Spatial distribution of postotic crest cells defines the head/trunk interface of the vertebrate body: embryological interpretation of peripheral nerve morphology and evolution of the vertebrate head
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The migration pathways and spatial distribution of neural crest cells largely depend on the embryonic architecture. At the preotic level in the chick embryo, cephalic crest adhere to even-numbered rhombomeres proximally, and populate each pharyngeal arch distally, thus prefiguring the morphology of the branchiomeric nerves. This distribution pattern is possible because of the absence of somites in the head. In the postotic region, however, somites and pharyngeal arches coexist at the same axial level. The caudalmost cephalic crest cell population, the circumpharyngeal crest cells, are derived from the postotic crest and their distribution covers the entire innervation areas of cranial nerves IX and X. In their proximal migration pathway, circumpharyngeal crest cells can exist along the dorsolateral pathway only where somites are absent. They divert around the occipital somites rostrally, making an arc that represents the caudal limit of the dorsolateral pathway of cephalic crest cells, or the head/trunk interface at the paraxial level. Ventrally, the circumpharyngeal crest cells localize in postotic pharyngeal arches as well as in an arc-shaped ridge, called the circumpharyngeal ridge. Since the circumpharyngeal ridge represents the caudal limit of the pharynx, it indicates the head/trunk interface at the level of the lateral body wall. These two interfaces of reverse orientation make an S-shaped, head/trunk interface together. Several structures unique to this region develop in this interface. Since the rhombomeric compartmentalization is distinct only in higher vertebrates, the rhombomere-dependent segregation of cephalic crest cells is more likely to be a secondary feature of the vertebrate head. The topographical configuration of the vertebrate crest cell distribution pattern does not support the idea that the vertebrate head evolved as a specialized trunk, but rather supports the idea that two distinct methods of segmental patterning have evolved in rostral and caudal parts of the vertebrate body, which resulted in the head and trunk, respectively. Postotic crest is located at the intermediate level between the trunk and the head, giving rise to both the cephalic and trunk crest cells. Its cephalic components circumpharyngeal crest cells, are distributed only rostral to the S-shaped interface.
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