Primordial Germ Cell Specification and Migration

Abstract

Primordial germ cells are the progenitor cells that give rise to the gametes. In some animals, the germline is induced by zygotic transcription factors, whereas in others, primordial germ cell specification occurs via inheritance of maternally provided gene products known as germ plasm. Once specified, the primordial germ cells of some animals must acquire motility and migrate to the gonad in order to survive. In all animals examined, perinuclear structures called germ granules form within germ cells. This review focuses on some of the recent studies, conducted by several groups using diverse systems, from invertebrates to vertebrates, which have provided mechanistic insight into the molecular regulation of germ cell specification and migration.

Causes and evolutionary consequences of primordial germ-cell specification mode in metazoans

“…Furthermore, the biological properties of germ plasm differ in some respects across taxa: Germ plasm can originate as a molecular accumulation in the oocytes before fertilization (D. melanogaster, D. rerio, Gallus gallus, and X. laevis) or clustering and concentration of P granules in the early embryos (C. elegans), and each of these mechanisms will ultimately drive formation of PGCs in early embryogenesis (92).…”

“…The length, number of cell divisions, and cellular behaviors during pre-PGC and PGC stages differ between induction and inheritance modes (2,15,19), and thus the rate of mutation might vary among modes. Furthermore, under inheritance, the PGCs typically form earlier (blastoderm stage) in embryogenesis than under induction (gastrulation), and thus may be mitotically and transcriptionally quiescent for an extended period after their specification (15,17,18,92), possibly favoring a lower mutation rate per generation. Some available mutation rate data indicate that the induction taxa mice and humans exhibit a higher germline mutation rate [∼38.00 (note: male germ line) and 12.85 × 10 −9 per site per generation, respectively] (101) than the inheritance invertebrates C. elegans (5.60 × 10 −9 ) (101) and D. melanogaster (4.65 × 10 −9 ) (101,108,109) [also see other factors, such as population size (101)].…”

Mitochondrial matters: Mitochondrial bottlenecks, self-assembling structures, and entrapment in the female germline

“…In all animals, mitochondria and other organelles are asymmetrically enriched within oocytes (reviewed in [16–18]). In animals that use maternal inheritance of germ plasm (the substance that contains determinants of the germline of the next generation) to specify the germline, mitochondria are enriched in areas where the germ plasm (the components that will specify the germ cells of the next generation) localizes [19] and reviewed in [17, 20]. Thus, subcellular enrichment of mitochondria has led to models whereby the oocyte selects the healthiest mitochondria for the next generation, by mechanisms that remain to be understood.…”

Zebrafishdazlregulates cystogenesis upstream of the meiotic transition and germline stem cell specification and independent of meiotic checkpoints

“…In many organisms the germline is among the first cell types to be set aside during early development (Ginsburg, 1994;Illmensee and Mahowald, 1976;Illmensee et al, 1976;Wolf et al, 1983). Early germ cells, called primordial germ cells (PGCs), can be specified by maternal factors or by inductive signals (Farrell et al, 2018;Lawson and Hage, 1994;Marlow, 2015;Nieuwkoop and Sutasurya, 1979). Once specified, PGCs ignore somatic differentiation programs and migrate to the site where the gonad anlage forms (Braat et al, 1999a;Gross-Thebing et al, 2017;Nieuwkoop and Sutasurya, 1979;Strome and Updike, 2015).…”

“…There the PGCs proliferate and eventually enter meiosis and differentiate to produce the gametes, sperm in males and oocytes in females. Although the earliest stages of PGC development in zebrafish have been extensively studied (Barton et al, 2016;Marlow, 2015;Paksa and Raz, 2015;Raz, 2003), the cellular events and molecular mechanisms involved in the transition from PGC to germline stem cell (GSC) and the earliest phases of gametogenesis are less well understood.…”