Biology Reference
In-Depth Information
divisions and ultimately give rise to over 200 different types of cells
making up a vertebrate body. The unique features of ESCs namely self-
renewal and totipotency provide enormous opportunities to experiment
and understand the processes like sex determination and differentiation
in fi shes, although such experiments are yet to be undertaken. The ES-like
cell cultures have been established during recent years, e.g.,
O. latipes,
D. rerio, Sparus aurata, Lateolabrax japonicus
(see MeiSheng et al., 2010).
In India, ES-like stem cell cultures have been successfully developed
for the leopard danio
D. frankei
(Routray et al., 2010) and
Lates calcarifer
(Parameswaran et al., 2007). Incidently, Wu et al. (2011) have described
a unique epigenetic profile in zebrafish sperm, as has been reported
for the mammalian sperm. According to Carrell (2011), the profi le in
zebrafi sh sperm includes 'bivalent poising' of key developmental genes
of embryogenesis. The bivalency marks a mechanism for establishment
of totipotency in the germ cells and maintenance of pluripotency through
embryogenesis into the presumptive ES cells. Techniques for generation of
fertile diploid nuclear transplants have been successfully developed using
embryonic cells as donors (e.g., Wakamatsu et al., 2001).
Generation of ESCs is limited to embryos in mammals and other higher
vertebrates but the ESCs are generated in both embryos and adults of
fi shes or at least until the female reaches reproductive senescence. Hence
fi shes are unique among vertibrates and provide ample opportunity to
identify and isolate the ESCs as well as to manipulate them for a better
understanding of the sex differentiation process. Many molecular markers to
identify PGCs and trace their migration are known, although only very few
molecular markers are now available to identify and techniques to isolate
them (Table 22, see also Xu et al., 2010). Germline ovarian stem cells (OSCs)
have been identifi ed by Nakamura et al. (2011) in adult ovaries of
O. latipes.
In the 'germinal cradle' located in the stromal compartment of the ovary
(Fig. 33), three types of cells buried in the multi-layered epithelium have
been detected: 1. Singly isolated germ cells, 2. Cyst forming cells and 3.
Large oocytes in the meiotic diplotene stage—all of them surrendered by
cells expressing
sox9b.
But the exclusive expression of
nanos
2 is limited to
the singly isolated germ cells, which are the OSCs.
Germ cells: Primordial Germ Cells (PGCs): The PGCs are the progenitors
of germ cell lineage and have the ability to differentiate into either oogenia
or spermatogonia. As these gonial cells carry heritable information to the
next generation, they are considered 'immortal'. Shinomiya et al. (2002)
generated chimeras of
O. latipes
by transplanting male (X
1
Y
1
) and female
(X
1
X
2
) specifi c PGCs into blastulae committed to differentiate into X
1
X
2
and
X
1
Y
2
genotypes, respectively. From these experiments, they concluded that
it is the Germ Cell Supporting Somatic Cells (GCSSCs) that determine the
