STRUCTURE AND FUNCTION OF THE CYTOSKELETON IN HELIOZOA .3. RAPID MICROTUBULE DISORGANIZATION DURING AXOPODIAL CONTRACTION IN ECHINOSPHAERIUM

The heliozoan Echinosphaerium feeds on various kinds of small protozoans by slow retraction or rapid contraction of microtubule-containing axopodia. The axonemal microtubules showed an intense positive birefringence that was clearly visualized when the organism was observed under a highly sensitive polarization microscope. In order to know if the axopodial contraction involves rapid disassembly of the cytoskeletal microtubules, dynamic changes of the axonemal birefringence were analyzed here during the process of rapid axopodial contraction by video microscopy with an image digitizing system. The number of microtubules in an axopodium was estimated from its birefringence by applying dielectric theories for shell-covered ellipsoids in concentrated suspensions. Quantitative analysis showed that the fractional volume occupied by the microtubules increased at the proximal region of the axopodium concomitantly with the axopodial contraction, and it was maintained for at least 10 s after the contraction. Electron microscopic observations showed that two bundles of axonemal microtubules crossed each other in the proximal part of a contracted axopodium. These results indicate that, during rapid axopodial contraction, the distal portion of the axonemal microtubules moves as a bundle into the axopodial base without a distinct disassembly of the microtubules themselves.