Neural crest cell migration

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Neural Crest Cell Migration

Neural crest cells (NCCs) are a transient, multipotent population that originates at the dorsal margin of the neural tube and undergoes a remarkable long-range migration throughout the embryo. Because of their pluripotency and wide-ranging migration, they are sometimes called "the fourth germ layer."

1. Origin and Induction

The neural crest arises at the lateral border (crest) of the neuroectoderm - the junction between the neural plate and surface ectoderm - during the third week of development.

Molecular induction requires a precise gradient of bone morphogenetic proteins (BMPs):

High BMP concentration → epidermal ectoderm
Intermediate BMP concentration (at the neural plate border) → neural crest induction
Low BMP concentration → neural plate (CNS)

Other signaling pathways critical for induction include Wnt/β-catenin (activates GBX2 homeobox gene), Notch, and FGF. Key transcription factors induced at this stage include FOXD3, SNAIL2 (Slug), SOX9, and SOX10.

(The Developing Human Clinically Oriented Embryology, p. 210)

Formation of neural crest from neural plate to neural tube, showing neural crest cells departing from the dorsolateral margins of the neural tube

Figure: Sequential formation of the neural crest - from notochord-induced neural plate, through neural folds and groove, to the closed neural tube from which neural crest cells delaminate.

2. Epithelial-to-Mesenchymal Transition (EMT)

Before migrating, NCCs must convert from epithelial to mesenchymal cells - a process triggered primarily by BMPs acting on neural epithelial cells:

BMPs induce expression of zinc finger transcription factors Snail, Slug, and Twist, which have a conserved role in promoting EMT.
These transcription factors upregulate cytoskeletal remodeling proteins and matrix metalloproteinases (MMPs) that degrade the basement membrane of the neural tube.
NCC expression of cadherins changes - cells loosen their adhesive contacts with the neural tube.
NCCs begin expressing integrins (receptors for laminins and collagens) to adhere to extracellular matrix along peripheral migratory routes.

"Neural crest cell migration does not rely on scaffolding (ie, radial glial cells or preexisting axon tracts) and thus is called free migration. This form of neuronal migration requires significant cytoarchitectural and cell adhesive changes and differs from most of the migratory events in the central nervous system." - Kandel's Principles of Neural Science, 6th Ed., p. 1188

3. Timing of Departure

Cranial (head) region: NCCs begin to migrate before the neural tube is completely closed.
Trunk region: Migration is delayed until after neural tube closure is complete.

(THIEME Atlas of Anatomy - General Anatomy)

4. Migratory Pathways

Neural crest cell migratory paths (A) and final positions of derivatives (B), with SEM of migrating cells

Figure 46-9 from Kandel's Principles of Neural Science: (A) Migratory paths of neural crest cells through the embryo, with SEM showing crest cells adjacent to neural tube and somites. (B) Final positions of major NCC derivatives - dorsal root ganglia, sympathetic ganglia, adrenal medulla, and melanocytes.

Trunk NCC Migration Routes (three main pathways):

Pathway	Route	Derivatives
Dorsal (dorsolateral)	Through the dermis, into skin via holes in basal lamina	Melanocytes (melanoblasts)
Ventrolateral	Through anterior half of somite	Sensory neurons of dorsal root ganglia
Ventral	Medial/ventral path, alongside dorsal aorta	Sympathetic chain ganglia, adrenal medulla chromaffin cells, enteric ganglia, preaortic ganglia

Somite Channeling - Segmentation

A key feature of trunk NCC migration is that cells pass through only the anterior (rostral) half of each somite, avoiding the posterior half. This segmented channeling is imposed by ephrin B proteins, concentrated in the posterior half of each somite. Ephrins act as repellant signals that interact with EphB class receptor tyrosine kinases on NCCs, blocking invasion of the posterior sclerotome. This creates the metameric (segmental) arrangement of the dorsal root and sympathetic ganglia.

(Langman's Medical Embryology)

Cross-sections and diagram showing NCC migratory paths: formation of dorsal root ganglia, sympathetic ganglia, suprarenal gland, preaortic and enteric ganglia

Figure 6.5 from Langman's Medical Embryology: (A-B) Cross-sections showing NCC delamination; (C) Final NCC-derived structures including sympathetic ganglion, developing suprarenal gland, preaortic ganglion, and enteric ganglia; (D) SEM showing neural crest cells on the dorsal surface of the closed neural tube.

5. Guidance Molecules

Several secreted factors promote and guide migration:

Factor	Role
BMPs (from dorsal aorta, later)	Promote EMT and continue to guide ventral migrants; induce Mash1 → sympathetic neuron fate
Ephrin B ligands	Repellant; confine migration to anterior somite halves (segmentation)
Wnt signals	From dorsal neural tube; induce Neurogenin → sensory neuron fate
FGF, TGF-β	Regulate proliferation, migration, and differentiation
Laminins/Collagens	ECM components in peripheral pathways; NCCs bind via integrins
Semaphorins/Neuropilins	Additional repulsive signals channeling cranial NCCs

(The Developing Human, p. 210; Kandel's Principles of Neural Science, p. 1188)

6. Fate Determination Along Migration Routes

Differentiation into specific derivatives depends on the combination of signals cells encounter along their journey and intrinsic predispositions that vary along the rostrocaudal axis:

Sensory neuron fate: Cells within the anterior sclerotome are exposed to Wnt from the dorsal neural tube/somites → express Neurogenin (bHLH TF) → sensory neurons. Further diversification yields nociceptive and proprioceptive subtypes.
Sympathetic neuron fate: Cells following a medial/ventral path are exposed to BMPs from the dorsal aorta → express Mash1 (bHLH TF) → sympathetic neurons.
Melanocyte fate: Cells taking the dorsolateral path into the epidermis → melanoblasts → melanocytes in skin and hair follicles.

7. Neural Crest Derivatives (Summary Table)

From Langman's Medical Embryology (Table 6.1):

Nervous system:

Spinal (dorsal root) ganglia
Sympathetic chain and preaortic ganglia
Parasympathetic ganglia of the GI tract (enteric ganglia)
Cranial nerve ganglia (CN V, VII, IX, X - partially)
Schwann cells and glial cells
Meninges (forebrain)
Adrenal medulla (chromaffin cells)

Craniofacial / skeletal:

Connective tissue and bones of the face and skull
Odontoblasts (dentin-forming cells)
Dermis in face and neck
Smooth muscle to blood vessels of the face and forebrain

Endocrine:

C cells (parafollicular cells) of the thyroid gland
Conotruncal septum in the heart

Pigment:

Melanocytes

8. Clinical Significance

Defective NCC migration or differentiation underlies a broad range of conditions ("neurocristopathies"):

Disease	NCC Defect
Hirschsprung disease	Failure of enteric NCCs to fully colonize the hindgut → aganglionic colon
DiGeorge syndrome (22q11.2 deletion)	Defective NCC migration into pharyngeal arches/pouches → absent thymus, parathyroids, conotruncal heart defects
Waardenburg syndrome	Mutations in SOX10, PAX3 → melanocyte/cochlear NCC defects → pigmentation + hearing loss
CHARGE syndrome	CHD7 mutations → NCC migration defects
Treacher Collins syndrome	Cranial NCC apoptosis → craniofacial defects
Melanoma, neuroblastoma, pheochromocytoma	NCC-derived tumors; tend to be highly malignant due to the intrinsic migratory/invasive capacity of NCC
Neurofibromatosis type 2 (NF2)	Transient change in NF2 promoter activity during NCC migration linked to Schwann cell tumors

(Fitzpatrick's Dermatology; Langman's Medical Embryology; THIEME Atlas)

"NCC are involved in at least one-third of all birth defects and many cancers such as melanomas, neuroblastomas, and others." - Langman's Medical Embryology

Sources: Kandel's Principles of Neural Science 6th Ed., p. 1188 | Langman's Medical Embryology | The Developing Human: Clinically Oriented Embryology | THIEME Atlas of Anatomy - General Anatomy | Fitzpatrick's Dermatology

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