Cytoskeletal modifications of the sensorimotor interneurons of Hydra vulgaris (Cnidaria, Hydrozoa), indicating a sensory function similar to chordotonal receptors of insects

The battery mother cell complexes in the tentacles of Hydra vulgaris contain a neuronal cell known as sensorimotor interneuron that is characterized by a modified cilium lying parallel to the mesoglea. The cilium is surrounded by up to three rings of microvilli. Microvilli and cilium arise in an unusual antiparallel orientation from the opposite poles of a central cellular cavity. The lumen of this cavity communicates with the extracellular environment by way of a straight channel-like opening that is encircled by the microvillar rings. The modified cilium extends into the channel and terminates outside in the intercellular space. The wall of the cavity and the channel are stabilized by bundles of microtubules. A prominent glycocalyx interconnects all microvilli and links the innermost microvillar ring to the cilium. Within this contact region approximately 0.7 Μm in length the ciliary axoneme is specifically modified: all nine microtubule doublets and up to six additional microtubules are embedded in electron-dense material. The microtubule doublets are connected to the ciliary membrane by ledges of Y-shaped cross-bridging elements. These axonemal modifications resemble those known from the hydrozoan cnidocil complex or from the outer segments of insect mechanoreceptor cells. Distribution and orientation of the sensorimotor interneuron within the tentacles indicate a mechanosensory function of the cell similar to that of chordotonal receptors of insects. © 1991 Springer-Verlag.