Vertebrate lateral line and vestibular systems center their function on highly mechanosensitive hair cells. Each hair cell is equipped with one kinocilium (which resembles a motile cilium) and 50-100 actin-containing stereocilia (which resemble microvilli) at the site of stimulus reception. This report describes electron-microscopic localization of calcium-binding sites on the sensory processes ofvertebrate hair cells. Using the Oschman-Wall technique for calcium localization [Oschman, J. L. & Wall, B. J. (1972) J. Cell Biol. 55,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73] together with electron-probe x-ray microanalysis of thin sections, we observed: (i) calcium-and ironcontaining deposits in the region of the ciliary necldace in goldfish lateral line hair cells, (ii) calcium deposits upon the surface of stereocilia of hair cells of the bullfrog inner ear, and (iii) calcium deposits upon stereocilia of hair cells of the guinea pig vestibular system.Vertebrate lateral lines and vestibular systems are equipped with hair cells of exquisite mechanosensitivity (1). Hair cells, capable of responding to a 0.5-nm stimulus (2) in less than 40 ,usec (3), are fitted with a single kinocilium and 50-100 microvillous "stereocilia" at the site of stimulus reception. It is interesting to note that the kinocilium, which resembles a "9+2" motile cilium, and the microvillous stereocilia, which contain core filaments of actin (4, 5), are both derived from primitive motile systems.Throughout the course of evolution, locomotor and sensory systems have been closely allied. Because ciliary and actinbased motile systems are, in part, activated and regulated by calcium ions, we ask: are calcium-binding sites associated with the kinocilia or stereocilia of vertebrate hair cells?Using the Oschman-Wall technique of calcium localization (6) in concert with electron-probe microanalysis ofthin sections, we have observed calcium-binding sites on the sensory cilia of hair cells of the goldfish lateral line, the bullfrog vestibular system, and the guinea pig vestibular system. MATERIALS AND METHODS Source of Tissues. Sense organs from the acousticolateralis system of animals from three vertebrate classes-a teleost fish, an amphibian, and a mammal-were studied. Mechanoreceptive hair cells within the following sense organs were investigated: (i) hair cells from the supraorbital canal of the lateral line system ofthe goldfish, Carassius auratus; (ii) hair cells from the macula sacculi of the vestibular system of the bullfrog, Rana catesbeiana; and (iii) hair cells from the macula sacculi and macula utriculi of the vestibular system of the guinea pig, Cavia porcellus.Electron Microscopy. Tissues were fixed by following the method of Oschman and Wall (6), a preparative procedure that localizes sites on cell membranes that have a high affinity for calcium ions. Experimental tissues were fixed overnight at 40C in a solution containing 2.7% (vol/vol) glutaraldehyde, 5% sucrose, and 5 mM CaCl2 buffered to pH 7.2 with 0.08 M ...
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