title: Diffusible signals and fasciculated growth in reticulospinal axon pathfinding in the hindbrain.
abstract: We have addressed the control of longitudinal axon pathfinding in the developing hindbrain, including the caudal projections of reticular and raphe neurons. To test potential sources of guidance signals, we assessed axon outgrowth from embryonic rat hindbrain explants cultured in collagen gels at a distance from explants of midbrain-hindbrain boundary (isthmus), caudal hindbrain, or cervical spinal cord. Our results showed that the isthmus inhibited caudally directed axon outgrowth by 80% relative to controls, whereas rostrally directed axon outgrowth was unaffected. Moreover, caudal hindbrain or cervical spinal cord explants did not inhibit caudal axons. Immunohistochemistry for reticular and raphe neuronal markers indicated that the caudal, but not the rostral projections of these neuronal subpopulations were inhibited by isthmic explants. Companion studies in chick embryos showed that, when the hindbrain was surgically separated from the isthmus, caudal reticulospinal axon projections failed to form and that descending pioneer axons of the medial longitudinal fasciculus (MLF) play an important role in the caudal reticulospinal projection. Taken together, these results suggest that diffusible chemorepellent or nonpermissive signals from the isthmus and substrate-anchored signals on the pioneer MLF axons are involved in the caudal direction of reticulospinal projections and might influence other longitudinal axon projections in the brainstem.
authors.affiliation: Institute for Neurobiology, Universidad Nacional Autónoma de México, Campus UNAM-UAQ Juriquilla, Querétaro, Mexico.
authors.forename: Hebert L
authors.initials: HL
authors.lastname: Hernández-Montiel
authors.forename: Esperanza
authors.initials: E
authors.lastname: Meléndez-Herrera
authors.forename: Ana C
authors.initials: AC
authors.lastname: Cepeda-Nieto
authors.forename: Carmen
authors.initials: C
authors.lastname: Mejía-Viggiano
authors.forename: Jorge
authors.initials: J
authors.lastname: Larriva-Sahd
authors.forename: Sarah
authors.initials: S
authors.lastname: Guthrie
authors.forename: Alfredo
authors.initials: A
authors.lastname: Varela-Echavarría
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D015415:Biomarkers; D002478:Cells, Cultured; D002642:Chick Embryo; D004058:Diffusion; D008954:Models, Biological; D009434:Neural Pathways / Q000196:embryology* / Q000502:physiology; D016501:Neurites / Q000502:physiology; D009474:Neurons / Q000166:cytology / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D012154:Reticular Formation / Q000033:anatomy & histology / Q000166:cytology / Q000196:embryology / Q000502:physiology*; D012249:Rhombencephalon / Q000033:anatomy & histology / Q000166:cytology / Q000196:embryology*; D015398:Signal Transduction; D013116:Spinal Cord / Q000502:physiology*
chemical_list: D015415:Biomarkers