The polarized apical cells of Sphacelaria rigidula display a well-organized cortical F-actin cytoskeleton. This consists of bundles of actin filaments (AFs), assuming definite patterns of organization in different regions of the cell cortex. At the tip region of the apical dome the AFs appear randomly oriented, showing a diffuse fluorescence. Immediately below, at the base of the apical hemisphere, the AFs form a ring-like band around the plasmalemma transverse to the polar cell axis. The rest of the cell cortex is traversed by AFs showing an axial or slightly inclined or helical orientation. Examination of the apical cells of S. rigidula in appropriate thin sections revealed that the wall has a multi-layered structure. In the tip region of the apical dome the cell wall bears randomly oriented cellulose microfibrils (MFs), while in the basal part of the apical dome it is reinforced by a layer of densely arranged transverse MFs. As the cell grows at the apex, the transverse MFs are continuously displaced towards the cell base. Below the transverse MF layer, an additional layer with axial or slightly oblique MFs starts being depositing internally, on the tubular part of the cell. Externally to them, the layer of transversely oriented MFs remains visible. The above observations were confirmed in apical cells of S. tribuloides. MF orientation in the innermost wall layer of the apical cells coincides with that of the cortical AFs observed by fluorescence. This mutual alignment between AFs and MFs in a cell that lacks cortical microtubules (MTs) suggests that the AFs are involved in the oriented deposition of MFs. Experimental disruption of AFs with cytochalasin B caused abnormal MF deposition, a fact strongly supporting the above hypothesis. The transverse MFs forming at the base of the apical dome define the diameter and consequently the cylindrical shape of the apical cells. It is suggested that in the brown algal cells examined the AFs play a morphogenetic role similar to that of cortical microtubules in higher plant cells.

The organization of F-actin in vegetative cells of the brown alga Sphacelaria rigidula was studied after staining with a modified rhodamine-phalloidin (Rh-Ph) protocol. The interphase vegetative cells display a well-organized F-actin cytoskeleton, consisting of cortical, endoplasmic and perinuclear arrays of actin filaments (AFs). The organization of these AFs changes slightly during mitosis, while they almost disappear at cytokinesis. The perinuclear AF population becomes more obvious during prophase, especially at the poles. At metaphase, an actin spindle is organized, co-localized with the microtubule spindle, while at anaphase an interzonal AF population appears, which persists at early telophase. At advanced telophase the image changes to a rather diffuse actin meshwork in the mid-area between the daughter nuclei. During post-telophase-early cytokinesis this AF system becomes gradually disassembled and a conspicuous actin plate is formed on the cytokinetic plane. This plate becomes more compact with the progress of cytokinesis, and seems to overcoat the patches of the forming diaphragm, and finally the daughter plasmalemmata. Treatment with cytochalasin B disturbs mitosis, not allowing the cells to proceed to anaphase, and prevents cytokinesis. These observations show that AFs are ubiquitous cytoskeletal elements of the vegetative cells of brown algae. The probable role of AFs during the cell cycle and particularly the unique actin configuration involved in cytokinesis are discussed.