FETAL VILLOSITY AND MICROVASCULATURE OF THE BOVINE PLACENTOME IN THE 2ND HALF OF GESTATION

The architecture of the fetal villous tree and its vasculature in the bovine placentome were studied in the second half of gestation using b oth conventional histology and histology of ink-filled blood vessels. These were compared with corrosion casts of plastic fillings of the va sculature, prepared for scanning electron microscopy. This combination of morphological methods allows perception of the villous tree throug hout gestation from broad-conical to tall-conical form where branch ra mification occurs mainly at right angles to the stem. The stem villus typically contains a single central artery and several peripheral vein s arranged in parallel. The proximal branches to the stem, the interme diate villi, contain a central arteriole and accompanying venules. The distal branches, the terminal villi, enclose capillary convolutions w hich consist of an afferent arterial capillary limb, capillary loops a nd efferent venous capillary limbs. Vascular interconnections exist wi thin the terminal villi, as capillaries or venules between the capilla ry convolutions, serially bridging them in up to 5 places, and as capi llary anastomoses between the capillary loops. Coiling and sinusoidal dilatations of these loops develop near the end of gestation. The intr aplacentomal rearrangement of villous trees with progressive gestation and their morphological vascular adaptations are discussed in relatio n to placental function, including the ever increasing need for transp lacental substance exchange. This adaptation allows the blood to trave rse the shortest possible arterioarteriolar route to the periphery of the trees where exchange takes place. The need for an increasing blood flow stimulates capillary growth and at the same time optimises the b lood flow reaching the placental barrier represented by the vessel cas t surface. The capillaries also carry the blood back into the very vol uminous system of venules and veins where back diffusion may occur. Th e total volume of terminal villi of bovine placentome, the 'working pa rt' of villous trees, hence distinctly increases with respect to the s tem and intermediate villi, the 'supplying part' of the villous tree. In morphological terms the efficiency of the bovine transplacental dif fusional exchange is higher than in the closely related 'co-ruminants' sheep and goats and distinctly higher when compared with the human pl acenta.