Particle movement in heliozoan axopods associated with lateral displacement of highly ordered membrane domains.

Freeze-fracture studies reveal that extrusive organelles displaying saltatory particle movements in centrohelidian axopod are attached to highly ordered domains within the plasma membrane. It is postulated that the motive force for lateral displacement of these membrane domains with the adhering organelle is located immediately underneath the plasma membrane being either part of the peripheral membrane proteins or attached filaments alined parallel to the axopodial microtubules. The attachment domain is interpreted as a "frozen" membrane area preventing untimely organelle discharge by membrane fusion.