A quantitative analysis of transcriptionally active syncytiotrophoblast nuclei across human gestation.

The syncytiotrophoblast (STB) epithelial covering of the human placenta is a unique terminally differentiated, multi-nucleated syncytium. No mitotic bodies are observed in the STB, which is sustained by continuous fusion of underlying cytotrophoblast cells (CTB). As a result, STB nuclei are of different ages. Morphologically, they display varying degrees of chromatin compaction, suggesting progressive maturational changes. Until recently, it was thought that STB nuclei were transcriptionally inactive, with all the mRNAs required by the syncytium being incorporated upon fusion of CTB. However, recent research has shown the presence of the active form of RNA polymerase II (RNA Pol II) in some STB nuclei. In this study, we confirm the presence of transcriptional activity in STB nuclei by demonstrating immunoreactivity for a transcription factor and an RNA polymerase I (RNA Pol I) co-factor, phospho-cAMP response element-binding protein and phospho-upstream binding factor, respectively. We also show, through immunoco-localisation studies, that a proportion of STB nuclei are both RNA Pol I and II transcriptionally active. Finally, we quantify the numerical densities of nuclei immunopositive and immunonegative for RNA Pol II in the STB of normal placentas of 11-39 weeks gestational age using an unbiased stereological counting tool, the physical disector. These data were combined with estimates of the volume of trophoblast to calculate total numbers of both types of nuclei at each gestational age. We found no correlation between gestational age and the numerical density of RNA Pol II-positive nuclei in the villous trophoblast (r = 0.39, P > 0.05). As the number of STB nuclei increases exponentially during gestation, we conclude that the number of transcriptionally active nuclei increases in proportion to trophoblast volume. The ratio of active to inactive nuclei remains constant at 3.9:1. These findings confirm that the majority of STB nuclei have intrinsic transcriptional activity, and that the STB is not dependent on CTB fusion for the provision of transcripts.