Small RNAs in Germ Cell Development - Current Topics in Developmental Biology - page 80

1.1. PGC specification: Preformation/epigenesis
The establishment of the germline is achieved by two different strategies.
Specification of germ cells is directed either by localized maternally
provided determinants in a process called “preformation” or epigenetically
by inductive signals emanating from surrounding tissues.
Many animals such as Caenorhabditis elegans , Drosophila melanogaster ,
Xenopus laevis , and Danio rerio employ germ plasm for specifying their
germline. The maternally provided germ plasm is composed of specific
RNAs and proteins that when incorporated into cells directs them to
develop as germ cells ( Fig. 4.1 A(a and b) and B(a and b)). In contrast to
this strategy, in other organisms (e.g., mammalians and Urodele amphi-
bians) germ cell progenitors are specified in an inductive process. Here,
signaling cues from surrounding tissues, involving proteins of the BMP
family, specify the germ cells ( Fig. 4.1 C(a); Lawson et al ., 1999; Ying and
Zhao, 2001; Ying et al ., 2001 ; for review, see Extavour and Akam, 2003 ).
A more detailed description of germ cell development and the factors
involved is provided in the following section.
1.2. Germline development in different model organisms
1.2.1. Germline development in the fly and the nematode
Cells forming at the posterior pole of the early D. melanogaster embryo (pole
cells) harbor the maternally provided pole plasm which directs them to
become progenitors of the germline ( Fig. 4.1 A(a and b); Technau and
Campos-Ortega, 1986 ). During embryonic development, a combination
of active and passive cell movements bring the PGCs to the region where
the gonad develops and where they associate with somatic gonad precursors,
reviewed in Richardson and Lehmann (2010) . During later stages of gonad
formation, GSCs are located in the “niche,” a position where they are
maintained as pluripotent cells by different signaling events that control
self-renewal and differentiation ( Kiger et al ., 2000; Tran et al ., 2000; Xie and
Spradling, 1998 ). One of the better-characterized signals controlling the
balance between maintenance of pluripotency and differentiation is the
BMP homologue Decapentaplegic (Dpp), constituting a key signal for
female GSC maintenance (Xie and Spradling, 1998, 2000) . The niche is
called “nest” in female, consisting of two GSCs associated with five or six
cap cells, which are part of the terminal filament ( Fig.4.1 A(c); Sahut-
Barnola et al ., 1995 ). In male gonads, the niche is referred to as “hub” and
consists of 5-9 GSCs and 10-18 somatic cyst progenitor stem cells ( Hardy
et al ., 1979 ). The early processes of gametogenesis in both the ovaries and
testes are very similar: a GSC divides, giving rise to two daughter cells, one
that remains in the niche and maintains a GSC fate, while the other becomes
located away from the niche and from the “stemness” signals and thus starts