C-myc expression in early human placenta--a critical evaluation of its localization

A tight association between the expression of the protooncogene c-myc and the proliferation of trophoblast in first trimester human placentae has been reported, supporting the view that c-myc is under close control of the cell cycle. However, this has not been verified in several other cells systems. Therefore we reexamined the exact localization of myc expression at the transcriptional and translational level in 20 first trimester and three term placentae. Myc mRNA and protein was sparse or absent at term but abundant in early gestation placentae. The proliferative cell columns and the villous cytotrophoblast contained the greatest amounts, revealing myc protein in around 60-70% of villous cytotrophoblast cells. Unexpectedly, a considerable fraction of the syncytiotrophoblast nuclei of early placentae (20%) also showed myc expression, and this was particularly evident on the protein level. The myc content estimated by immunohistochemistry decreased with increasing placental maturation. In addition, prominent myc expression was seen in decidual cells, suggesting a paracrine growth regulation of the gestational endometrium. Our findings do not support the notion that myc expression is closely cell cycle-dependent. On the contrary, it appears that in the human placenta, myc expression characterizes the phase of rapid organ development in the first trimester and is not restricted to the proliferative cytotrophoblast.