| Abstract: | Isolated melanophores of the angelfish, Pterophyllum scalare, have been used in a morphometric analysis and a quantitative study of their microtubule system. Using transverse sections spaced at regular intervals, the changes associated with the process of pigment aggregation have been determined. Upon the concentration of pigment granules in the central cell region, almost half of the cytoplasmic portion is also withdrawn from the peripheral cell regions. Counts of microtubules within a cell sector in cells with pigment aggregated and dispersed, respectively, reveal (a) a constancy of the number of microtubules in this sector regardless of the distance from the cell center, and (b) a reduction of microtubule number in cells with pigment aggregated by about 58%. On the basis of these counts, the total number of microtubules has been calculated. In the dispersed state, about 2,400 microtubules extend between the center and the periphery of the cell, while their number is about 1,000 in the aggregated state. Using a 13-protofilament model of a microtubule and relevant data on size and molecular weight of microtubule subunits, the amount of tubulin present as microtubules is calculated. In the average, the cells contain 1.95·108 monomers corresponding to 1.78·10?8 mg tubulin. A tentative estimation of the concentration of tubulin inside a melanophore yields values of 6.1 mg/ml for the whole cell and 16.5 mg/ml for the cytoplasm alone (excluding membrane-bound organelles). Based on this estimation, a comparison, with microtubule assembly in vitro is made. |
