Spinal commissural axons

A fundamental characteristic of the vertebrate central nervous system (CNS) is bilateral symmetry along the midline (Figure 3). This symmetrical structure divides neuronal projections into two types, uncrossed projections to ipsilateral targets and crossed projections to contralateral targets. Over the past decade, the mechanism involved in the formation of crossed projections has been extensively studied in spinal cord commissural projections. Spinal commissural axons, which derive from the dorsal part of the neural tube (alar plate), initially project ventrally along the circumferential axis and then cross the ventral midline [10]. These axons have been shown to be guided by a diffusible molecule released... 

Spinal commissural axons are shown to be guided toward the ventral midline by a chemoattractant from the floor plate. However, this can only explain the guidance mechanism near the ventral midline, raising the possibility that there may be another guidance mechanism. 

Commissural axons from the embryonic rat spinal cord extend preferentially toward floorplate explants in a manner suggestive of a chemotropic mechanism (Tessier-Lavigne et al., 1988 Placzek et al., 1990). Recently, two closely-related proteins isolated from embryonic chick brains, netrin-1 and netrin-2, have been shown to possess chemotropic activity in this spinal cord assay system (Serafini et al., 1994 Kennedy et al., 1994). 

M. (1994) Netrins are diffusible chemotropic factors for commissural axons in the embryonic spinal cord. Cell 78 425 35. 

Figure 4. Chemoattraction of spinal cord commissural axons by the floor plate. (A) Axon growth pattern of commissural neurons. (B, C) Collagen gel culture of an alar plate explant with a floor plate explant. Extensive outgrowth of commissural axons occurred toward the floor plate explant (B). Commissural axons were reoriented toward the floor plate explant (C].

Kennedy, T.E., Serafini, V., de la Torre, J.R. and Tessier-Lavigne, M. (1994). Netiins are diffusible chemotropic factors for commissural axons in the embryonic spinal cord. Cell 78, 425-435. 

If a spinal cord is cross-sectioned, the gray matter appears as a roughly H-shaped area in its middle which is, divided into dorsal (posterior), lateral, and ventral (anterior) horns. The horns are interconnected by a crossbar, the gray commissure. The rest of the spinal cord is the white matter, made up largely of tracts of myelinated nerve fibers (axons). Ascending tracts carry afferent sensory impulses towards the brain, descending tracts transmit motor impulses from the brain to the motor neurons in the ventral or lateral horns of the gray matter. 

Augsburger, A., Schuchardt, A., Hoskins, S., Dodd, J. and Buder, S. (1999). BMPs as mediators of roof plate repulsion of commissural neurons. Neuron 24,127-141. Bernhardt, R.R., Nguyen, N. and Kuwada, J.Y. (1992). Growthcone guidance by floor plate cells in the spinal cord of zebrafish embryos. Neuron 8, 869-882. Bernhardt, R.R., Patel, C.K., Wilson, S.W. and Kuwada, J.Y. (1992). Axonal trajectories and distribution of GABAergic spinal neurons in wild-type and mutant zebrafish lacking floor plate cells. J. Comp. Neurol. 326, 263—272. 

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