Developmental changes in rhesus monkey placental villi and cell columns. Scanning electron microscopy.

Developing rhesus monkey placentas were analyzed by scanning electron microscopy with special attention directed toward defining stages in the development of the villus branches. The initial phase in formation of villi was the conversion of reticulated trabeculae of syncytial trophoblast into chorionic villi by growth and proliferation of cell columns of cytotrophoblast. These villi were stout and unbranched. The second phase of development appeared to be the longitudinal splitting of the villi and cell columns to form groups of parallel branches but there was a common insertion of these into the basal plate. The third phase in formation of villi, which appeared to begin at about the same time as the longitudinal splitting occurred, was the outgrowth of large-diameter side branches in a zone nearer the chorionic plate. At about 38-40 days of gestation the next stage in villus formation occurred, characterized by the emergence of numerous, small syncytial sprouts. Continued proliferation of villi at later stages of gestation resulted in a decreased diameter of the terminal villi and an increasing complexity in the course of fetal capillaries.