Fetal circulation of the placenta: a comparative study of human and baboon placenta by scanning electron microscopy of vascular casts.

This study was undertaken to compare the ultrastructural appearance of the fetal microvasculature of the baboon and human placenta by the latex injection corrosion-cast technique. The general pattern of distribution of blood vessels in the placenta and the microvasculature were examined primarily by scanning electron microscopy using single and double latex (Cementex) injections of the placentae. The results of these latex injections of the baboon placenta demonstrate that (1) all the fetal arterial vessels of the baboon placenta at about three-quarters of the gestation period have a characteristic tortuosity while the venous vessels have a straight appearance, with no distinct coiling, (2) the arterial vessels exhibit a rough surface contour, (3) on the venous side the wide-calibre vessels are fairly straight and exhibit a smooth luminal surface contour, and (4) the capillary network on the arterial side is dense and highly convoluted, while on the venous side the capillary network is less tortuous. Moreover, no arterial-venous anastomosis was observed by scanning electron microscopy (SEM). The main findings based on observations of the SEM appearance of the human placental casts at three-quarter gestation and at term include the following: (1) the arterial vessels do not exhibit a marked tortuosity but do retain a rough surface contour, (2) stem vessels appear to branch at a horizontal plane before dividing into numerous smaller branches, (3) the venous side has straight, wide-calibre vessels with smooth surface contour, and (4) the capillary bed is densely tortuous on the arterial side and less tortuous on the venous side.