Responsiveness of chick statoacoustic ganglion neurons to morphogens
Abstract
The sensory organs of the chicken inner ear are innervated by the peripheral processes of statoacoustic ganglion (SAG) neurons. Sensory organ innervation depends on a combination of axon guidance cues and survival factors, located along the trajectory of growing axons and/or within their sensory organ targets. Morphogens are expressed within and surrounding the chick inner ear during stages of SAG axon outgrowth and pathfinding. The localized expression patterns of some members of the Wnt, BMP, Shh and FGF morphogen families makes them candidate tropic (guidance) cues for growing axons and candidate trophic (survival) factors for axons that have reached their targets. To test their involvement in these processes, three-dimensional collagen gel cultures were used to test the responsiveness of embryonic day 4 (E4) chick SAG neurons. SAG explants were cultured in the presence of purified proteins, presented either uniformly or as a point source, and evaluated for effects on neurite outgrowth. SAG neurons were unresponsive to Wnts -1, -4, -5a, -6 and -7b. Wnt bioactivity was confirmed using E6 chick spinal cord explants grown under comparable culture conditions. SAG neurons were also found to be unresponsive to FGF2, after confirming FGF2 bioactivity with a fibroblast survival assay. On the other hand, explants displayed enhanced neurite outgrowth when cultured in the presence of bath-applied purified BMP4, BMP7, a low concentration of Shh, FGF8, FGF10, or FGF19. The same molecules promoted outgrowth when presented as point sources, with the exception of BMP7. Purified protein cultures were further analyzed for effects on neuron survival and proliferation. Treatments that increased neurite outgrowth also yielded significantly fewer apoptotic cells, with no effect on cell proliferation. These results suggest that members of the BMP, Shh and FGF morphogen families may influence SAG neurons both trophically and tropically in vivo.
Degree
Ph.D.
Advisors
Fekete, Purdue University.
Subject Area
Biology|Neurosciences|Evolution and Development
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.