| In the central nervous system, specific signals, such as motor, sensory, visual and auditory signals, are conducted by different nervous pathways. Raphespinal Tract (RST) is one of the most important descending nervous pathways, and its axons are originated from the caudal raphe nuclei (CRN) localized along the midline of the brain stem. The neurons in raphe nuclei are also called serotonergic neurons because they secrete neurotransmitter 5-hydroxytryptamine (5-HT). Our previous studies have found that serotonergic axons are originated from CRN at E11.5, and serotonergic RST axons descend through the ventral spinal cord from E12.5 to E16.5 in mice. Sonic hedgehog (Shh) is a kind of morphogen that is expressed in the floor plate of the spinal cord, and can be secreted to the whole ventral spinal cord at embryonic stages to regulate the development and migration of many neurons. Recent studies have shown that Shh also plays a role in guiding axons'directed projection as an axon guidance cue. In the present study, in situ hybridization or immunohistochemical method was employed to detect the expression patterns of Shh or its receptors patched (Ptc) and smoothened (Smo) along the anterior to posterior (A-P) axis of developing spinal cord. The expression of Shh could be detected in ventro-floor plate of the spinal cord of mice between E12.5 and E14.5, and showed a decrement expression pattern along the A-P axis as the projection progression of serotonergic RST to caudal end. The expression of Ptc could be detected in serotonergic CRN of mice at E1 1.5, however, the expression of Smo could not be detected at this stage. The expressions of both Ptc and Smo could be detected in serotonergic CRN and RST axons of mice between E12.5 and E14.5. Our findings strongly suggest that Shh, as an axon guidance cue, may be involved in guiding the serotonergic RST directed projection along the A-P axis of developing spinal cord. |
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