Scanning electron microscopy study on corrosion cast of rat uterine vasculature during the first half of pregnancy.

Vascular changes in the rat uterus during the first half of pregnancy were studied by the microcorrosion casting/scanning electron microscope method. Arterioles supplying the endometrium were divided into two groups according to differences in their shape, distribution, and development. Circular impressions indicative of the presence of vascular sphincters were observed around the casts of each group of arterioles. Resin leakage, suggesting an increase in vascular permeability, was observed from capillaries composing the sub-epithelial plexus and the glandular 'baskets'. Although leakage was noted throughout the endometrium before implantation, it tended to be localised around each blastocyst after the onset of implantation. The present results suggest the existence of a special control mechanism of the uterine blood flow, and that the uterine vasculature is affected by the conceptus from the beginning of implantation.