Derived muscle arrangements and their shared innervation patterns of external and internal cheek pouches in rodents

Cheek pouches have evolved from the oral cavity in rodents and act as temporary food storage repositories. There are two types of opening, internal and external. Details about the complex cutaneous muscles controlling the pouches have still not been fully elucidated. To understand the shared and derived traits of the muscles surrounding the cheek pouch and their innervation, we carried out an evolutionary morphological study using two desert kangaroo rats (Dipodomys deserti) and three plains pocket gophers (Geomys bursarius) from each of the two families equipped with external cheek pouches, and four Syrian hamsters (Mesocricetus auratus) with internal cheek pouches. The most conspicuous derived trait of the muscles between the external and internal cheek pouches, the sphincter sacculi, surrounds almost all of the edge of the outer entrance of the pouch. It is present in both species with external pouches, but not in hamsters, which have internal pouches. Our neurological findings demonstrate that most pouch muscles are innervated by both the facial and the cervical nerves, regardless of the pouch type. In these dually innervated muscles, the ventromedial part of the muscles tends to be innervated dominantly or uniquely by the cervical nerves, which usually enter from the superficial or lateral aspect. As a trait shared with the cervical nerves innervating the propatagial muscles in aerodynamic mammals such as bats and flying squirrels, and panniculus carnosus in most mammals, our neurological evidence suggests that the cervical nerve has the potential to innervate derived cutaneous muscles in the cervicofacial region.