Sema3A chemorepellant regulates the timing and patterning of dental nerves during development of incisor tooth germ

Semaphorin 3A (Sema3A) axon repellant serves multiple developmental functions. Sema3A mRNAs are expressed in epithelial and mesenchymal components of the developing incisor in a dynamic manner. Here, we investigate the functions of Sema3A during development of incisors using Sema3A-deficient mice. We analyze histomorphogenesis and innervation of mandibular incisors using immunohistochemistry as well as computed tomography and thick tissue confocal imaging. Whereas no apparent disturbances in histomorphogenesis or hard tissue formation of Sema3A (-/-) incisors were observed, nerve fibers were prematurely seen in the presumptive dental mesenchyme of the bud stage Sema3A (-/-) tooth germ. Later, nerves were ectopically present in the Sema3A (-/-) dental papilla mesenchyme during the cap and bell stages, whereas in the Sema3A (+/+) mice the first nerve fibers were seen in the pulp after the onset of dental hard tissue formation. However, no apparent topographic differences in innervation pattern or nerve fasciculation were seen inside the pulp between postnatal and adult Sema3A (+/+) or Sema3A (-/-) incisors. In contrast, an abnormally large number of nerves and arborizations were observed in the Sema3A (-/-) developing dental follicle target field and periodontium and, unlike in the wild-type mice, nerve fibers were abundant in the labial periodontium. Of note, the observed defects appeared to be mostly corrected in the adult incisors. The expressions of Ngf and Gdnf neurotrophins and their receptors were not altered in the Sema3A (-/-) postnatal incisor or trigeminal ganglion, respectively. Thus, Sema3A is an essential, locally produced chemorepellant, which by creating mesenchymal exclusion areas, regulates the timing and patterning of the dental nerves during the development of incisor tooth germ.