VASCULAR INTERRELATIONSHIPS OF NEAR-TERM MINK PLACENTA - LIGHT-MICROSCOPY COMBINED WITH SCANNING ELECTRON-MICROSCOPY OF CORROSION CASTS

The microvasculature of the near-term zonary placenta of the mink has been studied using Light microscopy and scanning electron microscopy o f corrosion casts, prepared from maternal and fetal vessel systems, re spectively. The zonary area, most important for placental exchange, in cludes a characteristic labyrinth. The labyrinth is composed of lobule s oriented in a maternal-fetal direction. One maternal stem artery ser ves as the central axis of a lobule, and about six pairs of fetal stem arteries and stem veins of the chorionic primary villi mark the perip hery of the lobule. Viewed from the fetal side of the labyrinth, this lobular structure presents a roughly hexagonal pattern, with the centr al maternal stem artery and radially oriented arteriolar branches givi ng the lobule the shape of a star. These arterioles frequently form br idges to neighboring lobular systems; however, the majority continue i nto the fete-maternally oriented three-dimensional network of maternal capillary sinusoids, which converge on the outlets of the maternal st em veins on the maternal side of the labyrinth. Maternal main crypts a re delimited by the rays of the star-shaped lobules containing chorion ic primary villi. The latter penetrate into maternal crypts from the f etal side, and are characterized by their axial arterial and venous st em vessels. Fetal secondary villi are arranged at different levels fro m these stem vessels. The secondary villi are characterized by arterio les and venules branching in pairs from the stem vessels and supply th e tributary capillary complexes of terminal villi. The lobular structu re of the placental labyrinth provides a three-dimensional framework o f vessels where maternal capillary sinusoids and fetal capillaries mee t in a one-way cross-current arrangement. The blood flow conditions an d the peculiarities of the mink placenta interhemal membrane are compa red to those of other carnivores and discussed with respect to the eff iciency of the endotheliochorial placenta. (C) 1997 Wiley-Liss, Inc.