Comparative anatomy of the subscapularis, teres major and latissimus dorsi muscles from salamanders to mammals with special reference to their innervations from the brachial plexus.

Abstract

It has been reported that the ramification pattern of spinal motor nerves reflected the spatial orientation of motoneuron pools in the ventral horn of spinal cord and this topography of spinal motor nuclei was very similar in different vertebrates. Therefore, the ramification pattern of spinal nerves was an important criterion for discussing the phylogenetic homology of muscles. It has been reported that the human subscapularis muscle was innervated by several branches, the proximal branch of them was from the ventral layer of the dorsal cord and the distal one from the dorsal layer of the dorsal cord of the brachial plexus. This fact suggested the human subscapularis had different phylogenetic origins. In this study, I unveil the phylogenetic origin of the mammalian subscapularis. The animals observed were a chimpanzee, a lar gibbon, a cat, a fetal pig, a koala, a possum (mammals), a lizard, an iguana (reptiles) and salamanders (amphibians). The branches to the mammalian subscapularis were divided into proximal and distal groups based on the origin from the brachial plexus, just like the human subscapularis. In salamanders and lizards, the homologous branch with the mammalian proximal branch to the subscapularis was observed and the segmentally higher branch innervating the latissimus dorsi was homologous with the distal branch to the mammalian subscapularis. Conclusively, I suppose that the dorsal-most portion of the reptilian latissimus dorsi muscle differentiates to the mammalian teres major, and the segmentally higher portion of the reptilian latissimus dorsi contributes to the formation of the mammalian subscapularis.