Capillary patency and permeability in the endometrium surrounding the implanting rat blastocyst

The patency and permeability of the endometrial vasculature surrounding the implanting rat blastocyst on day six of pregnancy were investigated using vascular corrosion casting/scanning electron microscopy (SEM), light microscopy, transmission electron microscopy and the intravascular (i.v.) injection of Evans' blue dye and fluorescein isothiocyanate (FITC) tagged albumin. Vascular casting revealed an absence of subepithelial endometrial capillaries at the centre of the implantation site resulting in an ovoid space in the casts averaging approximately 420 microns long by 210 microns in diameter. Fluorescent microscope observations on sections of implantation sites cut after an i.v. injection of FITC albumin showed an absence of patent vessels within an area approximately 350 microns long by 200 microns wide of the blastocyst, while there had been extensive leakage of FITC albumin around this area due to a massive increase in vascular permeability in the surrounding endometrium. Similar sections from areas between implantation sites showed patent vessels throughout the endometrium and no leakage of FITC albumin into the interstitial tissue. Subsequent haematoxylin-eosin (H & E) staining of these same sections revealed that the avascular area surrounding each blastocyst was the primary decidual zone. Possible mechanisms by which the vessels closest to the blastocyst are shut down, and by which those nearby undergo the demonstrated large increase in permeability are discussed. Two possibilities as to why the maternal blood supply to the blastocyst is reduced at this critical time are firstly that it may weaken the maternal tissues, thus enhancing trophoblastic invasion, and secondly that it may be involved in the mechanism whereby the blastocyst gains immunological privilege from the mother.