The Questions of Developmental Biology

Snapshot Summary: Neural Crest Cells and Axonal Specificity
1. The neural crest is a transitory structure. Its cells migrate to become numerous different cell
types.
2. Trunk neural crest cells can migrate dorsolaterally into the ectoderm, where they become
melanocytes. They can also migrate ventrally, to become sympathetic and parasympathetic
neurons and adrenal medulla cells.
3. A portion of the anterior trunk neural crest enters the heart and forms the separation between
the pulmonary artery and aorta.
4. The cranial neural crest cells enter the pharyngeal arches to become the cartilage of the jaw and
the bones of the middle ear. They also form the bones of the frontonasal process, the papillae of
the teeth, and the cranial nerves.
5. The formation of the neural crest depends on interactions between the prospective epidermis
and the neural plate. Paracrine factors from these regions induce the formation of transcription
factors that enable neural crest cells to emigrate.
6. The path a neural crest cell takes depends on the extracellular matrix it meets.
7. Trunk neural crest cells will migrate through the anterior portion of each somite, but not
through the posterior portion of a somite. Ephrin proteins are expressed in the posterior portion of
each somite and appear to prevent neural crest cell migration.
8. Some neural crest cells appear to be capable of forming large repertoire of cell types. Other
neural crest cells may be committed to a fate even before migrating. The final destination of the
neural crest cell can sometimes change the specification of the neural crest cell.
9. The fates of the cranial neural crest cells are to a great extent controlled by the Hox genes.
10. Teeth develop through an elaborate dialogue between the neural crest-derived mesenchyme
and the jaw epithelium. The mesenchyme becomes the odontoblasts, while the epithelium
generates the ameloblasts.
11. The major signaling center of the tooth is the enamel knot. It secretes several paracrine factors
that regulate cell proliferation and differentiation in both the mesencyme and epithelium.
12. The specification of the motor neurons is done according to their place in the neural tube. The
LIM family of transcription factors plays an important role in this specification.
13. Targets of the motor neurons are specified before the motor neurons extend into the
periphery.
14. The growth cone is the locomotor organelle of the neuron, and it senses the environmental
cues. (It has been called a "neural crest cell on a leash" because the growth cone and the neural
crest cell both are migratory and sense the environment.)
15. Axons can find their targets without neuronal activity.