Commissural axons

Serafini, T., Colamarino, S.A., Leonardo, E.D., Wang, H., Bedding-ton, R., Skarnes, W.C., and Tessier-Lavigne, M. (1996) Netrin-1 is required for commissural axon guidance In the developing vertebrate nervous system. Cell 87 1001-1014. 

Henkemeyer, M. et al. (1996). Nuk controls pathfinding of commissural axons in the mammalian central nervous system. Cell 86, 35-46. 

Dickson BJ, Gilestro GF (2006) Regulation of commissural axon pathfinding by slit and its Robo receptors. Annu Rev Cell Dev Biol 22 651-675... 

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. 

Orientation of commissural axons in vitro to a floor plate-derived chemoattractant. Development 110 19-30. 

Tsakonas, S. (1990) Slit - an extracellular protein necessary for development of midline glia and commissural axon pathways contains both EGF and LRR domains. Genes Dev. 4 2169-2187. 

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... 

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].

Figure 5. Chemoattraction of alar plate commissural axons in the brain by the floor...

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. 

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. 

Placzek, M., Tessier-Lavigne, M., JesseU, T.M. and Dodd, T. (1990). Orientation of commissural axons in vitro in response to a floor plate-derived chemoattractant. Development 110, 19—30. 

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