Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 169

Figure 3.39 A model of GH-mediated
regulation of postnatal longitudinal bone growth.
Abbreviations : ALS (acid labile subunit), an insulin-
like growth factor binding protein; GH, growth
hormone; GHRH, growth hormone-releasing
hormone; IGF-I, insulin-like growth factor;
IGFBP3, IGF binding protein3.
Source : From Ohlsson et al. (2009) .

Hypothalamus

GHRH

Pituitary

GH

Endocrine IGF-I
- Liver derived (75%)
- Nonliver derived (25%)
- ALS/IGFBP3

(i)

IGFI

(ii)

(iii)

Bone

IGF-I

2. An indirect pathway, where the axis stimulates the liver (and, to a lesser extent, other tis-
sues) to secrete IGF-1, which stimulates bone growth.
Hormonal regulation of bone development is performed by the hypothalamus
through three neuroendocrine axes: hypothalamic pituitary-gonadal (HPG), hypo-
thalamic-pituitary-thyroid (HPT), and hypothalamic-pituitary (HP) (via action of the
pituitary thyroid-stimulating hormone (TSH). Biologists marvel at the highly accurate
symmetrical development of long bones. For instance, Wolpert et al. (1998) state:

In view of the complexity of the growth plate, it is remarkable that human bones in
the limbs on either side of the body can grow for some 15 years independently of
each other, and yet eventually match to an accuracy of about 0.2%.

This precise symmetrical development implies that information on the develop-
ment of both limbs is communicated to a higher organismic structure able to assess
and compare limb sizes and send signals for their symmetrical development. It is the
continual afferent input from bones that enables the CNS to assess their state, and,
via the above-mentioned neuroendocrine axes HPG, HPT, and HP, send instructions
about the size of their development. Besides, the peripheral sensory innervation of
different limbs, which are connected via the dorsal root ganglia, represents another
neural control system ( Sample et al., 2010; Wu et al., 2009 ).

Neural Control of Bone Remodeling
Like all other somatic cells, osteoblasts and osteoclasts have a limited life span:
about 3 months for osteoblasts and about 2 weeks for osteoclasts. The T account is