Minimal Concentrations of Retinoic Acid Induce Stimulation by Retinoic Acid 8 and Promote Entry into Meiosis in Isolated Pregonadal and Gonadal Mouse Primordial Germ Cells

Abstract

In the present study, we demonstrate that minimal concentrations (≤ 1 nM) of retinoic acid (RA), equivalent to the quantity contaminating serum-containing culture medium, are sufficient to promote meiotic entry and progression through meiotic prophase I (MPI) stages in isolated 12.5-days postcoitum (dpc) XX and XY mouse primordial germ cells (PGCs) in culture. Similarly, we found that the same low RA concentration up-regulated or induced stimulation by retinoic acid 8 (Stra8) in such cells, both at mRNA and protein level. In preleptotene/leptotene germ cells, STRA8 was localized in nuclear dots that disappeared at later MPI stages. In addition to Stra8, other meiotic genes such as Dmc1 and Rec8 appeared stimulated by RA directly in PGCs with similar concentration-dependent trends. Finally, we found that RA induced Stra8, Sycp3, Dmc1, and Rec8 transcripts, promoting meiotic entry in culture also in pregonadal 10.5-dpc PGCs of both sexes. When cultured isolated from somatic cells, such PGCs, however, were unable to progress through MPI stages, while after entering meiosis, they progressed through MPI when cultured within aorta/gonad/mesonephros tissues. We conclude that besides RA, germ cell intrinsic factors and other exogenous signals from the surrounding somatic cells are probably necessary for meiotic entry and progression in mouse PGCs.

Licensing of Primordial Germ Cells for Gametogenesis Depends on Genital Ridge Signaling

“…Based on these findings, the authors hypothesized that PGCs transition into meiotic germ cells (oocytes) in a gonad-independent, and therefore cell-autonomous, manner. This hypothesis was further supported by several in vitro studies [13,21–23], showing, for instance, that PGCs isolated from E10.5 mouse embryos of both sexes continue to develop in vitro and initiate meiosis at approximately the same time as meiotic entry occurs in vivo [13,22,23]. …”

“…The authors who proposed the cell-autonomous hypothesis considered E10.5 PGCs to be pre-gonadal germ cells [22,23]. However, we recently showed that the marker of genital ridge formation, GATA4, is expressed as early as E10.0 [2].…”

Activin A Accelerates the Progression of Fetal Oocytes Throughout Meiosis and Early Oogenesis in the Mouse

“…Fetal oocytes of mouse ovaries cultured in vitro progressing throughout the early stages of meiotic prophase I is slower than that in vivo, because in vitro culture conditions still are unable to fully meet the needs of oocyte development. Most interestingly, ActA appears to participate in the control of meiotic progression by modulating the expression of members of the RA system, namely through down-regulation of the RA degradation enzyme CYP26B1 and up-regulation of RARs [20][21][30][31]. According to the current more accepted model, female PGCs after arrival at the gonadal ridges are induced by the somatic environment, likely through RA, to activate the Stra8 gene [20,29].…”

“…According to the current more accepted model, female PGCs after arrival at the gonadal ridges are induced by the somatic environment, likely through RA, to activate the Stra8 gene [20,29]. STRA8 protein induces the mitotic-meiotic switch and drives female PGCs into meiosis [20][21][29][30]. Other unknown factors are then required for the correct progression throughout meiotic prophase I up to the diplotene stage [29][30].…”

“…STRA8 protein induces the mitotic-meiotic switch and drives female PGCs into meiosis [20][21][29][30]. Other unknown factors are then required for the correct progression throughout meiotic prophase I up to the diplotene stage [29][30]. This article has been peer-reviewed and accepted for publication, but has yet to undergo copyediting and proof correction.…”