Synaptophysin in the developing cochlea.

The immunoreactivity to SY38 (anti-synaptophysin antibody) was investigated in rat and guinea-pig cochleas during development. In rat pups SY38 reactivity first appeared in the inner spiral bundle (below inner hair cells) at postnatal day 3. Later on (days 10 and 15) the basal pole of outer hair cells (OHCs) was also reactive. In fetal guinea-pigs, the inner spiral bundle was reactive on day 45 of gestation, while the reactivity occurred below OHCs on day 62 of gestation. A preliminary electron microscopic finding (from a guinea-pig 62 days of gestation) indicated that SY38 immunoreactivity is localized within varicosities of efferent (olivo-cochlear) endings. Synaptophysin is thus present in the cochlea at the level where the two efferent systems terminate. Moreover, the occurrence of SY38 immunoreactivity, first at the ISB then at the OHC levels, is in accordance with the observation that the maturation of lateral efferents precedes that of medial efferents.