Novel membranous structures in apical and basal compartments of inner hair cells

Postfixation with a ferrocyanide-osmium tetroxide solution preserved a dense network of canaliculi extending from the apical to the upper lateral plasma membrane in cochlear inner hair cells (IHCs). Numerous Golgi bodies intermingled with this apical canalicular reticulum (CR). Osmium-ferrocyanide treatment also disclosed several previously unreported structures below the IHC nucleus. The first consisted of stacks of six or eight and sets of three parallel cisternae of rough endoplasmic reticulum spanning between clustered mitochondria. Some parallel cisternae ended with segmentation where they contacted mitochondria, and others terminated by transforming into blebs or continuing into canaliculi. A second feature was comprised of a complex of segmented cisternae and branching canaliculi with clustered mitochondria. Branching minicanaliculi with associated vesicles neighbored the complexes. A fourth entity consisted of synaptic-like vesicles that largely filled the subnuclear cytosol and congregated at synapses. An additional infranuclear structure was composed of slender canaliculi that collected near or streamed to plasmalemma, often next to a synapse. A paradoxical absence of rough endoplasmic reticulum above and Golgi zones below the nucleus provided evidence of atypical mechanisms for generating the membrane in CR and forming synaptic vesicles. The observations offer the view that IHCs are compartmentalized into an apical mechanoreceptor half and a basal half that affects neurotransmission. The apical CR provided a possible structural basis for sequestering the K+ known to influx apically and for directing its diffusion to the site of known efflux across the lateral plasmalemma. The codistribution of parallel cisternae, canalicular-mitochondrial complexes, and synaptic-like vesicles, all of which are unique to IHCs, implicated the cisternae and complexes in the genesis of the vesicles.