In the macaque cerebral cortex, the SPP1 (secreted phosphoprotein 1) gene is mainly expressed in corticospinal neurons. In this study, we found that SPP1 was principally expressed in motor neurons in lamina IX of the macaque spinal cord. The expression level varied among different spinal segments and correlated positively with neuron size. The expression was weak in Errγ-positive neurons, presumably gamma motor neurons, and in neurons in sacral Onuf's nucleus. These results suggest that SPP1 is a molecular characteristic of spinal motor neurons and is preferentially expressed in neurons with high conduction velocities.
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