Neurotrophin receptor expression during development of the chick spinal sensory ganglion.

The expression of mRNA for the trk neurotrophin receptors was studied in developing chicken dorsal root ganglion neurones using in situ hybridization histochemistry. trkC mRNA is expressed first, followed by trkB mRNA and finally trkA mRNA. The expression of each receptor begins very early during neurogenesis, and although initially quite widespread throughout the ganglion, the proportion of neurones expressing each receptor reduces as development proceeds. Expression patterns for each receptor become specifically restricted within the ganglion during this time, but during subsequent development the neurones migrate to their final site within the ganglion. From an assessment of the ganglion's ability to respond to neurotrophins in vitro, and from the results of earlier studies, both on the expression of receptors and gene targeting experiments, it is apparent that DRG neurones are dependent on these factors at very early stages of development and not, as previously thought, only after target innervation.