(apoptosis) are hypothesized mechanisms for the congenital dysmorphologies
associated with FAS.
PERIPHERAL NERVOUS SYSTEMSensory neurons of the spinal ganglia, as well as autonomic postganglionic
neurons and their supporting cells, are derived from neural crest.
Focal deficiencies in neural crest cell migration may result in lack of
innervation to specific organs or parts of organs. In Hirschsprung disease
(aganglionic megacolon), failure of neural crest cells to migrate to a portion
of the colon results in a localized deficiency in parasympathetic intramural
ganglia that may cause a loss of peristalsis and bowel obstruction.
DEVELOPMENT OF THE HEAD AND NECKThe cartilages and bones of the face (viscerocranium) develop from the pha-
ryngeal (branchial) arches.Each arch receives its blood supply from a spe-
cific aortic arch and its innervation from a specific cranial nerve (special or
branchial visceral efferent fibers). The third aortic archprovides most of the
adultblood supply to the head and neck.Theskeletal musclesof the head
and neck primarily arise from the pharyngeal arches and have a unique inner-
vation (special visceral efferent).
The face develops from a midline frontonasal prominenceand bilat-
eralmaxillary and mandibular prominences.Failure of the prominences
to fuse results in various facial clefts.
Teethoriginate from both ectodermal (enamel) and neurectodermal
(neural crest: dentin, pulp, cementum, and periodontal ligament) derivatives.
6 Anatomy, Histology, and Cell Biology
Pouch 1: Epithelial lining of middle ear
canals and tympanic membrane
Pouch 2: Epithelial lining of palatine tonsils
Pouch 3: Ventral portion: Epithelial components of thymus gland
Dorsal portion: Epithelial cells of inferior parathyroid glands
Pouch 4: Ventral portion: Epithelial “C” [parafollicular (interfollicular)
cells of the thyroid gland]
Dorsal portion: Epithelial cells of superior parathyroid glands
Clefts 1: External auditory canal
Clefts 2 →4: No derivativesDERIVATIVES OF PHARYNGEAL POUCHES AND CLEFTS