ABSTRACT
Neural crest cells originate at three discontinuous levels along the rostrocaudal axis of the chick rhomben-cephalon, centred on rhombomeres 1 and 2, 4 and 6, respectively. These are separated by the odd-numbered rhombomeres r3 and r5 which are depleted of migratory neural crest cells. Here we show elevated levels of apoptosis in the dorsal midline of r3 and r5, immediately following the formation of these rhombomeres at the developmental stage (10-12) when neural crest cells would be expected to emerge at these neuraxial levels. These regions are also marked by their expression of members of the msx family of homeobox genes with msx-2 expression preceding apoptosis in a precisely co-localised pattern. In vitro and in ovo experiments have revealed that r3 and r5 are depleted of neural crest cells by an interaction within the neural epithelium: if isolated or distanced from their normal juxtaposition with even-numbered rhombomeres, both r3 and r5 produce migrating neural crest cells. When r3 or r5 are unconstrained in this way, allowing production of crest, msx-2 expression is concomitantly down regulated. This suggests a correlation between msx-2 and the program-ming of apoptosis in this system. The hindbrain neural crest is thus produced in discrete streams by mechanisms intrinsic to the neural epithelium. The crest cells that enter the underlying branchial region are organised into streams before they encounter the mesodermal environ-ment lateral to the neural tube. This contrasts sharply with the situation in the trunk where neural crest pro-duction is uninterrupted along the neuraxis and the segmental accumulation of neurogenic crest cells is sub-sequently founded on an alternation of permissive and non-permissive qualities of the local mesodermal envi-ronment.
