In 1987, Nichols described in a few steps what actually happens to the neural crest cells before they migrate down different pathways:

1. Neural crest cells elongate and their organelles migrate closer to the basal region of the epithelium.
2. Cell to cell contact is lost and the cells begin to take on mesenchymal characteristics.
3. The basal cells release processes that penetrate the basal lamina that in effect degrades allowing the basal cells to finally break free and become mesenchymal.
4. The apical cells then form a new basal lamina.

Migratory Pathways

Weston identified a dorsolateral and a ventral population pathway. The pathways that he described are:

1. Along the dorsal trunk epidermal ectoderm (superficially)
2. More medially, along the lateral edges of the somites
3. Between somites
4. Through the anterior part of the somites (the posterior part contains chondroitin sulfate which neural crest cells can not travel along) and into the somitomeres
5. Between the neural tube and the somites

These cells go on to form pigment cells (melanocytes), spinal ganglia, and the sympathetic neurons.

The overall migration can be summarized by three pathways- a ventral pathway, a lateral pathway, and a dorsal route. Cells that go around the notochord and neural tube characterize the ventral pathway. Cells that go under the ectoderm and follow a superficial pathway characterize a lateral pathway. The dorsal route is across the caudal two thirds of each somite into a dorsal fin. In crest cell migration in humans, the cells migrate earlier when the neural folds have still not closed off to form the neural tube. In amphibians and birds, on the other hand, the migration is later when the neural folds have closed off to form the neural tube.

Patterns of Neural Crest Migration

Although migratory patterns of neural crest cells are not intrinsic, patterns emerge in cells that arise from a single rostral-caudal level of the neural crest but are destined to form several types of cells. For example:

1. Precursors of the dorsal root ganglia remain at the same rostro-caudal level of the neural tube from which they emerge. Therefore, the cells at a certain level will contribute to the dorsal root ganglion of that level.
2. Neurons that make up an individual sympathetic ganglion migrate two or three somite lengths rostrally and caudally, respectively
3. .Melanocytes, on the other hand, can migrate long distances along the embryo's axis.

Cessation of Migration

Cessation of migration is not as simple as the cells just hitting a barrier and stopping; it is more complex than that. Some such cessation methods involve increasing the production of chondroitin sulfate and decreasing the amount of collagen and other substrates. Other parts of the embryo secrete substances that make migration difficult. Migrating neural crest cells influence the patterning of other cell types.

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