Int. J. Mol. Sci. 2011, 12(12), 8539-8561; doi:10.3390/ijms12128539
Review

Crossing the Border: Molecular Control of Motor Axon Exit

1email and 1,2,* email
Received: 17 October 2011; in revised form: 5 November 2011 / Accepted: 8 November 2011 / Published: 29 November 2011
(This article belongs to the Special Issue Studies of Motor Molecules)
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract: Living organisms heavily rely on the function of motor circuits for their survival and for adapting to ever-changing environments. Unique among central nervous system (CNS) neurons, motor neurons (MNs) project their axons out of the CNS. Once in the periphery, motor axons navigate along highly stereotyped trajectories, often at considerable distances from their cell bodies, to innervate appropriate muscle targets. A key decision made by pathfinding motor axons is whether to exit the CNS through dorsal or ventral motor exit points (MEPs). In contrast to the major advances made in understanding the mechanisms that regulate the specification of MN subtypes and the innervation of limb muscles, remarkably little is known about how MN axons project out of the CNS. Nevertheless, a limited number of studies, mainly in Drosophila, have identified transcription factors, and in some cases candidate downstream effector molecules, that are required for motor axons to exit the spinal cord. Notably, specialized neural crest cell derivatives, referred to as Boundary Cap (BC) cells, pre-figure and demarcate MEPs in vertebrates. Surprisingly, however, BC cells are not required for MN axon exit, but rather restrict MN cell bodies from ectopically migrating along their axons out of the CNS. Here, we describe the small set of studies that have addressed motor axon exit in Drosophila and vertebrates, and discuss our fragmentary knowledge of the mechanisms, which guide motor axons out of the CNS.
Keywords: motor axon exit; axon pathfinding; spinal cord; motor exit point; dorsally-exiting motor neuron; ventrally-exiting motor neuron; Nkx2.9; Robo; Slit
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MDPI and ACS Style

Bravo-Ambrosio, A.; Kaprielian, Z. Crossing the Border: Molecular Control of Motor Axon Exit. Int. J. Mol. Sci. 2011, 12, 8539-8561.

AMA Style

Bravo-Ambrosio A, Kaprielian Z. Crossing the Border: Molecular Control of Motor Axon Exit. International Journal of Molecular Sciences. 2011; 12(12):8539-8561.

Chicago/Turabian Style

Bravo-Ambrosio, Arlene; Kaprielian, Zaven. 2011. "Crossing the Border: Molecular Control of Motor Axon Exit." Int. J. Mol. Sci. 12, no. 12: 8539-8561.

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