The neural crest: its relations with APUD and paraneuron concepts.

The neural crest cells give rise to a large variety of derivatives including neural, mesenchymal, APUD and/or paraneuron cell types. A better knowledge of these derivatives was partly achieved through studies using Le Douarin's quail/chick marker system. We review here evidences which were thus provided for a neural crest origin of calcitonin containing cells, carotid body, aortic paraganglia, adrenomedulla, and against a neurectodermal origin of enterogastric and respiratory tract endocrine cells. The role of neural crest cells in Pearse's APUD system is discussed. The results implicate that an explanation for the common properties of these cell types and their pathological and biochemical significance should not be looked for in a common embryological origin but at another level. The place of neural crest and, more generally, neurectoderm derivatives in the paraneuron concept of Fujita is examined. The relevance of the epithelial origin of these cell types to their "receptosecretory" function is stressed. Considering neural crest itself as a unique system is still questioned and discussed here. Its ubiquity and penetration of other systems is pointed out as a widespread phenomenon which is not restricted to APUD and paraneuron systems.