Projection-neurons that send axons through the lumbar spinal cord of the chick embryo are not obviously distributed in a segmentally repetitive pattern.

Earlier studies have shown that the formation of the segmentally repetitive pattern of spinal nerves in avian embryos depends upon the segmental arrangement of the somites and does not reflect an intrinsic pattern of the spinal cord. These studies could not, however, rule out the possibility that some elements of the central nervous system are intrinsically segmented. The question remained, for instance, whether projection-neurons are distributed in a segmentally repetitive pattern within the spinal cord. To address this question, horseradish peroxidase was injected into one or two segments of the lumbar spinal cord of chick embryos, thereby labeling projection-neurons that had ascending or descending axons passing through the injection site. In all stages examined, the number of labeled projection-neurons in the anterior and posterior halves of each segment did not differ significantly. In addition, segmentally repetitive peaks or troughs in the numbers of labeled projection-neurons were not detected in the center of each segment. Three subpopulations of projection-neurons, defined by their position along the dorsal-ventral axis of the spinal cord, were also not segmentally distributed by these criteria. While these results do not rule out the possibility that subpopulations of projection-neurons defined by dendritic morphology, functional class, or some other parameter are segmentally arranged or that there is a basic modular repetition of neural populations from segment to segment, they do show that projection-neurons as a class and subpopulations of these neurons defined by their dorsal-ventral position are not obviously distributed in a repetitive segmental pattern.