P93. Human germ cells development during the first trimester—X chromosome activity in human germ cells

Abstract

During the development of the human fetal germ cells, the first trimester human gonads include different subpopulation of germ cells because of the gradual and unsynchronized development. To elucidate human fetal germ cells development more precisely, human germ cells in the first trimester (9.0–12.5 weeks) were comprehensively analyzed by double and triple whole-mount immunostaining. Stage specific embryonic antigen 1 (SSEA1) has been commonly used for identifying mouse and human germ cells in vivo and in vitro, however, triple whole-mount immunostaining with SSEA1/cKIT/OCT4 clearly showed that both cKIT and OCT4 were specifically expressed in human germ cells in the gonads, however, SSEA1 expression pattern in human germ cells were very weak and speckled, importantly, not specific for human germ cells during the first trimester. By co-localization of cKIT with VASA in the first trimester human germ cells, we could identify several different subpopulations of immature human germ cells. This suggests that there is a transition from immature germ cells expressing cKIT to germ cells expressing VASA during the first trimester. Intriguingly, we observed cytoplasmic localization of VASA followed by prominent and transient Golgi localization of cKIT in human germ cells. Moreover, we observed drastic changes of nuclear morphology as the chromosomes begin to decondense in the human germ cells. This process of chromosome decondensation is manifested as signs of epigenetic reprogramming in human germ cells. One of the epigenetic features of female germ cell development is X chromosome reactivation. To further investigate the precise timing of X chromosome reactivation in human female germ cells, we examined tri-methylation of histone H3 lysine 27 (H3K27me3) accumulations on inactive X chromosome (Xi) in human female germ cells. Here we showed that dynamic decrease of H3K27me3 accumulation on Xi was observed according with increased cytoplasmic VASA expression in human female germ cells. These data provide the first evidence for epigenetic reprogramming and X chromosome reactivation in human fetal germ cells during the first trimester of development.