Development of the sensory nerves to the dorsum of the tongue in staged human embryos

To determine in selected, staged human embryos the time period and manner of formation of the sensory nerves to the mucosa on the dorsum of the tongue, cranial nerves V-3 and IX and their lingual branches were traced microscopically. These two cranial nerves along with cranial nerve VII are present in the dorsal part of their respective pharyngeal arch 1, 2, and 3 at Stage 13. All three nerves grow ventrally at Stage 16, and the lingual branches of V3 and IX become closer to the developing tongue region. The lingual fibers begin to enter the tongue substance at Stage 17 when they appear as large short branches that terminate 100-400 mum from the dorsal mucosal basement membrane; of the fibers that are in the tissue deep to the surface mucosa, those in the posterior tongue segment terminate the closest and those in the anterior segment the farthest. Chorda tympani nerve fibers are joined with V-3 at Stage 20. At this stage nerve fibers reach the basement membrane in the posterior tongue segment and by Stage 23 they terminate at the basement membrane throughout the tongue, being sparsest in the anterior segment. Primordial papillae appear as small mucosal projections throughout the tongue at Stage 20 and are innervated by Stage 23. The study found that innervation to the dorsum of the tongue develops during the second month of prenatal life. During the 4-week period, Stages 13-23, an innervation pattern was observed based on the relative time fibers terminate at the basement membrane. The pattern begins caudally in the root, then moves rostrally lateral to the middle tongue segment, proceeding then to the tip of the tongue. The last area to become innervated is the medial part of the middle segment or central portion of the tongue.