The three-dimensional feto-maternal vascular interrelationship during early bovine placental development: a scanning electron microscopical study

Both the fetal and maternal microvasculature of bovine placentomes was exam ined by scanning electron microscopy of vascular casts. So far the developm ent of the vascular architecture of the bovine placentome in early gestatio n has only been studied 2-dimensionally due to technical difficulties arisi ng from the fragility of the early placental blood vessels. Repeated experi ments led to the selection of the microvascular corrosion casts presented h ere. The vasculature of the maternal compartment is supplied by large carun cular stalk or spiral arteries, which release short maternal stem arteries. In the 3rd month of gestation, these arteries branch into several arteriol es at their base, thus providing the vascular framework for the lower part of the septal walls of the primary crypts. In the 4th month, due to progres sive longitudinal growth of the stem arteries, branching into arterioles oc curs not only at the base, but over the whole length of the stem arteries. These arterioles supply the capillary complexes of the septa which resemble the major part of the septal vasculature and face the secondary crypts. Fu rther indentation results in the formation of tertiary crypt capillary comp lexes, encircling the earlier secondary unit. From the 6th month of gestati on the architecture resembles the fully developed maternal placenta with st em arteries running directly to the fetal side to branch into 4 to 6 arteri oles, which turn back to enter secondary and tertiary septa. Maternal venul es, collecting the blood from the capillary bed of secondary and tertiary s epta, converge onto stem veins leaving the caruncle via branches of the ute rine vein. The fetal part of the placentome is supplied by the cotyledonary arteries, which branch into fetal stem arteries that are the tributary to single villous trees. Over their whole course towards the maternal side, th ese give off arterioles entering secondary villi. The tertiary or terminal villous vasculature consists of capillaries, which are organised in serial capillary loops. This system is progressively elaborated in the course of g estation. In the 4th month there are only finger-like loops, whereas from t he 6th month large fan-like structures can be observed. In early gestation the maternal and fetal blood vessels meet predominantly in a countercurrent fashion, changing to the less efficient crosscurrent exchange when the ter tiary unit develops. These results indicate the development of a highly ela borated fetomaternal villous-crypt exchange system, already established in the 1st half of gestation, thus meeting the increasing needs of the fetus.