Ultrastructural and morphometrical studies on the endothelial cells of arteries supplying the abdomino-inguinal mammary gland of rats during the reproductive cycle.

The ultrastructure of the endothelial cells of the deep circumflex iliac and caudal superficial epigastric arteries supplying the abdomino-inguinal mammary gland of female Wistar rats was studied throughout the reproductive cycle with an electron microscope and image analyzer and compared with that of mammary gland capillaries. The subendothelial space of the arteries was broader at the virgin and post-weaning stages but narrower in pregnancy and lactation. In some rats, the endothelial cells of the arteries radiated some blunt processes into the media through fenestrations in the internal elastic lamina. The density of pinocytotic vesicles (PV) in the arteries (number of PV per micron2 of endothelial cytoplasm) significantly increased during pregnancy and reached its maximum value during lactation, then subsequently decreased during the post-weaning stage. The mammary gland capillaries showed the maximum changes of PV during pregnancy and lactation, with twofold and fourfold increases during the respective periods. The density of mitochondria increased significantly in the capillaries during pregnancy and lactation. The length of the marginal folds and microvillous processes increased significantly during lactation in both the arteries and capillaries, and especially, increased twofold in the mammary gland capillaries. It is assumed that the ultrastructural changes of the endothelial cells of the arteries and capillaries are closely associated during reproductive cycle with the functional demand of the mammary glands.