The explanation for these partly discrepant results may relate to different procedures of tissue fixation and processing, as well as to the generally lower intensity of staining in the syncytiotrophoblast as compared to the fetoplacental endothelium. There was no evaluation of placentas of earlier gestational ages or of basal plate from the term placenta in the previous report, so the observation of EPO-R expression associated with extravillous cytotrophoblast, villous cytotrophoblast, and cytotrophoblast cell columns as reported herein is also new (Figs. 1-3). Further, the present results obtained with the m2her 16.5.1 antibody were observed with a sheep polyclonal antibody. Consistent with our observation of EPO-R expression by various populations of trophoblast cells in the human placenta is the finding of EPO-R immunoreactivity associated with the tro-phoblast giant cells in the mouse placenta.
A prominent band of 66 kDa was observed on Western blot for placental tissues and trophoblast cells, confirming the immunocytochemical results demonstrating EPO-R expression by these tissues (Figs. 5 and 6). Importantly, this detection of immunoreactive EPO-R by Western blot was achieved using another monoclonal antibody that recognized a different epitope from the one employed in the immunocytochemistry. Although intact placental tissues might be expected to express EPO-R by Western blot because they contain fetoplacental vascular endothelium and fetal erythroid precursors, the expression of EPO-R by Western blot was also observed in isolated and immuno-purified first-trimester and term trophoblast cells, as well as in the Jar trophoblast-derived choriocarcinoma cell line.