title: In vivo dynamics of CNS sensory arbor formation: a time-lapse study in the embryonic leech.
abstract: In the embryo of the leech Hirudo medicinalis, afferent projections of peripheral sensory neurons travel along common nerve tracts to the CNS, where they defasciculate, branch, and arborize into separate, modality-specific synaptic laminae. Previous studies have shown that this process requires, at least in part, the constitutive and then modality-specific glycosylations of tractin, a leech L1 homologue. We report here on the dynamics of growth of these projections as obtained by examining the morphology of single growing dye-filled sensory afferents as a function of time. Using 2-photon laser-scanning microscopy of the intact developing embryo, we obtained images of individual sensory projections at 3 to 30 min intervals, over several hours of growth, and at different stages of development. The time-lapse series of images revealed a highly dynamic and maturation-state-dependent pattern of growth. Upon entering the CNS, the growth cone-tipped primary axon sprouted numerous long filopodial processes, many of which appeared to undergo repeated cycles of extension and retraction. The growth cone was transformed into a sensory arbor through the formation of secondary branches that extended within the ganglionic neuropil along the anterior-posterior axis of the CNS. Numerous tertiary and quaternary processes grew from these branches and also displayed cycles of extension and retraction. The motility of these higher-order branches changed with age, with younger afferents displaying higher densities and greater motility than older, more mature sensory arbors. Finally, coincident with a reduction in higher order projections was the appearance of concavolar structures on the secondary processes. Rows of these indentations suggest the formation of presynaptic en-passant specializations accompanying the developmental onset of synapse formation.
authors.affiliation: Division of Biology, University of California, San Diego, California 92093, USA.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Baker
authors.forename: Brent
authors.initials: B
authors.lastname: Kauffman
authors.forename: Eduardo R
authors.initials: ER
authors.lastname: Macagno
authors.forename: Birgit
authors.initials: B
authors.lastname: Zipser
mesh_terms: D000818:Animals; D002490:Central Nervous System / Q000196:embryology; D003911:Dextrans; D004625:Embryo, Nonmammalian; D005456:Fluorescent Dyes; D020439:Growth Cones / Q000648:ultrastructure*; D007091:Image Processing, Computer-Assisted; D007865:Leeches / Q000196:embryology*; D018613:Microscopy, Confocal; D016501:Neurites / Q000648:ultrastructure; D009475:Neurons, Afferent / Q000502:physiology* / Q000648:ultrastructure; D013997:Time Factors
chemical_list: D003911:Dextrans; D005456:Fluorescent Dyes