Structural and functional aspects of porcine endometrial capillaries on days 13 and 15 after oestrus or mating.

The permeability of subepithelial capillaries in porcine endometrium was studied during midcycle and early pregnancy. Gilts were slaughtered on Day 13 or Day 15 of the oestrous cycle or pregnancy, 15 min after injection through the ear vein of 11.1 MBq of 125I-labelled human albumin in phosphate-buffered saline. The radioactivity of endometrial strips taken along the mesometrial and antimesometrial aspects of the uterine horn varied on average from 270 to 701 c.p.m./g and no difference (P greater than 0.05) was found between reproductive status, days of slaughter or sampling sites. The majority of the subepithelial capillaries showed ultrastructural evidence of increased vascular permeability, such as marked thinning of the capillary walls, especially on the side proximal to the epithelial basal lamina, multilayering and partial disparition of the endothelial basal lamina and abundant endothelial vesicles. Fenestrated pores were observed, but were rare. There was no obvious difference between reproductive status, days of sampling or sampling sites inside the uterus, suggesting that on Days 13-15 after oestrus the ultrastructural characteristics of porcine endometrial capillaries are little affected by the presence of attaching blastocysts and supporting the results obtained with radioactive albumin. Ferritin injected directly into a uterine artery of one gilt on Day 15 of pregnancy was carried through the capillary wall by endothelial vesicles, showing ultrastructural evidence of increased permeability.