Self-centring activity of cytoplasm.

Fish melanophore cells aggregate pigment granules at the centre or redisperse them throughout the cytoplasm. The granules move along radial microtubules by means of molecular motors. Cytoplasmic fragments of melanophores organize a radial array of microtubules and aggregate pigment at its centre. Here we report self-centring in microsurgically produced cytoplasmic fragments of black tetra melanophores. We observed rapid (10 min) formation of a radial microtubule array after stimulation of aggregation. Arrangement of microtubules in the fragments returned to random during pigment redispersion. Apparently, formation of the radial array does not depend on a pre-existing microtubule-organizing centre. The array did not form in granule-free fragments nor in fragments treated with inhibitors of the intracellular motor protein cytoplasmic dynein. We conclude that formation of the radial microtubule array is induced by directional motion of pigment granules along microtubules and present evidence that its position is defined by interaction of microtubules with the surface.