EVIDENCE FOR RAPID STRUCTURAL AND FUNCTIONAL-CHANGES OF THE MELANOPHORE MICROTUBULE-ORGANIZING CENTER UPON PIGMENT MOVEMENTS

Melanophores of the anglefish, Pterophyllum scalare, have previously been shown to display ~2,400 microtubules in cells with pigment dispersed; these microtubules radiate from a presumptive organizing center, the central apparatus (CA), and their number is reduced to ~1,000 in the state with aggregated pigment. In an attempt to elucidate the factors controlling this rapid reorganization of the microtubule apparatus, structure and function of the CA have been investigated under different physiological conditions. As a function of the state of pigment distribution, melanophores differ markedly with respect to CA organization. A complex of dense amorphous aggregates and associated fuzzy material, several micrometers in diameter, surrounds the centrioles in cells with pigment dispersed, and numerous microtubules emanate from this complex in a radial fashion. In the aggregated state, on the other hand, few microtubules are observed in the pericentriolar region, and the amount of fibrous material is greatly reduced. These changes in CA morphology as a function of the state of pigement distribution are associated with a marked difference in its capacity to initiate the assembly of microtubules from exogenous pure porcine brain tubulin in lysed cell preparations. After complete removal of preexisting microtubules, cells lysed in the dispersed state into a solution of 1-2 mg/ml pure tubulin have numerous microtubules associated with the CA in a radial fashion, while cells lysed in the aggregated state nucleate the assembly of only a few microtubules. We conclude that it is the activity of the CA that basically regulates the expression of microtubules. This regulation is achieved through a variation in the capacity to initiate microtubule assembly. Increase or decrease in the amount of dense material, as readily observed in the cell system studied here, seems to be a morphologic expression of such a physiologic function.