Primordial germ cells in mice

Abstract

Germ cell development creates totipotency through genetic as well as epigenetic regulation of the genome function. Primordial germ cells (PGCs) are the first germ cell population established during development and are immediate precursors for both the oocytes and spermatogonia. We here summarize recent findings regarding the mechanism of PGC development in mice. We focus on the transcriptional and signaling mechanism for PGC specification, potential pluripotency, and epigenetic reprogramming in PGCs and strategies for the reconstitution of germ cell development using pluripotent stem cells in culture. Continued studies on germ cell development may lead to the generation of totipotency in vitro, which should have a profound influence on biological science as well as on medicine.

Figure 1.

A schematic representation of germ cell development in mice. A brief outline of germ cell development in mice is shown schematically. Key events associated with each stage of germ cell development are also shown. (Left) PGC development (from E6.25 to ∼E12.5); (upper right) male germ cell development (from ∼E13.5); (lower right) female germ cell development (from ∼E13.5).

Figure 2.

A schematic representation of PGC development in mice. A brief outline of PGC specification, migration, and colonization of the gonads in mice is shown schematically. (Green) Blimp1- and Prdm14-positive cells that appear in the most proximal epiblasts. (Lower middle panel) A view of the transverse section of the position indicated by a red bar in the lower left panel. Epi, epiblast; (AVE, anterior visceral endoderm; ExE, extra-embryonic ectoderm; PGCs, primordial germ cells; EM, embryonic mesoderm; ExM, extra-embryonic mesoderm; Hg, hindgut; DA, dorsal aorta; NT, neural tube; NoC, notochord; IM, intermediate mesoderm; NeC, nephrogenic cord.

Figure 3.

Transcriptional regulation of PGC specification. A potential genetic network for PGC specification is shown schematically. Arrows and lines with terminal bars indicate genetic pathways for activation and for repression, respectively, as demonstrated by in vivo experiments (Kurimoto et al. 2008; Yamaji et al. 2008). Dotted arrows and dotted lines with terminal bars indicate genetic pathways for activation and for repression, respectively, as proposed based on in vitro experiments (Covello et al. 2006; West et al. 2009; Weber et al. 2010).

Figure 4.

The generation of PGC-like cells from ESCs/iPSCs in culture. A schematic representation of the generation of PGC-like cells (PGCLCs) from ESCs/iPSCs in culture (Hayashi et al. 2011). ESCs/iPSCs are induced into epiblast-like cells (EpiLCs) by the stimulation with ActivinA, bFGF, and KSR (knockout serum replacement) for 2 d. EpiLCs (about 1000 cells) are aggregated in a floating condition and are induced into PGCLCs by the stimulation with BMP4, BMP8b, SCF (stem cell factor), LIF (leukemia inhibitory factor), and EGF (epidermal growth factor). Blimp1-positive PGCLCs (green) are induced and form clusters at peripheries of the aggregate. PGCLCs are sorted by FACS (fluorescence activated cell sorting) according to their expression of Blimp1 transgene, or of surface markers, SSEA1 (stage-specific embryonic antigen 1), and Integrinβ3. PGCLCs are transplanted into neonatal testes lacking endogenous germ cells, leading to proper spermatogenesis. The ESC/iPSC-derived sperm are injected into oocytes by ICSI (intracytoplasmic sperm injection), and the resultant two-cell embryos are transferred to pseudopregnant females, resulting in the generation of fertile offspring.

Figure 5.

Regulatory network for EGC derivation from PGCs. Key signaling mechanisms (A) and events for the first 10 d during the derivation of EGCs from PGCs (B) are shown schematically. (A) Arrows indicate activation, whereas blunted arrows show repression. (B, upper panel) The requirement of the signaling molecules; (lower panel) gene expression dynamics.