Early germ cells contain telomerase, a reverse transcriptase which adds telomeres to 3-prime DNA ends, but telomerase activity declines in oocytes, fixing telomere length earlier during development.
To examine whether the subpopulation of putative early germ cells identified based on morphology also expressed marker genes known to be associated with PGCs, RT-PCR was performed.
Determining the identity of in vitro-generated PGCs has proven difficult given that a significant number of markers originally thought to be "germ cell-specific" have been shown to be expressed in both pluripotent stem cells and early germ cells.
In addition to the derivation of PGCs from murine ESCs, a recent study has reported that cells resembling early germ cells can also be differentiated from human ESCs based on the application of a monolayer culture, and that these cells express Vasa, Oct4, Stella, and Scp3 [20].
PHF7 is highly expressed in early germ cells and is required for GSC maintenance and spermatogonial differentiation [ 37].
Blimp1 and Prdm14 together are critical for epigenetic reprogramming during specification of PGCs and early germ cells.
In mammals, spermatogonial stem cells (SSCs) arise from early germ cells called gonocytes, which are derived from primordial germ cells during embryogenesis and remain quiescent until birth.
The early germ cells, or so-called primordial germ cells (PGCs), arise in the posterior primitive streak between mouse embryonic days E6.5 and E7.5 [ 42].
By contrast, the H3K27me3 modification associated with gene repression accumulated in early germ cells, and its signals decreased as the cells differentiated (Fig. 1E,I).
A transcriptomic analysis of CIS, early germ cells and several types of GCTs has indicated that CIS cells in fact resemble to PGCs/gonocytes (Sonne et al, 2009).
