Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 167

embryos. The intensity of the c-Fos expression in neurons was correlated with tem-
perature changes ( Jiménez-Trejo et al., 2011 ). In many reptiles, the perception in
the brain (thermosensitive neurons in the preoptic area) of the environmental tem-
perature leads to the expression of aromatase ( Milnes et al., 2002; Willingham et al.,
2000 ), which converts androgens into estrogen, determining the female sex when the
embryo is genetically male.
It is noteworthy that in human embryos as well, bundles of autonomic nerve fib-
ers arrive at the gonadal ridge 29-33 days after coitus, before sexual differentiation
into the ovaries or testicles occurs ( Møllgård et al., 2010 ).
New evidence shows that the sexual differentiation of the genital ridge is under
brain control in birds as well; the sexually dimorphic expression of genes in the
embryonic chick brain begins before the gonadal differentiation ( Lee et al., 2009 ).
Moreover, the effects of the temperature of incubation on the sex of chick embryos
are transmitted to the next generation ( Yılmaz et al., 2011 ). In the above-mentioned
cases, the temperature seems to trigger a neural mechanism that overrides the chro-
mosomal, or genetic, sex determination.
The role of the neural mechanisms in sex determination finds overwhelming sup-
port from numerous studies on the role of social factors in sex conversion in fish,
which have led investigators to conclude the existence of a central nervous mech-
anism in the hypothalamus that is responsible for “the induction of the dramatic
gonadal and behavioral transformations that are associated with sex change in her-
maphroditic fish” ( Elofsson et al., 1997b ).
These same studies also lead to the conclusion on the primacy of changes in the
brain over gonadal changes:
The initiation of the sex reversal is often controlled by social, behavioural factors, and
since the only way behaviour can affect gonads is through the brain, here must be a
central neuronal mechanism underlying the gonadal change [italics not in original].
Elofssson et al. (1997a)
Osteogenesis—Formation of Skeletal Bones
Central to the development of the skeleton in animals are three cell types: chondro-
cytes, the cell component of cartilage; osteoblasts, which are responsible for the
deposition of the bone matrix; and osteoclasts, which specialize in bone resorption
( Erlebacher et al., 1995 ). The first two come from a common progenitior cell, while
osteoclasts derive from hematopoietic stem cells.
The evolution of bones from cartilaginous structures is an innovation associated
with the appearance of vertebrates. It coincided with another crucial evolutionary
innovation of vertebrates: appearance of the neural crest.
First, let us focus on the development of the craniofacial bones because its mecha-
nism is better known. Cranial NCCs delaminate from the neural tube and migrate
to the pharyngeal arches to transform themselves and the local cell populations into