Summary. Using in vivo microscopy we investigated endometrial microvascular events occurring on days 5 and 6 of pregnancy at the time of implantation. Blood flow through the endometrium was visualized using incident-light fluorescence microscopy and a video image was recorded for subsequent analysis. At 17:00 h on day 5 of pregnancy it was not possible to identify the impending implantation site from the in vivo appearance of the subepithelial capillary plexus. At 09:00 h on day 6 of pregnancy the embryo implantation site was recognized as an avascular area surrounded by large diameter vessels. These were highly susceptible to haemorrhage when handled. Capillaries closest to the embryo had the greatest diameters, averaging 18·5 ± 2·5 μm, and capillary diameters decreased to 7·5 ± 0·4 μm by 2000 μm from the embryo. It was also observed that blood flow through larger diameter vessels was sluggish with frequent reversals and stoppages. Leucocyte rolling and adhesion were also common features in these larger vessels. These data indicate that changes in capillary diameter occur in response to local signals associated with the implanting rat embryo. The embryonic or local endometrial signals that mediate these major microvascular changes remain to be elucidated.

Keywords: endometrium; implantation; in vivo microscopy; microcirculation; rat