In anemones hair bundle mechanoreceptors are formed from stereocilia packed with actin filaments. The image to the right was taken using confocal microscopy of hair bundles stained with phalloidin, a dye that specifically binds actin.
The stereocilia are derived from a multicellular complex. A sensory neuron located at the center of the complex organizes the hair bundle. The sensory neuron is surrounded by two to four hair cells. Arising from the surface of the neuron are a single cilium surrounded by six to seven large diameter stereocilia. These are the longest stereocilia of the hair bundle. Approximately 50 to 100 small diameter stereocilia arise from the surface of each hair cell. Small diameter stereocilia converge onto the large diameter stereocilia such that the longest of these stereocilia are in contact with those from the sensory neuron. The stereocilia are progressively shorter across the hair bundle. The shortest stereocilia occur at the outermost boundary of the hair bundle.
Stereocilia are interconnected by extracellular linkages. Such lnkages ensure that the bundle behaves as a single mechanical unit. Perhaps most interesting ar the linkages known as tip links. Tip links interconnect the tip of one stereocilum with the distal region of the neighboring, taller stereocilium. According to the gating spring model developed for vertebrate hair cells, tip links are thought to directly attach to the ion channels that open when the hair bundle is deflected in a positive direction (towards taller stereocilia). A tip link is shown below interconnecting a pair of stereocilia. This image was taken with a transmission electron microscope. A group of tip links is shown at the bottom left (arrowheads). That image was taken using a scanning electron microscope.
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