Biology Reference
In-Depth Information
Figure 21 summarizes a generalized chronological sequence of gonadal
differentiation events in a teleost.
From relevant literature, Saito and Tanaka (2009) summarize a detailed
account on gonadal differentiation in the Japanese medaka
O. latipes
and
the same has been chosen as a model to present an overview. Manifestation
of Sertoli cells in males and granulosa cells in females is considered as
the 'starting point' for testicular and ovarian differentiation, respectively.
Sertoli cells are known to orchestrate development and morphogenesis of
testis, which then signals the sexual fate of the entire reproductive system
through production of hormones (Brennan and Capel, 2004).
Gonads: Gonadal development commences with the formation of sexually
undifferentiated primordium consisting of the germ cells namely Primordial
Germ Cells (PGCs) (Fig. 13) and supporting somatic cells. Differentiated
at the early blastula stage, PGCs migrate to colonize the gonadal anlage.
Then the PGC clusters are isolated from each other by one of the somatic
gonadal precursors expressing
sox9b
. Subsequently, a single gonadal
primordium consisting of the PGCs and somatic cells is separated into
right and left lobes. On the 4-5th day post-fertilization (dpf), a leaf-like
gonadal primordium is suspended at the mid-dorsal mysentery with
protruding right and left lobes (Hamaguchi, 1982). The number of PGCs
can be experimentally manipulated by disturbing the migration of PGCs
to the gonadal primordium (see also Table 22), which provides a useful sex
reversal model for understanding the role of PGCs in gonadal differentiation
(Saito and Tanaka, 2009).
Ovary: Following the two to four consecutive rounds of divisions, many
cysts each consisting of four, eight or 16 cells are formed in the female
embryo at 4-5 dpf (Saito et al., 2007). Hence the number of PGCs is larger
at hatching in females than in males, which is the fi rst known sign of
manifestation of ovarian differentiation. Subsequently, the germ cells
enter meiosis synchronously. Oogenesis persists throughout the gonadal
development and in the mature ovary (see Fig. 32).
The earliest manifestation of somatic cell differentiation is induced by the
female specifi c expression of
aromatase
gene at 1-2 weeks post-fertilization
(wpf) in the cluster of somatic cells located at ventral area of developing
gonad (Suzuki et al., 2004). These cells migrate dorsally and surround the
developing oocytes. They become morphologically recognizable by 3-4 wpf.
They are the precursors of the ovarian follicular granulosa cells, as detected
by
foxl2
, a granulosa marker gene (Nakamura S et al., 2008).
Testis: Genes, responsible for the earliest induction of gonadal differentiation,
have been identifi ed as
DMY/Dmrt1bY
in
O. latipes, tDmrt1
in
Oreochromis
niloticus
(see Fig. 13), and
amh
in
Danio rerio
. At the induction of one or the
