GENERAL HISTOLOGY

Detailed explanation-1: -As a morphogen, Shh participates in the patterning of the developing spinal cord. Following closure of the neural tube, commissural (crossing; contralateral) and association (same side; ipsilateral) neurons develop in the dorsal half of the cord, while inter-and motor-neurons form in the ventral half of the cord.

Detailed explanation-2: -The secreted ligand Sonic Hedgehog (Shh) organizes the pattern of cellular differentiation in the ventral neural tube. For the five neuronal subtypes, increasing levels and durations of Shh signaling direct progenitors to progressively more ventral identities.

Detailed explanation-3: -The notochord-a structure derived from the axial mesoderm-produces SHH, which travels extracellularly to the ventral region of the neural tube and instructs those cells to form the floor plate.

Detailed explanation-4: -Morphogens refer to substances thought to be involved in the patterning of cells during embryonic development. It is a term coined by Alan Turing in 1952 for a substance whose distribution by diffusion could determine the development of cells which would respond differently to different concentrations of the morphogen.