The trigeminal nerve consists of three major branches or rami (r) in jawed vertebrates: the ophthalmic (rV1), maxillary (rV2), and mandibular (rV3), with rV2 and rV3 innervating the upper and lower jaws, respectively. This three-branch configuration is conserved in the lamprey, a representative of the cyclostomes, the sole survivor of jawless vertebrates. While the first branch is widely accepted to be homologous to that of jawed vertebrate rV1, the homology of the other two remains unclear, so they are called rV1, rV2/3A, and rV2/3B. In this study, we investigate evolutionary modifications of the trigeminal nerve by comparing its organization in lamprey and jawed vertebrate embryos. By detailed neuronal tracing analysis of rV2/3B, which innervates the velum, we demonstrate that it consists solely of motor components. Conversely, we identify both sensory and motor fibers in rV2/3A. These results make a striking contrast to the trigeminal nerve of the jawed vertebrates, as their rV2 is purely sensory, but rV3 is a mixed nerve. Furthermore, through comparative analyses of sensory projection inside the brain, we clarify that the principal sensory nucleus of the trigeminal nerve (PrV) of anamniotes (shark, catfish, and Xenopus) lacks a somatotopic organization, suggesting that somatotopy evolved in at least the last common ancestor of amniotes. Last, by gene expression analysis of Drg11, a marker for the PrV and the spinal trigeminal nucleus (SpV), we show that a PrV-equivalent region is not present in the lamprey. These findings indicate significant structural changes in the trigeminal nerve associated with jaw acquisition, including shifts in innervation territories, the disappearance of the branch innervating the velum, and the emergence of novel brain nuclei.
Nakanishi et al. (Sun,) studied this question.