Surgical anatomy of ovine facial and hypoglossal nerves for facial nerve reconstruction and regeneration research: An experimental study in sheep

Background: The lack of a clinically relevant animal model for facial nerve research is a challenge. The goal of this study was to investigate the anatomy of the ovine facial and hypoglossal nerves to establish a clinically relevant facial nerve research model. Materials and methods: Six cadaver female Merino sheep (33.5 ± 3 kg, approximately 3 years old) and three anesthetized female Merino sheep (30 ± 3 kg, approximately 3 years old) were used. In cadaver sheep, a right side preauricular to submandibular incision was made. Dimensions of the face, neck, and length of facial nerve were measured. In anesthetized sheep, each facial nerve branch and hypoglossal nerve in the right side was stimulated. The number of myelinated fibers was analyzed histologically. Results: The facial nerve exited the stylomastoid foramen and divided into upper and lower branches. The lower branch then subdivided into buccal and marginal mandibular branches. The hypoglossal nerve was observed behind the digastric posterior belly. Stimulation revealed the temporal, zygomatic, buccal, marginal mandibular, and cervical branch innervated the forehead, orbicularis, upper lip and nasal, lower lip, and platysma, respectively. The number of myelinated fibers of the main trunk, upper, buccal, lower branch, and hypoglossal nerve was 11 350 ± 1851, 4766 ± 1000, 5107 ± 218, 3159 ± 450, and 7604 ± 636, respectively. The length of the main trunk was 9.2 ± 1.5 mm, and distance of the marginal mandibular branch to the facial artery was 94 ± 6.8 mm. Conclusions: Due to the similarity in nerve anatomy and innervation, the ovine model can be used as a clinically relevant and suitable model for facial nerve research.