Lymphatics of the rat mammary gland during virgin, pregnant, lactating and post-weaning periods.

The distribution and ultrastructure of the lymphatics of the rat mammary gland in virgin, pregnant, lactating and post-weaning periods were examined by enzyme-histochemistry for 5'-nucleotidase (5'-Nase) and transmission electron microscopy. Enzyme-histochemistry for 5'-Nase stained lymphatics in dark brown. In the lactating period, lymphatics abounded in the interlobular connective tissues, but in other periods they were few. The interlobular lymphatics drained into collecting lymphatics running along the mammary ducts. Gaps between lymphatic endothelial cells were significantly wider in lactating period than in other periods, while both number and area of vesicles in the lymphatic endothelial cells were significantly larger in the virgin period than in other periods. In the pregnant and lactating period, the lymphatics contained many lymphocytes and lipid droplets. The results show that during lactating period, the interlobular lymphatics increase and that gaps between lymphatic endothelial cells serve as a major route through which tissue fluids and particulate matters enter the lymphatics, while vesicles seem to be main trans-endothelial transport route during the virgin period. The results will provide basic information for our next investigation on lymphangiogenesis in association with breast cancer.