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 vascular 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 to prepare vascular casts, which were examined qualitatively and/or quantitatively. The maternal component of the placental vascular system evolves from preimplantation blood vessels between the endometrial glands, into which the initial feto-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-dimensionally, the sinusoids are arranged as vascular crypts. This implies a villous-crypt type of interdigitation for the mink, but since the fetal capillaries surround the maternal sinusoids as a dense network a labyrinth is formed. The vascular crypts are supplied by very short arterioles, branching from maternal stem arteries, which arise from branches of the uterine artery and move straight to the surface of the endometrium. Venous outlets of the sinusoids converge onto venules and large stem veins in the deepest portion of the endometrium. This architectural pattern persists until term. Morphometry was used to confirm the qualitative observations in vascular casts. The diameter of materal vascular crypts significantly increased from 137.3±21.9 µm in early gestation up to 217.8±80.9 µm in the intermediate stage and 431.8±119.5 µm near-term, when compared to the paraplacental zone in early gestation (82.2±19.5 µm). The capillary or sinusoidal diameter also increased significantly from intermediate stage (42.9±11.8 µm) to near term (60.1±16.7 µm), whereas the difference in the paraplacental zone (7.3±2.1 µm) and early gestation (13.0±3.2 µm) was not statistically significant.