In our studies on the fetal testes, we have never found a germ cell that coexpressed C-KIT and MAGE-A4, and our preliminary observations (unpublished) suggest that although C-KITpos and MAGE-A4pos CIS cells can be detected, the two proteins are not coexpressed in the same cell. One interpretation of these findings is that cells with a CIS phenotype can develop not only from gonocytes but also from the OCT4neg/C-KITneg/ MAGE-A4pos population (prespermatogonia). Alternatively, it is possible that some CIS cells undergo partial differentiation that is characterized by loss of expression of OCT4 and onset of expression of MAGE-A4.
In conclusion, our findings extend those of Fukuda et al., who used morphological criteria to identify three populations of human germ cells in testes obtained during the second trimester. In agreement with their findings, we propose that the human germ cells should be classified as gon-ocytes, intermediate cells, and prespermatogonia, depending on their appearance and the expression of three of the proteins evaluated by immunohistochemistry. The characteristics of the cell types are outlined in Table 3.