Variations in receptor cell innervation in the saccule of a teleost fish ear

Using transmission electron microscopy, we quantitatively analyzed the afferent and efferent synapses on 67 sensory hair cells along the saccular epithelium of the oscar (Astronotus ocellatus), a cichild fish with a non-specialized ear. The synaptic profile (number of afferent and efferent synapses per cell) varied considerably among cells. The number of synapses per hair cell ranged from three to 24, and all but six of the 67 hair cells had both afferent and efferent synapses. Statistical analysis showed that the synaptic profiles did not significantly vary anywhere on the saccular epithelium except at the edges. There, hair cells had significantly fewer efferent synapses than hair cells in other epithelial regions. This statistical variation in efferent synapse distribution in different epithelial regions corresponds with the lengths of ciliary bundles in these regions. The synapses on hair cells showed a regional specificity in position. In all cells synapses were never located more apically than the top of the nucleus. On hair cells towards the periphery, the most apical synapse on the hair cells tended to be afferent. On more centrally located cells, the most apical synapse was efferent in 92% of the cells.