Small RNAs in Germ Cell Development - Current Topics in Developmental Biology - page 82

differentiating ( Fig. 4.1 A(c); reviewed in Davies and Fuller, 2008 and Xie
et al ., 2008 ).
In the nematode C. elegans , the germ plasm is aggregated into the
“P-granules,” which are evenly distributed throughout the oocyte cyto-
plasm before and shortly after fertilization. During the first division, the
granules become localized to the posterior pole of the zygote and are then
asymmetrically segregated and stabilized during the following divisions,
such that the germ plasmmaterial is inherited by the P4 blastomere, defining
the single germline precursor ( Deppe et al ., 1978; Hird et al ., 1996; Seydoux
and Fire, 1994; Strome and Wood, 1982 ). The P4 blastomere gives rise to
the two PGCs, Z2 and Z3, that together with two somatic gonadal cells (Z1
and Z4) give rise to the gonad (reviewed in Kemphues and Strome, 1997;
Seydoux and Braun, 2006 ).
1.2.2. Germline development in zebrafish
The mechanisms responsible for PGC specification in zebrafish were
revealed when the first early molecular marker for these cells, the RNA
encoding for the Vasa protein, was identified ( Braat et al ., 1999; Yoon et al .,
1997 ). vasa mRNA, which resides within an electron-dense germ plasm,
localizes to the first cleavage planes in the early embryo ( Fig. 4.1 B(a)) and is
later incorporated into four cells. Cells that maintain the germ plasm in them
during subsequent cell divisions develop into PGCs ( Knaut et al ., 2000 ).
While the topology of germ plasm distribution differs, this scenario is similar
to that described for the invertebrate species discussed above, as well as for
cells located away from it further divide and differentiate. The somatic stem cells (SSCs)
give rise to the follicle cells that envelope the developing cyst. (B) PGC development in
the zebrafish. (B(a)) Maternally inherited germ plasm accumulates at the edges of the
first two cleavage furrows and is later incorporated into the four cells that become the
PGCs. (B(b and c)) After specification, the PGCs actively migrate toward the region
where the gonad develops, eventually forming two bilateral clusters on either sides of
the body, positions where they interact with somatic cells and differentiate into
gametes. (C) Germ cell development in mouse. (C(a)) Epiblast cells of the early
mouse embryo are induced by signals from the extraembryonic ectoderm (EXE) to
become progenitors of the PGCs (pPGCs) at E6.0, persistent signaling specifies the
PGCs at E7.25; TE (trophoectoderm), VE (visceral ectoderm), PE (primitive endo-
derm). (C(b)) In the females the oogonia give rise to the primary oocytes that are
arrested at meiosis I. Postnatally, the oocyte completes the meiosis, and dependent on
FSH signal gives rise to the secondary oocyte, which undergoes folliculogenesis and
becomes surrounded by the Zona pellucida (ZP), granulosa- and theca cells. Upon LH
stimulus, ovulation occurs and the somatic support cells differentiate to form the
Corpus luteum (Cl). In males, the spermatogonia divide postnatally into one daughter
cell retaining the spermatogonia cell fate, and another that differentiates into the
spermatocyte. Spermatocytes undergo meiosis and give rise to spermatids that interact
with the supporting sertoli cells facilitating their development into mature sperm.