Interconnections between the stereovilli of the fish inner ear

Summary The maculae utriculi and sacculi of the inner ear from the European roach (Rutilus rutilus) were investigated by transmission electron microscopy. The stereovilli of peripherally and centrally located sensory cells differ in several features that suggest a developmental gradient. The stereovilli of the peripheral sensory cells, shown to be differentiating cells by other research groups, are short and less steeply graded in height than in central hair cells. All stereovilli in both kinds of hair bundles are interconnected. In the central bundles of stereovilli basal, tip, and vertical connectors are separated by unconnected regions. In contrast, filaments and sometimes other additional structures connect the stereovilli of peripheral bundles over their entire length, but vertical connectors are usually absent. Osmiophilic material occurring inside peripheral stereovilli is interpreted to be monomeric actin. Central and peripheral hair bundles also differ in their reaction to ruthenium red and cationized ferritin. Only the stereovilli of the central cells can be fused by these polycations. Ruthenium red also discriminates between supporting and sensory cells indicating differences in amount or distribution of extracellular material. Hair bundles, intermediate in properties and position between central and peripheral sensory cells, were also found, so that it became possible to propose a scheme of developmental steps leading from microvilli or microvillus-like stereovilli to the fully differentiated hair bundle.