MICRASTERIAS CELLS AS A MODEL SYSTEM FOR RESEARCH ON MORPHOGENESIS

Micrasterias species have been the subject of numerous experimental st udies on cell shape formation in the last 40 years. Chemical and physi cal treatment during different development states, as well as investig ations of ultrastructure by means of various different preparation met hods, have yielded information about some principles of morphogenesis in the symmetric, highly ornamented Micrasterias cell. The basic symme try of a Micrasterias cell is determined prior to mitosis and is estab lished without nuclear control thereafter. Normal cell development, ho wever, may occur only under the conditions of continuous protein synth esis throughout the cell cycle. A prepattern for the later cell shape seems to be present at the plasma membrane at the early stages of sept um formation. It is realized by a local, patterned distributed incorpo ration of cell wall material that is delivered by Golgi-produced vesic les. The areas where fusions take place between the primary wall mater ial containing vesicles and the plasma membrane are defined by the inw ard ionic currents that are carried at least in part by calcium. These areas develop into lobes during the following course of cell growth. Cell shaping in Micrasterias cells is thus mediated by both an enhance d extension of the cell wall and an additional incorporation of wall m aterial in the areas of the lobes. Numerous studies have indicated tha t actin plays an important role in morphogenesis, whereas microtubules do not participate in this process but are involved mainly in nuclear migration. The present review shows that although a wealth of details concerning Micrasterias morphogenesis has already been elucidated, tw o main questions, i.e., the method of septum formation and the splitti ng of the lobes, remain to be answered.