J. Comp. Neurol. 328: 437-48 (1993)

A nonrandom interneuronal pattern in the developing frog spinal cord.

R. D. Heathcote & A. Chen

In the developing spinal cord of the frog, Xenopus laevis, a population of interneurons assumed a pattern that represented a previously undescribed level of organization. Glyoxylic acid treatment and immunocytochemistry showed that the neurons contained catecholamines and their synthetic enzyme, tyrosine hydroxylase. Cells were located within the ependymal layer of the floor plate region of the larval spinal cord. The cells had several processes including a long one that projected toward the brain without fasciculating with other labeled processes. In addition, the cytoplasm of the catecholaminergic cells extended into the central canal, showing that they were a population of cerebrospinal fluid contacting neurons.

The spatial domain of catecholaminergic neurons started abruptly at the boundary between the hindbrain and spinal cord and continued to the tip of the tail. The neurons occupied two longitudinal columns within the sheet of floor plate cells, which included cells that did not exhibit the catecholaminergic phenotype. Unlabeled cells were intercalated between catecholaminergic cells in each column, giving the labeled cells the appearance of being spaced along the length of the spinal cord. This general arrangement was evident at the time of hatching. Spatial analysis showed that the position of cells along a column was not random. The nonrandom behavior was due to cells being restricted from the area immediately surrounding other catecholaminergic cells. Further analysis showed that the cellular pattern lacked segmental or other periodic repeats. Ultimately, the location of a cell within a column, depended upon the position of its closest catecholaminergic neighbor.