Figure 8.

Revised scheme for premeiotic development of germ cells in adult human testis. It is hypothesized that small and rare ES-like cells detected for the first time in testis undergo asymmetric cell division to give rise to A_dark spermatogonial cells with cytoplasmic Oct-4B. These cells undergo extensive nuclear reprogramming by epigenetic changes because pluripotent cell machinery is shut OFF and specific genes necessary for committed cell fate are turned ON. This may result in the dark appearance of their nuclei and later transform into A_pale spermatogonial stem cells (SSCs) with normal pale nucleus and decreased expression of Oct-4B. A_pale SSCs also proliferate as indicated by studying c-KIT staining, reported earlier by our group (Unni et al. 2009), undergo further differentiation and meiosis to complete the spermatogenesis cycle.