Innervation of mouse molars during the early states of tooth germ development.

The topography of nerves and first molar tooth germs in 11-14-day embryos was studied using silver-impregnated serial sections. Nerve fibers growing toward the developing tooth germ became visible on the 12th day, while the first sign of molar tooth differentiation was found as a thickening of the oral epithelium in 11-day embryos. From the 12th to 13th day the nerve fibers spread, forming a plexus close to the base of the tooth bud, and on the 14th day some entered into the dental follicle of the tooth germ at the early cap stage. However, no nerve fiber was found growing into the dental papilla during the observation period. The observations showed that the earliest nerve fibers running toward the tooth forming area appeared after the histogenesis of the tooth germ started, and the timing and pattern of the innervation of the tooth germs revealed that tooth germs are a useful model for investigating the mechanism of nerve growth into developing peripheral organs.