Marginal neurons in the urodele spinal cord and the associated denticulate ligaments

Marginal neurons have been described in the spinal cords of a variety of vertebrates including lamprey, reptiles, birds, and mammals but not in amphibians. There has been speculation about a motor function for these neurons but recent experimental evidence in lampreys indicates that they are intraspinal mechanoreceptor neurons. Additional evidence on reptiles and birds demonstrates that the marginal neurons are closely associated with the denticulate ligaments. In the present investigation, we have examined the spinal cords of Necturus, Ambystoma tigrinum, and A. mexicanum with light and electron microscopic techniques. Marginal nuclei were found in the ventrolateral position immediately internal to the pia and to the denticulate ligament. The marginal neurons were scattered in a continuous column of neuropil without segmental accumulation. They were approximately 30 to 50 μm in diameter and fusiform with dendrites extending from the poles, parallel with the length of the spinal cord. Neuronal fingerlike processes, like those found in peripheral mechanoreceptors and in the marginal nuclei of reptiles, were also found in the three species of urodeles studied. The structure of the denticulate ligaments, similar in the three different amphibians, was composed of collagen, elastin, and fibroblasts, all of which were concentrated in the segmental lateral processes.