Variation and evolution of mammalian corticospinal somata with special reference to primates

The morphology of the somata originating the corticospinal tract was examined in 24 species of mammals to identify commonalities and major sources of variation among the different species. Horseradish peroxidase was applied to a hemisection of the spinal cord at the C1-C2 junction. After tetramethylbenzidine processing, the labeled somata throughout the cerebral cortex were plotted and counted. Then, 23 morphological characteristics of the corticospinal somata were examined, including their number, size, and density across the cortical surface. The results show that morphological characteristics of corticospinal somata are closely related to an animal's body, brain, and cerebral cortex size. That is, mammals with large neocortical surfaces tend to have larger as well as more corticospinal somata; mammals with large bodies tend to have corticospinal somata that are less densely distributed. Moreover, the probable increase in the ratio of local noncorticospinal somata to corticospinal somata implies that the evolution of the corticospinal tract was accomplished by an increase in "support" or "server" cells as well as an increase in the size of the tract itself. The results also show that several characteristics are reliably related to an animal's taxonomic classification and hence its ancestry. Comparisons among three mammalian lineages indicate that some characteristics may have changed uniquely in the anthropoid primate lineage, and thus, presumably, in the human lineage. The results suggest that if morphological characteristics of the corticospinal tract important in the evolution of the specialized motor abilities in anthropoid primates are sought, then examination of the role of changes in soma diameter, rostral (motor)/caudal (sensory) ratios of density, concentration, surface density, and volume density may be more instructive than examination of the total number of corticospinal neurons alone.