Syncytial knots and intervillous bridges in the human placenta: an ultrastructural study.

An electron microscopic study has shown that the syncytial knots of the villi of the human placenta contain aggregated nuclei which exhibit marked degenerative changes; within the cytoplasm there is an abundance of cytoplasmic filaments and many stacks of annulate lamellae. It is suggested that syncytial knots are a sequestration phenomenon in which senescent nuclear material is aggregated and removed from metabolically active areas of the syncytiotrophoblast. Intervillous bridges appear to be formed chiefly by fusion of syncytial knots from adjacent villi, and it seems reasonable that the effete material in a syncytial knot should be used for this purpose. The intervillous bridges hava a fine structure which suggests that they have a mechanical function, and this lends support to the theory that they form an internal strut system within the placenta.