Many different cell types possess microtubule patterns which appear to be polarized and oriented, in part, by cytoplasmic factors not directly associated with a centrosome. Recently, we demonstrated that cytoplasmic extensions ("arms") of teleost melanophores will reorganize their microtubule population outward from their centers after surgical isolation (McNiven, M. A., M. Wang, and K. R. Porter. 1984. Cell. 37:753-765). In the study reported here, we examine microtubule dynamics within the centrosome-free fragments and find that, after severing, microtubule reorganization is initiated at the proximal (cut) end of an arm and migrates distally with the aggregated pigment mass until it becomes permanently positioned at the middle of the arm. Computer-aided image analysis demonstrates that this middle position is located at the arm centroid, implicating the action of a cytoplasmic gel in this process. Morphological studies of arms devoid of pigment reveal that microtubules do not emanate from a single site or structure within the centroid area, but from a more generalized region. Taken together, these findings suggest that factors distributed throughout cytoplasm participate in microtubule assembly and organization.

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