Abstract
During embryonic development, complex events, such as cellular proliferation, differentiation, survival, and guidance of axons, are orchestrated and regulated by a variety of extracellular signals. Receptor tyrosine kinases mediate many of these events, with several playing critical roles in neuronal survival and axonal guidance. It is evident that not all the receptor tyrosine kinases that play key roles in regulating neuronal development have been identified. In this study, we have characterized the spatial-temporal expression profile of a recently identified receptor tyrosine kinase, anaplastic lymphoma kinase (ALK), in embryonic chick by means of whole-mount in situ hybridization in conjunction with immunohistochemistry. Our findings reveal that Alk is expressed in sympathetic and dorsal root ganglia as early as stage 19. In addition, mRNA is expressed from stage 23/24 (E4) to stage 39 (E13) in discrete motor neuron subsets of chick spinal cord along with a select group of muscles that are innervated by one of these particular motor neuron clusters. Expression within the spinal cord is coincident with the onset and duration of motor neuron programmed cell death and during the period of musculature innervation and synapse formation. Hence, the data presented here identify ALK as a novel candidate receptor for regulating critical events in the development of neurons in both the central and the peripheral nervous systems.