Ontogenesis of substance P in the rat peripheral vestibular system.

Ontogenesis of substance P (SP) in rat vestibular receptors between gestational day 17 (GD 17) and postnatal day 17 (PD 17) was investigated by immunocytochemistry. SP immunoreactivity was first detected in the afferent nerve fibers of the utricular and saccular maculae on GD 19 and thereafter at birth (PD 0) in the cristae. SP immunoreactivity presented a characteristic pattern. It was strictly confined to the slopes of cristae and peripheral regions of the maculae. The pattern of SP immunoreactivity changed during the maturation of the afferent innervation pattern between PD 4 and PD 9, resulting in fewer fibers being stained. Then, the number of immunostained calyces and small bouton endings increased. On PD 17 the distribution of SP was similar to that of the adult stage. We suggest that SP, present in the epithelium at critical stages of development, may be involved in the maturation of vestibular receptors.