The neuronal population of the marginal zone (lamina I) of the rat spinal cord. A study based on reconstructions of serially sectioned cells.

Complete series of silver-stained semithin transverse sections were used to reconstruct 177 nerve cells of rat lamina I. According to the three-dimensional shape of the perikarya and the number and orientation of primary dendritic trunks, lamina I cells formed four distinct groups: (1) Fusiform cells with long rostrocaudal axis and having 1-4 primary dendrites oriented rostrocaudally or ventrally, which were the most numerous (50%) and predominated in the lateral third of lamina I. (2) Flattened cells (12%) which were thin discs of angular contour, spread out parallel to the lamina dorsal border; they emitted thick lateral and medial, but no dorsal or ventral, primary dendrites, and were mainly located in the middle third. (3) Multipolar cells (20%) with polyhedric somata emitting 4-12 primary dendritic trunks in several directions, which were practically confined to the medial third of the lamina. (4) Prismatic, wedge-shaped cells (18%), partly situated or encased, in the white matter, emitting one dorsal interstitial dendrite and several transversely oriented dendrites, which were distributed throughout the whole dorsal border of lamina I, though more abundant in its lateral portion. A subpopulation of large cells was identified in all groups, except in the multipolar one. These four cell types may help establish a basic morphologic classification of the neuronal population of lamina I, and may explain the different appearances under which local cells have previously been described in preparations using different planes of section and varied staining methods.