Afferent innervation patterns in crista ampullaris of the mouse during ontogenesis

The development of vestibular afferent innervation patterns was studied by labeling the peripheral terminations between gestation day 17 and postnatal day 10. Extracellular injections of horseradish peroxidase were performed into vestibular ganglia in mouse otocysts maintained in vitro for several hours. At gestation days 17 and 18, the afferent innervation patterns were characterized by the presence of a few collaterals that arose from the parent fiber and sometimes ended by swellings or by enlargement extended by filopods rising to the epithelium surface. At the 20th gestation day the first endings differentiated into boutons or calyces were seen. At birth, the afferent innervation consisted of collaterals that could terminate either in boutons or in incomplete thin calyces. Starting on postnatal days 1 and 2, boutons and calyces became more pronounced and 3 afferent innervation patterns could be distinguished, i.e., calyx, dimorphic, and bouton. By postnatal day 5, the filopods had disappeared and the characteristic endings on type I or II cells were clearly individualized. On postnatal day 10, the afferent innervation patterns were comparable to those in the adult. These results are discussed in relation to ultrastructural data concerning the synaptogenesis and to the physiological development that have been described during the first postnatal days.