THE ROLE OF THE CYTOSKELETON IN POLARITY AND MORPHOGENESIS OF ALGAL CELLS

Selected algal species continue to serve as model organisms for the st udy of cell growth and cellular morphogenesis. Recent improvements in immunohistochemical and microinjection methods have helped to consolid ate our views of the role of the cytoskeleton as a generator of spatia l patterns in the cytoplasm before cellular morphogenesis. Progress ha s also been made in the discovery and characterization of molecular co mponents of both the cytoskeleton and the extracellular matrix (ECM). Studies on the oocytes of fucoid brown algae have demonstrated that th e ECM serves an active role in controlling cell shape and in defining the developmental fate of a cell. Actin, transmembrane proteins of the beta-integrin type, and vitronectin-like proteins in the ECM have bee n discussed as important elements in polar axis formation in the early steps of post-fertilization development. The mechanism of cell expans ion has been investigated in the large coenocytic cells of the siphono clad green algae. It was shown that the alignment of cell wall microfi brils in these cells depends on the degree of order in the cortical mi crotubule system. However, in contrast to earlier hypotheses, microtub ules do not appear to function as physical boundaries guiding the path s of cellulose synthesizing terminal complexes in the plane of the pla sma membrane. Recent work on the giant unicellular green alga Acetabul aria has revealed dynamic reorganizations of the actin cytoskeleton du ring the course of apical morphogenesis. Actin has also been suggested to play a role, in more subtle ways, in the establishment of membrane prepatterns during cellular morphogenesis of the desmid green alga Mi crasterias, prepatterns that predict regions of future surface expansi on.