi apparatus, and inside the lumen within 2-5 min. A significant label accumulated at the apex of the cell after 30-60 min. Biotin labeling did not modify the time of transit of human transferrin, as attested by comparison with the time of transit of native transferrin. Human transferrin was never detected inside vesicles containing casein micelles. In contrast, rabbit milk transferrin was immunocytochemically detected inside vesicles containing casein micelles. These results indicate that transcytosis of human transferrin follows a pathway different from vesicles that carry casein micelles.