Ultrastructural observations on cell death by apoptosis in the "resting" human breast.

For the first time the process of epithelial cell deletion was studied within the parenchymal component of the "resting" human breast. The dying cells were initially recognised by specific nuclear changes involving peripheral condensation of the chromatin and nucleolar disintegration. At this stage the cells were retracted from the lumen and had lost desmosomal connections with their neighbours. Within the cytoplasm, there was evidence of ribosomal detachment from the endoplasmic reticulum with the formation of ribosome aggregates. The majority of dying cells were phagocytosed at this stage although a few underwent further morphological changes. These involved blebbing and fragmentation of the nucleus followed by cytoplasmic fragmentation. The dying cells and cell fragments were phagocytosed by epithelial or myoepithelial cells as well as mononuclear phagocytes and undergo lysosomal digestion within the phagosomes. These progressive morphological changes were consistent with cell deletion occurring by apoptosis.