Are pioneer axons guided by regulatory gene expression domains in the zebrafish forebrain? High-resolution analysis of the patterning of the zebrafish brain during axon tract formation.
Although the principles of axon growth are well understood in vitro the mechanisms guiding axons in vivo are less clear.  It has been postulated that growing axons in the vertebrate brain follow borders of neuroepithelial cells expressing specific  regulatory genes. In the present study we reexamined this hypothesis by analysing the earliest growing axons in the  forebrain of embryonic zebrafish. Confocal laser scanning microscopy was used to determine the spatiotemporal  relationship between growing axons and the expression pattern of eight regulatory genes in zebrafish brain. Pioneer axons  project either longitudinally or dorsoventrally to establish a scaffold of axon tracts during this developmental period. Each  of the regulatory genes was expressed in stereotypical domains and the borders of some were oriented along dorsoventral and  longitudinal planes. However, none of these borders clearly defined the trajectories of pioneer axons. In two cases axons  coursed in proximity to the borders of shh and pax6, but only for a relatively short portion of their pathway. Only later  growing axons were closely apposed to the borders of some gene expression domains. These results suggest that pioneer  axons in the embryonic forebrain do not follow continuous pathways defined by the borders of regulatory gene expression  domains.
Hjorth
J T
JT
Department of Anatomy and Cell Biology, University of Melbourne, Victoria 3052, Australia.
Key
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B
D000818:Animals; D001369:Axons / Q000502:physiology*; D019521:Body Patterning; D001921:Brain / Q000196:embryology*; D004625:Embryo, Nonmammalian / Q000502:physiology; D004627:Embryonic Induction; D005136:Eye Proteins; D018507:Gene Expression Regulation, Developmental; D005809:Genes, Regulator; D053823:Hedgehog Proteins; D018398:Homeodomain Proteins / Q000235:genetics; D017404:In Situ Hybridization, Fluorescence; D018613:Microscopy, Confocal; D009419:Nerve Tissue Proteins / Q000235:genetics; D009474:Neurons / Q000502:physiology*; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D016548:Prosencephalon / Q000196:embryology*; D011506:Proteins / Q000235:genetics; D012097:Repressor Proteins; D015534:Trans-Activators; D015027:Zebrafish / Q000196:embryology
D005136:Eye Proteins; D053823:Hedgehog Proteins; D018398:Homeodomain Proteins; D009419:Nerve Tissue Proteins; D000071841:PAX6 Transcription Factor; D051761:Paired Box Transcription Factors; D011506:Proteins; D012097:Repressor Proteins; D015534:Trans-Activators