Annexin II (calpactin I) in the mouse mammary gland: immunolocalization by light- and electron microscopy.

To elucidate the putative role of annexin II (calpactin I) in the secretory function of mammary tissue its immunolocalization in the mammary gland of pregnant and lactating mice was investigated by light- and electron microscopy using the immunoperoxidase technique. A low level of fairly uniform annexin II staining was evident throughout the gland despite its mixed composition during pregnancy. In lactating tissue it was revealed that apparently mature alveoli contained a concentration of annexin II staining outlining their epithelium. The staining was localised by immuno-electron microscopy to the apical membrane of these alveolar epithelial cells and their microvillar extensions. There was also an apparent association of annexin II with vesicles of a range of sizes located near, or actually fused with, the apical membrane. Many of the small, stained vesicles could clearly be identified as casein-containing vesicles while the large vesicles were apparently associated with either casein granules or possibly lipid. The appearance of a selective concentration of annexin II in apparently actively secreting mammary epithelial cells, as revealed in this study, is consistent with a possible structural and/or functional role for this protein at the membranes participating in the secretion of protein and possibly lipid from these secretory cells.