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.