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NEURONAL SPECIFICATION IN THE SPINAL CORD: INDUCTIVE SIGNALS AND TRANSCRIPTIONAL CODES
Thomas M. Jessell
Neural circuits are assembled with remarkable precision during embryonic development, and the selectivity inherent in their formation helps to define the behavioural repertoire of the mature organism. In the vertebrate central nervous system, this developmental program begins with the differentiation of distinct classes of neurons from progenitor cells located at defined positions within the neural tube. The mechanisms that specify the identity of neural cells have been examined in many regions of the nervous system and reveal a high degree of conservation in the specification of cell fate by key signalling molecules.
PROPRIOCEPTION
The part of the somatosensory system that relays information about trunk and limb position.
ROSTROCAUDAL
The axis of the vertebrate embryo that runs from head to tail. Also referred to as the anteriorposterior axis at early stages of neural development.
DORSOVENTRAL
The axis of the vertebrate embryo that runs from back to stomach.
Many telling insights into vertebrate neuronal patterning have come from attempts to trace the pathways by which inductive signals commit cells to specific fates15.
This article summarizes progress in defining some of these pathways through the analysis of cell fate specification in just one region of the central nervous system (CNS), the spinal cord. The physiology and connectivity of neurons within the mature spinal cord have been particularly well delineated6, providing a clear end point for studies of the development of these circuits. Spinal neurons serve two main functions: they relay cutaneous sensory information to higher centres in the brain and they integrate proprioceptive input and motor output. These two functional systems are also segregated anatomically. The neurons and circuits that process cutaneous sensory input are concentrated in the dorsal spinal cord, whereas circuits involved in PROPRIOCEPTION
and motor control are largely confined to the ventral spinal cord6. Progress in defining mechanisms of dorsal patterning has been discussed elsewhere7, and so this article focuses solely on neuronal specification in the ventral spinal cord.
The allocation of cell fate in the spinal cord, as in other regions of the CNS, depends on two signalling systems that are activated together with the more basic program of neural induction8. These two signalling sys-
tems intersect along the...