Morphology of pig uterine subepithelial capillaries after topical and systemic oestrogen treatment

The ultrastructure of subepithelial capillaries in pig endometrium was studied after intrauterine and systemic oestradiol treatment. Gilts were killed on day 13 or 19 (n = 4 per day per treatment) after surgical introduction of Silastic beads containing either oestradiol or cholesterol into the uterine lumina on day 10 after oestrus. An additional group of gilts was injected i.m. with 5 mg oestradiol valerate on days 11-15 of the oestrous cycle and killed on day 13 or day 19 (n = 4 per day). Light and electron microscope studies of endometrial samples revealed that cholesterol beads did not appear to affect subepithelial capillary structure. Both intrauterine and systemic treatment of gilts with oestradiol were associated with regional modification of the capillary wall involving attenuation and fenestration of the face directly underlying the uterine epithelium, accumulation of caveolae in the opposite face of the capillary, and the occurrence of discontinuous, multilayered capillary basal laminae. The similarity of these structural changes to those observed in capillaries at the time of embryonic attachment suggests that oestrogens of blastocyst origin may function to modify capillary morphology and possibly facilitate increased transcapillary traffic during the establishment of pregnancy.