Variability in maps of identified neurons in the sea lamprey spinal cord examined by a wholemount technique.

A technique is described for preparing toluidine blue-stained wholemounts of lamprey spinal cords. By this technique virtually all the neurons in the spinal cord can be studied with respect to their soma size and shape, their primary dendrites, and sometimes secondary dendrites and proximal portions of the axon. Several cell types previously studied physiologically and described by others in cross section are described in wholemounts. These are dorsal cells, giant interneurons, edge cells and lateral cells. In addition, several unique cell types are noted in wholemount which were previously unremarked upon. These include obliquely oriented bipolar cells, trident-shaped cells located mostly in the rostral two-thirds of the spinal gray column, and small neurons with cell bodies in the dorsal and ventral axon tracts. Edge cells, which had previously been described as having large cell bodies close to the lateral edge of the lateral axon tracts with large medially oriented dendrites, are shown to be quite heterogeneous in size, location of soma and dendritic tree configuration. By use of the wholemount technique, 4 spinal cords of large sea lamprey larvae, close to transformation, were mapped for lateral cells, giant interneurons and dorsal cells. Considerable variability was noted in numbers and locations of these cells. The possible significance of this finding for the development of the vertebrate nervous system is discussed.