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/

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 p roximally, and populate each pharyngeal arch distally, thus prefigurin g the morphology of the branchiomeric nerves. This distribution patter n is possible because of the absence of somites in the head. In the po stotic region, however, somites and pharyngeal arches coexist at the s ame axial level. The caudalmost cephalic crest cell population, the ci rcumpharyngeal crest cells, are derived from the postotic crest and th eir 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 are that represents the caudal limit of the dorsolateral pathway of c ephalic crest cells, or the head/trunk interface at the paraxial level . Ventrally, the circumpharyngeal crest cells localize in postotic pha ryngeal arches as well as in an are-shaped ridge, called the circumpha ryngeal ridge. Since the circumpharyngeal ridge represents the caudal limit of the pharynx, it indicates the head/trunk interface at the lev el of the lateral body wall. These two interfaces of reverse orientati on make an S-shaped, head/trunk interface together. Several structures unique to this region develop in this interface. Since the rhombomeri c compartmentalization is distinct only in higher vertebrates, the rho mbomere-dependent segregation of cephalic crest cells is more likely t o be a secondary feature of the vertebrate head. The topographical con figuration of the vertebrate crest cell distribution pattern does not support the idea that the vertebrate head evolved as a specialized tru nk, but rather supports the idea that two distinct methods of segmenta l patterning have evolved in rostral and caudal parts of the vertebrat e body, which resulted in the head and trunk, respectively. Postotic c rest is located at the intermediate level between the trunk and the he ad, giving rise to both the cephalic and trunk crest cells. Its cephal ic components circumpharyngeal crest cells, are distributed only rostr al to the S-shaped interface.