Close association of olfactory placode precursors and cranial neural crest cells does not predestine cell mixing.

Vertebrate sensory organs originate from both cranial neural crest cells (CNCCs) and placodes. Previously, we have shown that the olfactory placode (OP) forms from a large field of cells extending caudally to the premigratory neural crest domain, and that OPs form through cell movements and not cell division. Concurrent with OP formation, CNCCs migrate rostrally to populate the frontal mass. However, little is known about the interactions between CNCCs and the placodes that form the olfactory sensory system. Previous reports suggest that the OP can generate cell types more typical of neural crest lineages such as neuroendocrine cells and glia, thus marking the OP as an unusual sensory placode. One possible explanation for this exception is that the neural crest origin of glia and neurons has been overlooked due to the intimate association of these two fields during migration. Using molecular markers and live imaging, we followed the development of OP precursors and of dorsally migrating CNCCs in zebrafish embryos. We generated a six4b:mCherry line (OP precursors) that, with a sox10:EGFP line (CNCCs), was used to follow cell migration. Our analyses showed that CNCCs associate with and eventually surround the forming OP with limited cell mixing occurring during this process.