Non-selective reinnervation by regenerating facial motoneurons after peripheral nerve crush in the developing rat

To study the regeneration process after the facial nerve crush in the neonatal period, the location of cranial motoneurons supplying the posterior belly of the digastric muscle (PDG) and the vibrissae follicular muscles was examined using retrograde tracing methods in the rat with facial nerves that had been surgically injured 1-2 day(s) after birth. In different stages after the induced facial palsy was functionally recovered, i.e. 4-20 weeks after the surgical operation, Fluoro-Gold (FG) was injected into the PDG, while wheat germ-agglutinated horseradish peroxidase (WGA-HRP) was injected into the vibrissae follicular muscles. Distribution of neurons retrogradely labeled with FG and/or WGA-HRP was then examined in the brainstem. On the intact side, used as a control, FG-positive neurons were restrictedly seen within the accessory facial nucleus (Acs7), while HRP-positive neurons were found in the main facial nucleus (FMN) proper. On the injured sides with any survival periods: 1) FG-positive neurons were diffusely seen not only in the Acs7 but also in the FMN, where the normal myotopical representation seemed to be no longer maintained. 2) HRP-positive neurons were also seen scattered not only in the FMN but in the Acs7. 3) The volume and cell number in both the Acs7 and the FMN were notably reduced. Thus, the present results indicate: 1) that the misdirectional regeneration of nerve fibers occurs widely across the facial motoneuron pools, and 2) that the non-selective distribution can never be restored to the selective one when severely damaged even in the neonatal periods.