Novel membranous structures in apical and basal compartments of inner haircells

Postfixation with a ferrocyanide-osmium tetroxide solution preserved a dens e network of canaliculi extending from the apical to the upper lateral plas ma membrane in cochlear inner hair cells (IHCs). Numerous Golgi bodies inte rmingled with this apical canalicular reticulum (CR). Osmium-ferrocyanide t reatment also disclosed several previously unreported structures below the IHC nucleus. The first consisted of stacks of six or eight and sets of thre e parallel cisternae of rough endoplasmic reticulum spanning between cluste red mitochondria. Some parallel cisternae ended with segmentation where the y 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 complexe s. A fourth entity consisted of synaptic-like vesicles that largely filled the subnuclear cytosol and congregated at synapses. An additional infranucl ear structure was composed of slender canaliculi that collected near or str eamed to plasmalemma, often next to a synapse. A paradoxical absence of rou gh endoplasmic reticulum above and Golgi zones below the nucleus provided e vidence of atypical mechanisms for generating the membrane in CR and formin g synaptic vesicles. The observations offer the view that IHCs are compartm entalized into an apical mechanoreceptor half and a basal half that affects neurotransmission. The epical CR provided a possible structural basis for sequestering the ICI known to influx epically and for directing its diffusi on to the site of known efflux across the lateral plasmalemma. The codistri bution of parallel cisternae, canalicular-mitochondrial complexes, and syna ptic-like vesicles, all of which are unique to IHCs, implicated the cistern ae and complexes in the genesis of the vesicles. (C) 1999 Wiley-Liss, Inc.