In the present study, consistent with these observations, we noted that germ cells with distinctive morphologies (small, round, and single vs. larger, peripheral groups) were present in second-trimester samples. OCT4 (POU5F1 ), a transcription factor that is expressed in pluripotent embryonic stem cells, was immunolocalized to the nuclei of most germ cells in first-trimester testes, consistent with reports that pluripotent embryonic germ (EG) cell lines can be derived by culturing PGCs. Notably, we found that expression of OCT4 persisted in a subpopulation of germ cells during the second trimester and that these cells also expressed C-KIT on their surface.
Morphologically, this germ cell population was small and round, with a high nuclear:cytoplasmic ratio, and this fit the criteria proposed by Fukuda et al. for classification as a gonocyte. Fluorescent colocalization studies revealed that the OCT4/C-KIT-positive cells were distinct from those that were MAGE-A4-positive. The morphology of the MAGE-A4-positive cells was strikingly similar to those classified as prespermatogonia by Fukuda et al., and we believe these represent a population of fetal germ cells that have differentiated from the pluripotent OCT4-positive gonocytes. canadian family pharmacy