Neural crest

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The neural crest, a transient component of the ectoderm, is found at in between the neural tube and the epidermis (or the free margins of the neural folds) of an embryo during neural tube formation. Neural crest cells quickly leave this during or shortly after neurulation.

It has been referred to as the fourth germ layer, due to its great importance. The neural crest can give rise to neurons and glia of the peripheral nervous system (PNS); some skeletal elements, tendons and smooth muscle; chondrocytes, osteocytes, melanocytes, chromaffin cells, and supporting cells and hormone producing cells in certain organs. Diseases due to defects in the neural crest induction, formation or migration are referred to as neurocristopathies, and genes that cause some of these like piebaldism and Hirschprung's disease have been cloned in mice models.


Induction

Cells fated to become neural crest tissue are induced by BMP, Wnt, and FGF signaling to express the proteins Fox3D], RhoB and Slug, and to lose expression of E-cadherin.

  • RhoB is likely to signal cytoskeletal changes required for migration.
  • Slug is a repressor that leads to an activation of factors that dissociate tight junctions.

Function

There are four main categories of neural crest based upon function:

Cranial neural crest

  • The cranial neural crest arises in the anterior and populates the face and the pharyngeal arches giving rise to bones, cartilage, nerves and connective tissue.
Other Migration Locations:
  • Into the pharyngeal arches and play an inductive in thymus development.
  • Into the pharyngeal arches and form the parafollicular cell or ultimobranchial bodies of the thyroid gland.
  • Into the pharyngeal arches and play an inductive role in parathyroid gland development.
  • Facial ectomesenchyme of the pharyngeal arches forming skeletal muscle, bone, and cartilage in the face.
  • Odontoblasts (dentin-producing cells) of the teeth.
  • Into the optic vesicle and the developing eye and contributes to many anterior eye elements such the cornea, sclera, and ciliary muscle. It also contributes to the attaching skeletal muscles of the eye.
  • Into the otic placode and participates in the inner ear development.
  • Sensory ganglia of the fifth, seventh, ninth and tenth cranial nerves.

Vagal and sacral neural crest

  • The vagal and sacral neural crest arises in the neck and tail and populates the gut, forming the parasympathetic neurons that regulates peristalsis and control blood vessel dilation.
Other Migration Locations:

Trunk neural crest

  • The trunk neural crest lies between the vagal and sacral neural crest and gives rise to two groups of cells. One group migrates dorsolateral and populates the skin, forming pigment cells and the other migrates ventrolateral through the anterior sclerotome to become the epinephrine-producing cells of the adrenal gland and the neurons of the sympathetic nervous system. Some cells remain in the sclerotome to form the dorsal root ganglia
Other Migration Locations:

Cardiac neural crest

  • The cardiac neural crest overlaps the vagal neural crest and migrates to populate the pharyngeal arches 3, 4 and 6 (producing structures in the head) and to the heart, forming connective tissue that separates the great vessels of the heart.
Other Migration Locations:

Migration

Neural crest cells require extracellular matrix to migrate through interactions between integrins and fibronectin and laminin. Migration is directed by inhibitory and attractive signals from cells. Ephrin is an inhibitory ligand in posterior sclerotome that affects ventral pathway trunk neural crest cells and causes them to migrate through the anterior sclerotome instead. Thrombospondin promotes migration through the anterior sclerotome. Another signal, stem cell factor is involved in specifying the destination of migration. If expressed in the wrong locations, pigment cells migrate to that site and proliferate there.

Plasticity

Neural crest cells show varying degrees of plasticity. Some trunk neural crest cells are pluripotent. Cranial neural crest cells can give rise to trunk neural crest cells if transplanted. However, heart neural crest cells are committed before migration. Individual neural crest cells can take on a new fate, however groups of neural crest cells cannot.


References

  • Anderson RB, Newgreen DF, Young HM. "Neural crest and the development of the enteric nervous system." Adv. Exp. Med. Biol. 2006;589:181-96.
  • Basch ML, Bronner-Fraser M. "Neural crest inducing signals." Adv. Exp. Med. Biol. 2006;589:24-31.
  • Hox domains in chicks
  • Diagram at umich.edu
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