Kathryn W. Tosney
Research interest:
Axon Guidance
in the embryo


A major puzzle in development is how neurons extend to their targets with such precision. To help solve this puzzle we examine axonal guidance at three levels, the in vivo level, the cellular level and the molecular level, each of which proffers its own methods and insights.

The in vivo level of analysis shown on this page is the most basic; it defines all subsequent questions.

Here we identify the embryonic populations that guide. Without knowing which tissues guide, it is difficult to ask reasonable cellular or molecular questions. We use the simple strategy of challenging neurons by surgically excising or moving tissues in the intact chicken embryo or in representative “slices” in culture. This approach has identified several tissues that guide multiple populations and one tissue that guides a specific subset of motoneurons.


Three principles have emerged from this level of analysis.

1) Guidance cues come in two classes, general and specific. General cues guide several different populations, acting like broad highways that delineate the prospective gross anatomical nerve pattern. For instance, cues that dictate the segmental pattern of spinal nerves guide sensory neurons, motoneurons and neural crest cells. In contrast, specific cues act like "signposts" along the highways, directing axons to specific destinations. One population-specific cue is provided by the developing myotome. If it is removed, the neurons that normally innervate fail to grow out.

2) General and specific cues act independently. For instance, motor axons innervate limb muscle with their normal precision, even after we have removed the cues that impose a segmental pattern on their initial outgrowth.

3) General guidance cues themselves come in two types: permissive cues that comprise the paths themselves and inhibitory cues that lie adjacent to the paths. Both play active roles in guidance. For instance, our surgeries prove that axons avoid entering inhibitory tissues by actively responding to an inhibitory influence rather than by turning in response to a signal resident only within paths.


Figure 1 shows a live culture preparation called a "slice" that retains embryonic tissue relations but supplies great accessibility. We explant portions of embryos large enough to simulate a real internal environment but simple enough to support detailed analysis. A flourescent label was injected into the spinal cord (SC); it labeled motor axons that have extended into the spinal nerve (arrow) and reached the base of the limb buds (L).

Figure 2 shows, at higher magnification, that some axons have diverged from their fellows in response to cues specific to them; they will innervate muscle developing from the myotome (m). Such living axons can be optically recorded. Two tissues inhibit axon advance: P, the pelvic girdle precursor, which prevents axon entry into limb except in restricted positions, and the perinotochordal mesenchyme, pm, which prevents divergence ventrally. n: notochord


Using culture preparations like this, we have shown that sensory and motor axons are guided by both diffusible and contact-mediated interactions
(Hotary and Tosney, 1996).


Publications

  • Tosney, K. W. , A. Wagnitz, D. Dehnbostel, and K. J. Balazovich (2010). Evidence that growth cones exert mechanical force as they exit the spinal cord. under revision

  • Hotary KB and Tosney KW (1996). Cellular interactions that guide sensory and motor axons identified in an embryonic slice preparation. Dev. Bol. 176: 22-35
  • Tosney, K.W., K.B. Hotary and C. Lance-Jones (1995). Specifying the target identity of motoneurons. BioEssays, 17: 379-382.

  • Tosney KW (1992). Growth cone navigation in the proximal environment of the chick embryo. In The Nerve Growth Cone. eds, P.C.Letourneau, S.B.Kater, E.R.Macagno, Raven Press, NY. pp 387-403.

  • Tosney KW (1992). Neuronal Pathfinding in the Developing Chick Embryo. In Proceedings of the Course on Developmental Neurobiology at the International School of Neuroscience. Fidia Foundation, Thieme Medical Publishers, Inc., NY. pp 46-52.

  • Tosney, K.W. (1991). Cells and cell interactions that guide motor axons in the developing chick embryo. BioEssays. 13: 1-7

  • Oakley RA. and Tosney KW (1991). Peanut agglutinin and chondroitin-6-sulfate are molecular markers for tissues that act as barriers to axon advance in the avian embryo. Dev. Biol. 147: 187-206

  • Tosney KW and Oakley RA (1990). Perinotochordal mesenchyme acts as a barrier to axon outgrowth in the chick embryo; implications for a general mechanism of axon guidance. Exp. Neurol. 251:232-244.

  • Tosney KW and Hageman M(1989). Different subsets of axonal guidance cues are essential for sensory neurite outgrowth to cutaneous and muscle targets in the dorsal ramus of the chick. J. Exp. Zool. 251: 232-244

  • Tosney KW , Schroeter S, Pokrzywinski JA(1988). Cell death delineates axon pathways in the hindlimb and does so independently of neurite outgrowth. Dev. Biol. 130:558-572

  • Tosney KW (1988). Somites and axon guidance. Scan. Microsc. 2:427-442.

  • Tosney KW (1988). Proximal tissues and patterned neurite outgrowth at the lumbosacral level of the chick embryo: Partial and complete deletion of the somite. Dev. Biol. 127:266-286.

  • Tosney KW (1987). Proximal tissues and patterned neurite outgrowth at the lumbosacral level of the chick embryo: Deletion of the dermamyotome. Dev.Biol. 122:540-588

  • Tosney KW (1987). "Axonal pathfinding." In From Message to Mind: Directions in Developmental Neurobiology, pp.91-95. Edited by S.Easter, K.Barald and B.Carlson, Sinauer, Sunderland, MA.

  • Tosney KW, Watanabe M, Landmesser L and Rutishauser U (1986). The distribution of NCAM in the chick hind limb during axon outgrowth and synaptogenesis. Dev. Biol. 114:437-452.

  • Tosney KW and Landmesser LT(1985). Development of the major pathways for neurite outgrowth in the chick hindlimb. Dev. Biol. 109:193-214.

  • Tosney KW and Landmesser LT (1985). Specificity of motoneuron growth cone outgrowth in the chick hindlimb. J. Neurosci. 5:2336-2344.

  • Tosney KW and Landmesser LT (1985). Growth cone morphology and trajectory during outgrowth into the chick limb. J. Neurosci. 4:2345-2358.

  • Tosney KW and Landmesser LT (1984). Pattern and specificity of axonal outgrowth following varying degrees of chick limb bud ablation. J. Neurosci. 4:2158-2527.

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