The development of the endotheliochorial mink placenta: light microscopy and scanning electron microscopical morphometry of maternal vascular casts

The development of the mink endotheliochorial placenta has been studied by means of light microscopy and scanning electron microscopy of maternal vasc ular corrosion casts. The placental blood vessels of three groups of mink, representing early, intermediate and near-term gestational ages were either perfusion fixed for histology, or instilled with liquid plastic in order t o prepare vascular casts, which were examined qualitatively and/or quantita tively. The maternal component of the placental vascular system evolves fro m preimplantation blood vessels between the endometrial glands, into which the initial fete-maternal contact is made. The influence of highly invasive syncytiotrophoblast provokes a transition of the maternal capillaries into extensively anastomosing sinusoids with a subsequent modification of their endothelial cells into large cells with luminal protrusions. Three-dimensi onally, the sinusoids are arranged as vascular crypts. This implies a villo us-crypt type of interdigitation for the mink, but since the fetal capillar ies surround the maternal sinusoids as a dense network a labyrinth is forme d. The vascular crypts are supplied by very short arterioles, branching fro m maternal stem arteries, which arise from branches of the uterine artery a nd move straight to the surface of the endometrium. Venous outlets of the s inusoids converge onto venules and large stem veins in the deepest portion of the endometrium. This architectural pattern persists until term. Morphom etry was used to confirm the qualitative observations in vascular casts. Th e diameter of materal vascular crypts significantly increased from 137.3+/- 21.9 mu m in early gestation up to 217.8+/-80.9 mu m in the intermediate st age and 431.8+/-119.5 mu m near-term, when compared to the paraplacental zo ne in early gestation (82.2+/-19.5 mu m). The capillary or sinusoidal diame ter also increased significantly from intermediate stage (42.9+/-11.8 mu m) to near term (60.1+/-16.7 mu m), whereas the difference in the paraplacent al zone (7.3+/-2.1 mu m) and early gestation (13.0+/-3.2 mu m) was not stat istically significant.