Human germ cell development is poorly understood with most of our knowledge derived by extrapolation of studies in mice. Nonetheless, although human germ cell development undoubtedly shares similarity to that of the mouse given their similarities in embryology, it is also clear that genetic requirements for human germ cell development are unique. This is illustrated by a number of observations. First, there are several Y chromosome genes, including the DAZ
genes, that are absent in mice (26
). Thus, understanding their role and that of related homologs, such as DAZL
whose functions appear to overlap at least minimally, is difficult (or impossible) on a genetic background that lacks key regulators. Second, similarly in the case of X chromosome, women require two X chromosomes for oocyte development, whereas mice are fertile with just a single X (30
). Third, it has been observed that reproductive genes and proteins may evolve rapidly (33
). Indeed, the human genes, STELLAR
, encode examples of extremely divergent proteins that are expressed in germ cells. Mouse and human Stella
homologs are just 30% identical at the amino acid level and have distinct differences in expression (14
). Fourth, it is clear that humans are remarkably infertile compared with other species, with nearly half of the infertility cases linked to faulty germ cell development (36
). Finally, humans are remarkably imprecise in carrying out some key aspects of germ cell development that are reportedly the most highly conserved. For example, meiotic chromosome missegregation is rare in most model organisms. In the common yeast, chromosome missegregation occurs in ~1/10 000 cells. In flies, missegregation occurs in 1/1000 to 1/2000 cells and in mice in ≤1/100. In humans, meiotic chromosome missegregation occurs in 5–20% or more of cells depending on sex and age (37
). Thus, there are fundamental differences in the genetics, biology and pathology of human germ cell development, compared with model organisms, that merit studies of human germ cell development per se
. Yet, given the timeline of human germ cell development in vivo
in the first trimester, it has not been possible to probe gene function on a human genome background.
Recently, the differentiation of hESCs has been used to probe gene function in human germ cell development (15
). In these studies, human germ cell development was assessed following silencing and overexpressing of genes of the DAZ
gene family that encode germ cell-specific cytoplasmic RNA-binding proteins (not transcription factors). Results indicated that human germ cell formation and developmental progression could be modulated by the DAZ
gene family with human DAZL
shown to function in PGC formation, whereas the Y chromosome homolog, DAZ
, and closely related autosomal homology, BOULE
, promoted later stages of meiosis and development of haploid gametes. In spite of these successes, however, further genetic analysis is clearly justified; the use of iPSCs would allow us to take advantage of naturally occurring human genetic variants, including complex deletions and rearrangement, for further analysis.
In this study, we show that human adult and fetal somatic cell-derived iPSC lines can differentiate to PGCs in a similar manner to hESCs, with some differences noted. In addition, we observed that like hESCs, germ cells differentiated from iPSCs entered meiosis, a functional marker of germ cell formation and differentiation, when DAZ family proteins were overexpressed. Furthermore, data indicate that the iPSC lines can differentiate to haploid cells with characteristic staining of ACROSIN for spermatid. With these results, we suggest that iPSCs may provide a useful platform for the study of human germ cell development and infertility defects. Indeed, our data indicate that germ cell differentiation may occur more spontaneously in iPSCs than in hESCs. This may be linked to the process of reprogramming, enhanced expression of pluripotency markers or a preferential differentiation to germline rather than somatic cell derivatives.
Finally, we note that although a major cause of infertility is the production of few or no germ cells, often associated with meiotic defects, today's treatments for infertility are largely ineffective for those with few or no germ cells (38
). We envision that the production of germ cells from iPSCs may enable direct screening and assay not only of genetic factors but also for chemicals or small molecules that promote germ cell survival or demise. Ultimately, this may contribute to new strategies for the diagnosis and treatment of infertility, a common health problem that affects 10–15% of reproductive-age couples.