An ultrastructural study of lactation in the human breast.

In this study the morphological features of lactation in the human breast were examined by scanning and transmission electron microscopy. The lactating lobules comprised large numbers of interconnecting acini which were lined by a single layer of epithelial cells with underlying myoepithelial cells. Marked variations were noted in the shape of the epithelial cells. The myoepithelial cells formed an open meshwork of interconnecting cytoplasmic processes packed with myofibrils. The basal cytoplasm of the epithelial cells was packed with rough endoplasmic reticulum while the apical cytoplasm contained a hypertrophic Golgi body, numerous vacuoles (a few of which contained casein micelles), a number of lipid droplets and small coated and uncoated vesicles. The lipid droplets were released by progressive protrusion from the apical surface. They remained covered by the plasmalemma and were finally budded off into the lumen. In certain cases a portion of cytoplasm was released with the lipid droplet. The vacuoles and small vesicles fused with the plasmalemma and released their contents by exocytosis. Within the samples the majority of epithelial cells were actively lactating although examples of undifferentiated "resting" and dead (lysed) cells were also identified.