PGCs derive from polar plasm, which is assembled at the posterior of the oocyte during oogenesis.
The pathway of polar plasm assembly involves the initial localization of osk mRNA to the posterior pole of the oocyte.
How much Osk is present dictates the amount of polar plasm to be assembled and the number of PGCs formed (Ephrussi and Lehmann, 1992; Smith et al., 1992).
For example, Hsp83 RNA is a component of the posterior polar plasm [ 57], and Hsp90 protein is required for localization of maternal mRNA to the posterior pole, which is essential for development of germ cells in the Drosophila embryo [ 58].
Germ granules differ significantly in their morphology and functions in various species, for example, polar granules or pole plasm in D. melanogaster, C. elegans, X. laevis, nuage and sponge body in D. melanogaster, P granules in C. elegans, CB and intermitochondrial cement (IMC or pi-body) in mammals., CB is thought to be cognate to the germ cell specification structure, nuage, in Drosophila.
More recently, eiIF4a was identified as a new component of the polar granules in the germ plasm at the posterior of the oocyte (Thomson et al. 2008).
As part of the germ plasm, Oskar induces polar (or germ) granule formation and in doing so interacts with a number of genes that characterise these polar granules, in particular tudor (tud), vasa (vas) and valois (vls) [ 3, 103].
Germ plasm, or pole plasm, contains the polar granules which form during oogenesis and are required for germline development.
In Drosophila embryos germ cells are specified by the pole plasm, a specialized cytoplasmic region containing polar granules.
Germline cells among different species are characterized by the presence of a morphologically unique organelle called the germ plasm (also referred to as nuage, polar granules or mitochondrial cloud) [ 1, 2].
The establishment of transcriptional quiescence in fly PGCs is mediated by at least three maternally deposited pole plasm determinants, germ cell-less (gcl ), polar granule component (pgc ), and nanos (nos) (Asaoka et al. 1998, 1999; Deshpande et al. 1999, 2004; Leatherman et al. 2002; Martinho et al. 2004).
