During human embryological development and differentiation, primordial or rudimentary stages of the organ and peripheral systems are first formed. The ectoderm of the embryo gives rise to the epidermis, the central nervous system, the peripheral nervous system and several non-neuronal cells of the head and heart. Formation of the neural ectoderm occurs during the third week of gestation (Le Douarin & Kalcheim, 1999; Wilson & Hemmati-Brivanlou, 1997).
A segment of the dorsal ectoderm serves as the template of the neural ectoderm to form the region of the embryo known as the neural plate. This process of neural plate formation is usually called neural induction. After neural induction, three derivatives are then produced from the ectoderm from three independent processes (Liem et al. 1995; Le Douarin & Kalcheim, 1999). One process called neurulation leads to the formation of the neural tube which serves as the primordial of the central nervous system made up of the brain, spinal cord and neurons to name a few.
Another process leads to the formation of the outer ectoderm also referred to as ectodermis which later on differentiates to skin, hair, nails, mouth epithelium and related structures. The third process produces the neural crest which subsequently differentiates into the peripheral nervous system, adrenal medulla, melanocytes, facial cartilage and tooth dentine (Le Douarin & Kalcheim, 1999). The neural crest is an intermediate structure produced by the interaction between the neural plate and the surface ectoderm. The neural crest is sometimes considered as the fourth germ layer.
Neural crest cells delaminate from the dorsal neural tube when the dorsal tips of the neural folds join together. After delamination, these cells migrate from their origins in the neuraxis by traversing designating paths to settle on peripheral targets (Le Douarin & Kalcheim, 1999; Wilson & Hemmati-Brivanlou, 1997). Crest-derived cells have the potential of differentiating into many types of tissue cell types such as Schwann cells or glial cells, adrenal medullar cells, epidermal pigment cells and head connective tissue cells.
Schwann cells or glial cells make up sensory, sympathetic and parasympathetic systems as well as enteric nervous systems. Neural crest cells express only the characteristics that are suitable for the organ to which they have colonized (Knecht & Bronner-Fraser, 2002). The process of induction by the non-neural ectoderm on the neural plate lateral cells was shown to be facilitated by the Wnt gene family members, more specifically the Wnt-6.
Neural crest cells, upon induction, express slug, a transcription factor that controls delamination or dissociation of cells from the epithelial layer of the embryo. This transcription factor is hypothesized to activate factors that detach the taut connections between crest cells, permitting changes in the shape and other characteristics towards less of the neuroepithelial cells and more of mesenchymal cells (Le Douarin & Kalcheim, 1999). Neural crest cells in the head delaminate prior to the neural folds fusion.
Neural crest cells migrating during the epithelial-to-mesenchymal differentiation cease the synthesis of the cell surface adhesion molecule but synthesize it again during their aggregation for the spinal and sympathetic ganglia formation (Le Douarin & Kalcheim, 1999; Wilson & Hemmati-Brivanlou, 1997). Neural crest migration pathways are initiated from designated sites along the axis of the cranial-caudal region of the dorsal neutral tube referred to as the neuraxis. These migration routes guide the dividing cells derived from the neural crest starting from the origin into the targets.
After reaching the targets, crest-derived cells discontinue their division and initiate the expression of the phenotypes appropriate for the target. This means that the origin of the crest cell in the neuraxis dictates the target it will colonize (Knecht & Bronner-Fraser, 2002). During migration, crest-derived cells come across signaling molecules such as growth and trophic factors as well as fibronectin, laminin and collagen IV. These molecules enhance the continuous migration and proliferation of crest-derived cells.
Receptors that are needed for interaction with environmental conditions are developed during this migration. There is a significant increase in the number of crest-derived cells upon reaching their specific target (Liem et al. 1995; Le Douarin & Kalcheim, 1999). Cranial development traces its origin in the cranial neural crest. Crest-derived cells present in the head region form craniofacial mesenchyme which later differentiates into cartilage and bone tissues, neurons and glia of the cranial region and facial connective tissues.
Other crest-derived cells on the other hand follow routes along pharyngeal structures to form thymus connective tissue, tooth primordial odontoblasts, and middle ear and jaw bones. On the other hand, the crest-derived cells in the trunk region differentiate into the following derivatives: sympathic adrenal cells, trunk sensory neurons and glia and pigment cells by following three respective pathways (Le Douarin & Kalcheim, 1999; Wilson & Hemmati-Brivanlou, 1997). The vagal and sacral neural crests cell progenies form the enteric nervous system.
The pathway of the vagal crest-derived cells is along the length of the bowel. On the other hand, the pathway of the sacral crest-derived cells is through the post umbilical gut. Blockage or failure of the neural crest-derived cells to arrive on the colon from these sector leads to the absence of enteric ganglia which consequently leads to the lack of peristaltic movements in the concerned segment of the colon, a condition referred to as Hirschsprung’s disease(Le Douarin & Kalcheim, 1999).
Cardiac neural crest cells follow pathways through the pharyngeal arches 4 and 6 where they form the septum that divides the pulmonary artery from the aorta. Failure or blockage of the migration and colonization of cardiac neural crest-derived cells through this route leads to thymus, thyroid, parathyroid gland defects and a condition called common cardiac outflow tract (Osumi-Yamashita & Eto 1990; Le Douarin & Kalcheim, 1999).