Wartenberg classified the fetal germ cells into different subpopulations, these being gonocytes (i.e., the PGC population after it was resident in testis cords) and prospermatogonia. Wartenberg divided the latter into M prospermatogonia (mitotically active), T1 prospermatogonia (nondividing/resting), and T2 prosper-matogonia (those that had resumed mitotic activity). Holstein et al. identified a germ cell type in 8-, 10-, and 13-wk embryos that had a large nucleus and a prominent nucleolus that they called ‘‘gonocytes.’’ In more mature fetuses (13 and 22 wk), those authors noted that some germ cells had features distinct from the gonocytes and suggested that these be classified as ‘‘fetal spermatogonia.’’ Fukuda et al. examined fetuses between 9 and 30 wk of gestation and reported that they were able to identify three morphologically distinct germ cell types, which they called gonocytes, intermediate cells, and fetal spermatogonia.
The aims of the present study were to evaluate the patterns of protein expression in testes from the first and second trimesters, to determine whether one or more populations of germ cells could be identified in paraffin-fixed material by means of immunohistochemical evaluation, and to evaluate whether the pattern of protein expression was correlated with changes in cell morphology and/or differentiation status of germ cells. Germ cells were identified using antibodies directed against proteins reported to be expressed in human fetal germ cells or testicular germ cell tumors. These proteins included those known to be expressed in PGCs, such as placental alkaline phosphatase (PLAP), C-KIT (the membrane tyrosine kinase receptor for the kit ligand ), and OCT4, which is a marker of pluripotency. Other proteins were selected because they are believed to be involved in regulation of the cell cycle (e.g., CHK2, and PCTAIRE-1 ). The pattern of expression of MAGE-A4 (melanoma antigen A4) was of particular interest, because this protein is a member of a testis-cancer family of antigens and because, in the adult testis, intense immunostaining for MAGE-A4 has been detected in the nuclei and cytoplasm of spermatogonia.