The Impact of Trophoblast Turnover on Syncytiotrophoblast Specialization

The structurally and functionally specialized areas of syn-cytiotrophoblast described above very unlikely are stable structures. Rather, the comparison of these subtypes of syncytiotrophoblast with the events that take place during trophoblast apoptosis and turnover leads us to suspect that they are an expression of a highly dynamic process of differentiation and degeneration.

Taken together, the data lead us to suggest the following cycle of structural and functional changes within in the villous trophoblastic cover (Fig. 6.21):

•  Upon syncytial fusion, syncytiotrophoblast with well-developed rough endoplasmic reticulum, focused on protein metabolism, is generated.

•  Within days to a very few weeks, due to missing transcription activity of the syncytial nuclei, the ribosomes are lost and the respective area is transformed into syncytiotrophoblast with smooth endoplasmic reticulum and tubular mitochondria, focused on steroid metabolism.

•  As a consequence of progressive apoptotic degradation of the syncytiotrophoblastic cytoskeleton, the aging nuclei are focally aggregated, forming apoptotic knots, which shortly are pinched off from the syncytial surface and are deported into the maternal circulation. The sorting process, which on the one side accumulates the syncytial nuclei and on the other side generates enucleated, thin layers of syncytiotrophoblast, is probably driven by pressure of the underlying fetal capillaries and sinusoids.

•  After aggregation and extrusion of the nuclei result enucleated areas of syncytiotrophoblast, which mostly span over (1) fetal sinusoids (vasculosyncytial membranes, specialized in diffusional exchange) or (2) undifferentiated cytotrophoblast in G0-stage (syncytial lamellae covering cytotrophoblast, possibly specialized in steroid metabolism).

•  Formation of the latter structures should be investigated as a signal for the cytotrophoblast to enter the cell cycle, later to differentiate and to fuse.

•  By syncytial fusion, the vasculosyncytial membranes and the lamellae covering cytotrophoblast are again equipped with new syncytial nuclei, cell organelles, and RNA so that the turnover cycle can start anew.

As discussed before, the length of such a cycle is estimated to last 3 to 4 weeks.

Also throughout the first 4 months of pregnancy, when the respective syncytiotrophoblast specialization is not demonstrable, similar turnover processes take place within the villous trophoblast (for review, see Huppertz et al., 2002). They are possibly less impressive due to (1) much higher relative numbers of villous cytotrophoblast, (2) higher turnover rates, and (3) reduced villous capil-larization as the speculated driving force for nuclear sorting of the differentiation processes within the syncy-tiotrophoblast, and thus escape electron microscopical and histochemical detection.

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