Microtubles and actin filaments in teleost visual cone elongation and contraction

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Abstract
Teleost retinal cones contract in light and elongate in darkness. This paper describes the disposition of microtubules and cytoplasmic filaments in cone cells of 2 species of fish (Haemulon sciurus and Lutjanus griseus). In Haemulon, the neck-like “myoid” region of the cone changes in length from 5 μ to 75 μ. Maximal observed rates of elongation and contraction are comparable to that of chromosome movement in mitosis (2–3 μ/min). Microtubules presumably participate in cone elongation, since numerous longitudinal microtubules are present in the myoid region, and colchicine blocks dark-induced elongation. Myoid shortening, on the other hand, appears to be an active contractile process. Disruption of microtubules in dark-adapted cones does not produce myoid shortening in the absence of light, and light-induced myoid shortening is blocked by cytochalasin-B. Cone cells possess longitudinally-oriented thin filaments which bind myosin subfragment-1 to form arrowhead complexes typical of muscle actin. Myoid thin filaments are clearly observed in negatively stained preparations of isolated cones which have been disrupted with detergent after attachment to grids. These myoid filaments are not, however, generally preserved by conventional fixation, though bundles of thin filaments are preserved in other regions of the cell. Thus, actin filaments are poorly retained by fixation in precisely the region of the cone cell where contraction occurs. Cone cells also possess longitudinally-oriented thick filaments 130–160Å in diameter. That these thick filaments may be myosin is suggested by the presence of side-arms with approximately 150 Å periodicity. The linear organization of the contractile apparatus of the retinal cone cell makes this cell a promising model for morphological characterization of the disposition of actin and myosin filaments during contraction in a nonmuscle cell.