Freeze-fracture images of mammary glands of lactating mice

Summary

Thin section as well as freeze-fracture images of lactating mouse mammary glands were examined with the electron microscope. The well-developed tight junction, consisting of 5–8 strands, is present at the apical part of the lateral plasma membranes of the adjacent glandular cells. The secretory products in this cell are classified into two types, milk fat globules without cores and proteinous granules having dense cores. Some proteinous granules fuse with each other to become larger structures. Membrane particles of the limiting membranes of both adjacent granules are cleared off from the initial site of the fusion. A large apocrine process including a large fat droplet, a few small proteinous granules, and little cytoplasmic matrix is sometimes seen. In addition, some proteinous granules are released by exocytosis. At exocytosis, the membrane particles are cleared off from the fusion site of the plasma membrane and the granule limiting membrane just before the fusion occurs.