Actin cortex and microtubular system in morphogenesis: cooperation and competition.

Actin cortex and microtubules determine two different types of morphological organization of the cytoplasm. Cooperation and competition between these two organizations may produce a diversity of final forms. Actin cortex alone, without the other cytoskeletal systems, is sufficient for the organization of vectorized pseudopod-forming cytoplasmatic units termed 'actinoplasts'. Reversible segregation of cytoplasm into actinoplasts and microtubule-rich stable domains ('tubuloplasts') is suggested to play an important role in many types of morphogenesis. Segregation of this type can be induced in fibroblasts of certain lines by the tumour promotor, tetradecanoyl phorbol-acetate (TPA). Self-organization of each actinoplast leads to the development of microfilament bundles associated with focal contacts. Analysis of the evolution of these bundle-contact structures during spreading of mouse fibroblasts suggests that their patterns are determined by the degree of centripetal tension within the actin cortex; the microtubular system stabilizes these patterns. Pseudopodial activity of the edges of the actinoplasts can be controlled by environmental factors. One particular type of control, i.e. the effect of cell-cell contact, is discussed. Actin cortex and microtubules seem to have alternative effects on the distribution of vimentin-containing intermediate filaments.