Cytoskeletal elements in insect sensilla

Abstract

Insect sensilla have evolved prominent cytoskeletal elements as part of their functional specialization. The cytoskeleton present in sensory cells as well as in auxiliary cells may play an important role in sensilla function. The scolopale, the characteristic cytoskeletal component in the innermost auxiliary cell of mechanosensitive scolopidia and thermo-/hygrosensitive sensilla, is mainly composed of bundles of 10 nm filaments. Cytochemical approaches for light and electron microscopy identified these structures as actin filaments that exhibited a unique filament orientation and uniform filament polarity. None of these approaches has provided evidence for the presence of myosins in the scolopale. In contrast, tropomyosin and the microtubule-associated protein 2 are associated with the actin filament bundles in the scolopale of scolopidia. All data taken together suggest that the actin filaments of scolopale have a stabilizing rather than a contractile function. In scolopidia, in addition to cellular stabilization, filament elasticity would appear to be important during stimulation. Owing to the high number of microtubules, the scolopale in thermo-/hygrosensitive sensilla seems more rigid than in scolopidia and may protect sensory dendrites from mechanical forces. In sensory cells of scolopidia, regularly cross-striated ciliary rootlets are additional prominent cytoskeletal structures. Immunohistochemistry reveals that these rootlets contain the Ca²⁺-binding protein centrin, which forms contractile filaments in other systems, e.g., unicellular green algae. Accordingly, contractions of ciliary rootlets may also be part of the filament function in insect sensilla.