Does timing of axon outgrowth influence initial retinotectal topography in Xenopus

摘要: The question of whether timing is involved in generating the topographic organization earliest embryonic projection from retina to tectum has been examined Xenopus laevis. First, normal schedule axonal outgrowth was characterized. Groups axons originating either dorsal or ventral extremes were labeled by vitro incubation sectors (one-quarter one-third) eye primordia [3H]proline and their time courses determined using light microscope autoradiography. Comparisons growth nerve fiber populations showed that those leave first, grow along optic pathway, reach roughly 6 hr ahead retina. This stereotyped sequence schedules development initial retinotectal map: first ventrolateral receives input (stage 37/38), then dorsomedial part 40). Second, test accurate axon target invasion defines spatial ordering connections, retinal altered substituting halves young stage 21 primordia, with [3H]proline, for older 27 embryos. These heterochronic transplants resulted retarded such original pioneer fibers reached at least 9 later than normal, arriving after fibers, thereby reversing tectal invasion. Despite this, pattern innervation remained spatially normal. It concluded map generated not temporal but, rather, selective means neuronal recognition.