INVOLUTION OF THE SHEEP MAMMARY-GLAND

Changes in the ovine mammary gland epithelium during initiated involut ion were studied by light and electron microscopy. Apoptosis of the du ct and alveolar epithelial cells was first identified at 2 d after wea ning, reached a peak at 4 d and then progressed gradually thereafter. Apoptotic cells were phagocytosed by intraepithelial macrophages and a lveolar epithelial cells. Occasional apoptotic epithelial cells were o bserved in the alveolar and duct lumina. The highly vacuolated cells i n the alveolar and duct lumina were confirmed to be macrophages as the y were CD45(+), MHC class II+. Changes in myoepithelial cells involved shrinkage and extension of cytoplasmic processes into the underlying stroma and no apoptosis was observed. Regression of the blood capillar ies was also by apoptosis. The resulting apoptotic bodies were either taken up by adjacent endothelial cells or were shed into the capillary lumen to be phagocytosed later by mural endothelial cells or blood mo nocytes. The mammary glands were completely involuted by 30 d after we aning. It was concluded that the mammary gland involutes by apoptosis, a process which allows deletion of cells without the loss of the basi c architecture and the integrity of the epithelial lining of the gland .