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surgical procedures. In addition to vasa, other conserved germ line genes between
invertebrates and Danio were subsequently identified including nanos1 and dazl
(boule in Drosophila) (Koprunner et al. 2001 ; Hashimoto et al. 2004 ).
The most significant biological processes that distinguish germ cell development
are early segregation of the germ cells precursors (pPGCs) from somatic cell lin-
eages at extragonadal locations, migration of PGCs to the developing gonadal pri-
mordium and finally, differentiation into germ cells influenced by the somatic
gonadal environment (Braat et al. 1999c). Across different species, PGCs are mor-
phologically very similar. Ultrastructural analysis shows the presence of RNAs and
protein aggregates that appear as discrete, electron dense cytoplasmic inclusions, in
close association with mitochondria, endoplasmic reticulum, and Golgi (Eddy
1975 ). In Danio rerio, germ plasm assembly starts before the end of oogenesis
(Olsen et al. 1997 ). Many of the components that constitute the germ plasm and
specify germ cell identity are conserved evolutionarily (Table 8.1). PGCs lineage
separates from somatic cell lineages early in development and display a unique
regulation over their basic cell functions. These include a selective responsiveness
to cell signaling, RNA stabilization, and posttranscriptional regulation. In this way,
PGCs avoid being differentiated into somatic cells and thus preserve the capacity to
give rise to the next generation. It follows that the major goals of research into the
germ line are to understand the mechanisms that allow PGCs formation, directional
migration, and differentiation into mature gametes within the gonads.
8.2.2 Germ Cell Development in Zebrafish
Danio rerio belongs to the class Pisces and displays partial or meroblastic cleavage.
The blastodisc undergoes division as it forms on top of a large yolky mass which
does not divide (Gilbert 2006 ). The precursor of primordial germ cells (pPGC)
refers to blastomeres containing germ plasm that, during early cleavages, divide
unequally as stem cells. One daughter cell will receive germ plasm and maintain the
germ stem cell character while the other blastomere will enter the somatic lineages.
PGCs correspond to the founders of the germ line that will give rise to two daughter
cells, both of them contributing to the germ cell lineage. The term germ cell (GC) is
reserved for the stem cells of the germ line that reside in the gonad producing cells
that will eventually give rise to gametes (Braat et al. 1999a).
Oogenesis: The process of oogenesis has been divided into four different stages
as described by Selman et al. ( 1993 ). The earliest stage Ia oocyte appears as a
transparent ball of cytoplasm surrounding a central germinal vesicle (GV) with no
yolk and forming nests of interconnected oocytes. At Stage Ib the germ plasm
aggregates to form the Balbiani body. In situ hybridization analysis shows that the
germ plasm mRNAs vasa, nanos1, and dazl are localized to the Bb (Braat et al.
1999c; Kosaka et al. 2007 ). One important feature at this stage is that the axis
marked by the Bb and nucleus (germinal vesicle) is the first indicator of the animal–
vegetal polarity that will determine the anterior–posterior axis later on during
T. Aguero et al.