Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 203

Experiments by Hegstrom et al. have shown that the innervation “knows” the
time when the muscle fiber will express the ECD receptor to become receptive to the
hormone.
FSH is the pituitary hormone induced by the hypothalamic neurohormone GnRH.
Although FSH via blood and other body fluids circulates throughout the body, its
action is restricted to a circumscribed group of granulosa cells in the ovarian follicle
only. The FSH action is restricted in only granulosa cells, because it is there that
the local sympathetic innervation releases the neurotransmitters norepinephrine (NE)
and vasoactive intestinal peptide (VIP), which induce expression of the FSH receptor
( Mayerhof et al., 1997 ). Then, granulosa cells transform androgens into estradiol that
stimulates the development of the ovarian follicle.
Besides the above mechanisms of inducing the expression of receptor molecules,
local innervation is involved in other ways of gene expression. This is experimen-
tally demonstrated not only directly but also indirectly in experiments of denervation
where it is observed that denervated tissues cannot express genes that they normally
do. A typical example is the development of the laryngeal muscle in the African
clawed frog, Xenopus laevis . In juvenile individuals of both sexes, the muscle is
similar, but after metamorphosis the muscle in males takes its male-characteristic
form, as a result of both the global action of androgens and the male-specific local
innervation:
[I]nnervation and androgen effects on fiber size are independent and additive. Fiber
number is regulated by the interaction of nerve and hormone.
Tobias et al. (1993)
The Source of Information for Selecting Sites
of Histone Modification
Chromosomes are transient structures that form during cell division apparently to
facilitate equal distribution of genes to daughter cells. Chromosomes are transcrip-
tionally inert because of their highly compact state. Chromatin organization of the
genome is required for gene expression. Switching from chromosomal to chromatin
structures after cell division, and the reverse, results primarily from the epigenetic
changes in the nucleosomal histone molecules ( Figure 4.2 ).
Chromatin structure is less compact and resembles loose threads of DNA and his-
tone proteins. Elementary structural units of chromatin are nucleosomes ( Figure 4.3 ).
Each nucleosome contains a DNA segment wound around an octamer consisting of
one pair of four core histone molecules, H2A, H2B, H3, and H4, creating the image
of a bead on a string. N-terminal tails of these histones may be acetylated, methyl-
ated, phosphorylated, and so on. On the outside, the nucleosome contains another
histone molecule, H1 ( LaVoie, 2005 ).
Chromatin appears in two main forms, heterochromatin and euchromatin. In
the first form, the DNA/genes are tightly packed as a result of methylation and