Initial tract formation in the vertebrate brain

Abstract

This chapter summarizes the recent work in tract formation in the vertebrate brain. The biggest step forward in the study of early axonogenesis came with the introduction of fluorescent tracers: the procion dyes Individual cells were injected with fluorescent tracers directly and the outgrowth of the axons and dendrites were monitored over time. This is perhaps the ideal way to study tract development, particularly because these fluorescent tracers can often be made electron-dense, allowing electron microscopic examination of identified processes. Some investigators have been successful in injecting individual cells in the vertebrate CNS but most of the recent reports on axonogenesis have employed markers applied extracellularly, either horseradish ∼ peroxidase (HRP) or carbocyanine dyes applied locally or antibodies against molecules associated with early axons applied more broadly. The development of the first tracts is investigated in the presumptive fore- and mid-brains in zebra fish. Embryonic chick and mouse brains are also examined with methods similar to those used on zebrafish and frog; i.e. fixing them lightly in formaldehyde and reacting them with neuron-specific antibodies in whole-mounts and sections. In the mouse, the work has shown that the first TuJ1-positive cells appear in the neural plate on E8.5, prior to neural tube closure, and are almost certainly the precursors of the mesencephalic nucleus of the trigeminal nerve.