Development of the spinal nerves in the lamprey: I. Rohon-Beard cells and interneurons.

Two types of large neurons are described in the caudal one-third of the spinal cord of larval lampreys (26-day, 13-mm), Lampetra japonica. Rohon-Beard cells (RB cells) are large neurons (8-14 micron in diameter) consistently positioned superficially in the dorsolateral part of the medullary wall and separated from the extramedullary space only by a basal lamina and a thin glia limitans and from the central canal lumen by the ependyma. The RB cell contains an extensive Golgi apparatus and granular endoplasmic reticulum. Its two processes arise from the basal part of the perikaryon and join the dorsolateral tract (DLT) to extend rostrally and caudally within the DLT, which is formed exclusively by contribution of RB cell processes at least in the caudal one-third of the tail spinal cord. RB cells receive no synaptic nerve endings on their surfaces. The early developing RB cells exhibits an apical process that reaches the luminal surface of the central canal. Interneurons (15-19 microns in diameter) lie in the lateral region of the cord between the DLT and ependyma and receive from the DLT many synaptic nerve endings that contain aggregates of small, clear vesicles. They contribute axonal processes to the ventrolateral tract (VLT). Some developing interneurons have an apical process that contains abundant free ribosomes and granular ER. A presumptive model of how these cell classes are interconnected within the cord is presented.