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(FRAME-INFO (STRING "Differentiation of early neurons in the developing spinal cord. \(Image courtesy of Dr. H. Roelink, Dept. of Biological Structure, University of Washington\). These photographs show cross-sections of the developing spinal cord in chick embryo. The canal centrally is all that remains of the lumen of the neural tube; cells in the walls of the tube proliferate to give rise to the gray matter of the spinal cord. Immediately below the developing cord is the circular profile of the notochord; it releases the signaling molecule Sonic Hedgehog \(stained red in panels A, B, and C\) which induces the formation of the floor plate. The floor plate \(the triangular region in the ventral midline of the spinal cord in panels A, B, and C - see schematic in Frame 5 also\) in turn becomes a source of Sonic Hedgehog and is therefore also labeled red. The green fluorescent cells are cells whose distribution in the developing spinal cord is dependent on Sonic Hedgehog. Green cells in Panel A will differentiate into several types of interneurons, which are excluded from the ventral spinal cord by Sonic Hedgehog released from the notochord and floor plate. Green cells in panels B and C are motor neurons induced in the ventral neural tube by Sonic Hedgehog. Panel D shows motor neurons in red and motor neuron precursor cells destined for the anterior \(ventral\) horn in green. "))

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