Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 99

morphology of sexual organs, a decrease in neuropeptide Y (NPY) secretion releases
the central neurobiological brake. This, as well as the secretion of the hormone kiss-
peptin by other hypothalamic neurons, triggers the activation of the GnRH pulse
generator via the signal cascade.

Hypothalamic GnRH → pituitary FSH → ovarian E2.

This stimulates oogenesis and folliculogenesis. Development of ovarian follicles
is stimulated by neurally induced pituitary gonadotropins (FSH and LH). FSH stimu-
lates the development of the primary follicle into a secondary follicle, and the pitui-
tary LH induces the transformation of the secondary follicle into the tertiary follicle.
In the ovary, oogonia develop into primary oocytes, which arrest growth at the pro-
phase I of the cell cycle. The first meiotic division results in the production of a large
secondary oocyte and a small, first polar body ( Figure 2.26 ).
The second meiotic division of the oocyte produces a larger haploid mature egg,
and the second meiotic division of the polar body results in two polar bodies. The

Primary oocyte

Primary oocyte
(commencing
maturation)

Secondary
oocyte

First polar
body

Mature
ovum

Polar bodies

Figure 2.26 Diagram showing the reduction in the number of chromosomes during
maturation of the ovum.
Source : From http://commons.wikimedia.org/wiki/File:Gray5.svg.