Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 168

Interparietal

Paraxial
mesoderm

Supra-
occipital

Parietal

Frontal

Neural
crest

Exoccipital
Stapes
Malleus
Incus
Tympanic
bulla

Petrosal

Alisphen-
oid

Lac

Orbitosphenoid

Lateral
mesoderm

Pre
sphenoid

Ethmoid

Basioccipital

Sqm.
temporal

Vomer

Ptr

Maxilla

Palatine

Hyobranchials
Thyroid
Arytenoid
Cricoid

Pre
maxilla

Jugal

Mandible

Basihyoid

Figure 3.38 Schematic mouse skull showing the contributions of neural crest and paraxial
and lateral mesoderms to the cranial skeleton. Abbreviations : Lac, lacrimal; Ptr, pterygoid.
Source : From Noden and Trainor (2005) .

cartilages and bones of the craniofacial region and entire the pharyngeal skeleton,
including jaws, hyoid, and gill structures ( Knight and Schilling, 2006 ) ( Figure 3.38 ).
The cranial crest takes over the functions of the mesoderm in this region, and its
cells form skeletogenic cell types when they reach the presumptive bone regions.
The neural crest-derived mesenchyme is the source of bone-patterning informa-
tion; it has an instructive function in shaping the skull, while the local epithelium
“tends to play a more permissive role, heeding instructions from mesenchyme”
( Jheon and Schneider, 2009 ). Before leaving the neural tube, NCCs are supplied with
the migration program and the patterning information they enact in target sites:

… the proper program of events governing the migration of crest may need first
to be established in the hindbrain, to allow migratory crest cells to interpret and
respond to environmental signals set up through a series of tissue interactions [ital-
ics not in original].

Trainor et al. (2002)

In the above context, remember that the evolutionary precursor of the neural crest
is the epidermal nerve plexus of protochordates ( Gans and Northcutt, 1983 ) and in
protochordates, the neural tube seems to have played a neural crest-like function
( Meulemans and Bronner-Fraser, 2007 ).
During postnatal growth as well, bone development is under neural control via
two pathways ( Figure 3.39 ):

1. A direct pathway via the hypothalamic-pituitary axis, where release by specialized hypo-
thalamic GHRH neurons induces pituitary secretion of the growth hormone, which stimu-
lates bones to secrete IGF-1.