The avian spinal cord is characterized by an absence of motor nerves and sensory 
nerves and ganglia at its caudalmost part. Since peripheral sensory neurons 
derive from neural crest cells, three basic mechanisms could account for this 
feature: (i) the caudalmost neural tube does not generate any neural crest 
cells; (ii) neural crest cells originating from the caudal part of the neural 
tube cannot give rise to dorsal root ganglia or (iii) the caudal environment is 
not permissive for the formation of dorsal root ganglia. To solve this problem, 
we have first studied the pattern of expression of ventral (HNF3beta) and dorsal 
(slug) marker genes in the caudal region of the neural tube; in a second 
approach, we have recorded the emergence of neural crest cells using the HNK1 
monoclonal antibody; and finally, we have analyzed the developmental potentials 
of neural crest cells arising from the caudalmost part of the neural tube in 
avian embryo in in vitro culture and by means of heterotopic transplantations in 
vivo. We show here that neural crest cells arising from the neural tube located 
at the level of somites 47-53 can differentiate both in vitro and in vivo into 
melanocytes and Schwann cells but not into neurons. Furthermore, the neural tube 
located caudally to the last pair of somites (i.e. the 53rd pair) does not give 
rise to neural crest cells in any of the situations tested. The specific 
anatomical aspect of the avian spinal cord can thus be accounted for by limited 
developmental potentials of neural crest cells arising from the most caudal part 
of the neural tube.