The development of trigeminal nerve fibers to the oral mucosa, compared with their development to cutaneous surfaces , ,

Abstract Small cutaneous branches of all three divisions of the trigeminal nerve pass toward epithelial surfaces in 14 mm human embryos. By 18 mm, such fibers are nearer the epithelium of the upper lip (14 μ., maxillary division) than the lower lip (31μ, mandibular division). At 20.5mm and 20.7mm, the age when Fitzgerald and Windle ('42) and Hooker ('52, '58) first found the perioral area sensitive to stimulation, the nerve fibers are about the same distance from the epithelium of both upper and lower lips (13–14 μ in rima oris area). Maxillomandibular fibers of the trigeminal nerve reach the basement membrane of the oral cavity epithelium earlier in development than they touch the basement membrane of the cutaneous epithelium about the mouth. Nerve fibers also penetrate the basement membrane of oral mucosal surfaces earlier than that of perioral cutaneous surfaces and form crude Merkel's end discs in mucosal epithelium earlier in development (mucosal epithelium, ten weeks; cutaneous epithelium at 12 weeks according to Hogg, '41). Ophthalmic fibers approach the upper eyelid epithelium later in development than the maxillomandibular fibers near perioral areas and touch the basement membrane of the eyelid epithelium a week later than they do that of the lips. Hooker did not observe reflexes from eyelid stimulation before ten weeks, when ophthalmic fibers reach the basement membrane, although reflexes from perioral stimulation were elicited when the nearest nerve fibers identified are still 13–14 μ from the basement membrane of the perioral epithelium. Additional correlations are made between the human fetal reflexes recorded by Hooker and the degree of development of the sensory peripheral nerve fibers.