In vivo observations of timecourse and distribution of morphological dynamics inXenopus retinotectal axon arbors

Abstract

Changes in neuronal structure can contribute to the plasticity of neuronal connections in the developing and mature nervous system. However, the expectation that they would occur slowly precluded many from considering structural changes as a mechanism underlying synaptic plasticity that occurs over a period of minutes to hours. We took time‐lapse confocal images of retinotectal axon arbors to determine the timecourse, magnitude, and distribution of changes in axon arbor structure within living Xenopus tadpoles. Images of axons were collected at intervals of 3 min, 30 min, and 2 h over total observation periods up to 8 h. Branch additions and retractions in arbors imaged at 3‐ or 30‐min intervals were confined to shorter branches. Sites of additions and retractions were distributed throughout the arbor. The average lifetime of branches was about 10 min. Branches of up to 10 μm could be added to the arbor within a single 3‐min observation interval. Observations of arbors at 3‐min intervals showed rapid changes in the structure of branchtips, including transitions from lamellar growth cones to more streamlined tips, growth cone collapse, and re‐extension. Simple branchtips were motile and appeared capable of exploratory behavior when viewed in time‐lapse movies. In arbors imaged at 2‐h intervals over a total of 8 h, morphological changes included longer branches, tens of microns in length. An average of 50% of the total branch length in the arbor was remodeled within 8 h. The data indicate that the elaboration of the arbor occurs by the random addition of branches throughout the arbor, followed by the selective stabilization of a small fraction of the new branches and the retraction of the majority of branches. Stabilized branches can then elongate and support the addition of more branches. These data show that structural changes in presynaptic axons can occur very rapidly even in complex arbors and can therefore play a role in forms of neuronal plasticity that operate on a timescale of minutes. © 1996 John Wiley & Sons, Inc.

Stabilization of Axon Branch Dynamics by Synaptic Maturation

“…Interstitial branches are typically small (Ͻ5 m), highly dynamic, and shortlived (Witte et al, 1996). A fraction of these interstitial branches will be maintained and serve as sites for further branch addition (Kaethner and Stuermer, 1992;O'Rourke et al, 1994;Witte et al, 1996;Alsina et al, 2001;Ruthazer et al, 2003;Hua et al, 2005).…”

Cell-Autonomous TrkB Signaling in Presynaptic Retinal Ganglion Cells Mediates Axon Arbor Growth and Synapse Maturation during the Establishment of Retinotectal Synaptic Connectivity

“…Both types of processes undergo dynamic remodeling during axon arborization (Cohen-Cory and Fraser, 1995;Witte et al, 1996). To determine whether the high rate of branch turnover in GFP-TrkB.T1 RGC axon arbors was attributable to increased filopodial-like dynamics rather than increased turnover of more established branches, we reexamined RGC axon branch dynamics but excluded processes shorter than 5 m (filopodial-like processes) from the analysis.…”

Visualizing Morphogenesis with the Processing Programming Language

“…Axonal arbors are dynamic structures, with remodeling of branches and synapses occurring throughout life, in correlation with cognitive processes such as learning and memory (Meyer and Smith 2006). Optic axonal arbors that establish visual connectivity are peripherally located in the brain, and can be imaged in intact, living animals, including lower vertebrates such as tadpoles of frogs or fish (Figure 2a; Marshak et al, 2009;Witte et al, 1996;Harris et al, 1987). From in vivo timelapse videos of optic axonal arbors, previous researchers quantified the rates of branch additions and retractions, and the mean lifetimes of branches that underlie the remodeling of these arbors (Figure 2a; Marshak et al, 2007;Witte et al, 1996).…”

“…Optic axonal arbors that establish visual connectivity are peripherally located in the brain, and can be imaged in intact, living animals, including lower vertebrates such as tadpoles of frogs or fish (Figure 2a; Marshak et al, 2009;Witte et al, 1996;Harris et al, 1987). From in vivo timelapse videos of optic axonal arbors, previous researchers quantified the rates of branch additions and retractions, and the mean lifetimes of branches that underlie the remodeling of these arbors (Figure 2a; Marshak et al, 2007;Witte et al, 1996). Another study measured and modeled the mean branching angle of axon arbors in vitro (Shefi et al, 2005).…”

“…Morphometric measurement further defines the quantitative and statistical relationships between the cell dynamics that underlie morphogenesis (Kim and Davidson 2011;Marshak et al, 2007;Elul et al, 1997;Witte et al, 1996). Mathematical decomposition of cell dynamics additionally facilitates the computational modeling of cellular mechanisms that drive morphogenesis (Satulovsky et al, 2008).…”