Are paragriseal cells in the avian lumbosacral spinal cord displaced ventral spinocerebellar tract neurons?

In lumbosacral segments the spinal cord of birds contains numerous paragriseal neurons lying in the white matter of lateral and ventral funiculus. These paragriseal cells project to the cerebellum. Neurons of the dorsal horn (mainly Clarke's column) make up a dorsal spinocerebellar tract and neurons of the ventral horn (mainly spinal border cells) are at the origin of a ventral spinocerebellar tract. It was the aim of this investigation to look for the distribution of spinocerebellar ventral horn neurons and paragriseal cells in the thoracolumbosacral spinal cord of pigeons and to compare this distribution with that of the cervical enlargement. Neuroanatomical tracers were injected into the anterior cerebellum of pigeons and labeled spinal neurons were counted throughout the length of the spinal cord. In the cervical enlargement the number of spinocerebellar ventral horn neurons increases more rostral than that of dorsal horn neurons but the number of both groups of neurons decreases simultaneously at the caudal end of the enlargement. In the ventral horn of thoracolumbosacral segments the number of spinocerebellar ventral horn neurons and paragriseal cells increases again more rostrally than that of dorsal horn cells. However, the number of ventral horn cells decreases whereas that of paragriseal cells and of dorsal horn cells is maintained. This shows that the number of ventral horn cells decreases in favor of paragriseal cells, which supports the suggestion that paragriseal cells are displaced ventral horn spinocerebellar neurons. It is discussed whether the paragriseal neurons migrate toward their input.