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In-Depth Information
were independently investigating cell nucleus. In Belgium, Edouard van Beneden
(1846-1910) found that the egg and the sperm contribute equal numbers of chromo-
somes to the zygote. Around the same time, the young American physician Walter
Sutton (1877-1916) demonstrated that each of the gametes contributes half of the
normal set of chromosomes, leaving the zygote with the normal diploid set (
Galun,
2003
). After the rediscovery of Mendel's laws of inheritance (1900), Sutton, at the
University of Columbia, and Boveri, at the University of Würzburg, developed the
chromosome theory of heredity, in which chromosomes are carriers of the Mendelian
factors (
Anlagen
) (
Petronczki et al., 2003
).
Gametes are produced by specialized organs—the ovaries and testicles, respec-
tively. Egg cells are produced in the ovaries and sperm cells in the testicles. These
gametes are haploid (from ancient Greek απλός (haploos), single) cells (i.e., cells
containing only one of each pair of chromosomes and genes of the parent). Such
gametes have only half the normal set of chromosomes and are known as a
haploid
number of chromosomes
in order to stress the distinction from the diploid number of
chromosomes that is characteristic of somatic cells. Thus, parents contribute equally
to the development of the offspring.
The halving of the number of chromosomes in gametes is known as meiosis (from
ancient Greek μείον (meion), less, and from the proto-Indo-European root *
mei
,
small).
Production of germ cells, egg cells, and sperm cells represent the coronation of
the biphasic process of meiosis. Germ cells derive from a cluster of approximately
20 primordial germ cells (PGCs), which in mammals are specified by BLIMP1
(B-lymphocyte maturation-induced protein 1), early in the gastrulation process by
the embryonic day 7. BLIMP1 suppresses the somatic differentiation of PGCs by
repressing the premature expression of some germ cell-specific genes. Then PGCs
migrate and on reaching the genital ridge by the embryonic day 12 (E12), they
undergo epigenetic reprogramming, including the erasure of parental imprinting, as
may be concluded from demethylation of many loci. Now PGCs activate the mater-
nally inactivated X chromosome. At this early stage, when the sex of the embryo
is determined, the oogonia and the primary oocytes establish their own epigenetic
marks, especially DNA methylation.
Production of Egg Cells—Oogenesis
In vertebrates, PGCs migrate to reach the ovary, where they start dividing mitoti-
cally to form large numbers of oogonia. During the embryonic stage, oogonia enter
meiotic divisions to arrest the cycle at prophase I until at the outset of sexual matu-
rity. Inhibition of meiosis is primarily a result of the activation of gama-aminobutyric
acid or dopamine central neurobiological brake, which happens soon after birth, that
hold in check the secretion of GnRH by the GnRH pulse generator. Experimental
release of the brake enables the induction of meiotic divisions.
The GnRH pulse generator is a network of 3000-4000 hypothalamic neurons.
At the outset of puberty, coincident with substantial changes in the physiology and
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