Characterization of Human Spermatogonial Stem Cell Markers in Fetal, Pediatric, and Adult Testicular Tissue

Altman  et al., 2014 (Oct). Nam Tran Lab

This study characterizes germ cell membrane markers of male gonocytes, prespermatogonia, and SSCs in humans via FACS. THY1 and c-KIT are transient markers of gonocytes but not in prespermatogonia and post-natal SSCs. c-KIT is detected in gonocytes and THY1 is detected in somatic compnent in addition to its presence in gonocytes. SSEA4 is only present in gonocytes, prespermatogonia, SSCs, and Sertoli cells from fetal through adult human stage.

Male testis contains two populations of germ cells defined by the relative expression of VASA:
At 13 weeks, VASA dim (VASAD) and VASA bright (VASAB) were present as two germ cell populations. VASAB population represent prespermatogonia, but VASAD represent gonocytes. Both VASAD and VASAB express MAGEA confirming that they are primitive germ cells. In adult testis, VASAD a germ cells were in the basement mmbrane where primitive spermatogonia and SSC are located. In adult testis, VASAB were seen near the lumen where mature spermatocytes are located.

SSEA4 is a common membrane marker for gonocytes, prespermatgonia and Sertoli cells: 
During second and third trimesters, SSEA4 is expressed in all seminiferous cord cells. Both VASAD and VASAB express SEEA4 at similar levels, indicating that SSEA4 is a common marker for both gonocytes and prespermatogonia. SSEA4-positive/VASA-negative are identified as Sertoli cells. In adult testis, SSEA4-positive/VASA-postive were only detected in VASAD germ cells, indicating that SSEA4 remained to be the marker for primitive spermatogonia.

Gonocytes transiently express c-KIT and THY1:
THY1+ cells are arranged in clusters of 3-6 cells and expressed low expression levels of VASAD; however, VASAB never expressed THY1. SSEA4+/THY1+ are considered primitive gonocytes. At 19 weeks of gestation, 10% of SSEA+ cells were THY1+ (gonocytes), and 90% of SSEA4+ cells were THY1- (prespermatogonia and Sertoli cells). In contrast, >90% of SSEA4- cells express THY1.

After second trimester, THY1 expression was detected only in somatic cells outside of seminuferous cords while c-KIT was not detected in either VASAD or VASAB cells. Even though THY1 was found in early fetal gonocytes, THY1 expression in adult men was solely expressed in somatic cells, including Sertoli cells, peritubular interstitial cells, and cells making up the lamina propria.

SSEA4 continues to be the membrane marker or spermatogonial stem cells (SSCs) postnatally:
Both SSC and Sertoli cells from 4 month  and 4 year old boys continued to express SSEA4 within the seminiferous cord, as seen in fetal testis during third trimester. Neither THY1 nor c-KIT expression was detected within the seminiferous cord of these postnatal testis. And less than 10% of spermatogonia were VASAD postnatally.