Apical cells of brown algae with particular reference to Sphacelariales, Dictyotales and Fucales

Brown algae show a significant diversity in thallus forms, giving a great number of model systems for the study of many important morphogenetic mechanisms. Thallus growth in brown algae is diffuse, intercalary or apical. The latter takes place by means of one or more apical cells. Among the brown algal groups, Sphacelariales, Dictyotales and Fucales give the best examples of apical growth, and have been repeatedly used for the study of the morphogenetic role of apical cells. In Sphacelariales the apical cells appear strongly polarized, the polarity expressed also on the organization of the microtubule cytoskeleton. These cells show a type of growth that can be compared with tip growth of root hairs, moss protonemata, pollen tubes and fungal hyphae, and is called ‘tip-like growth’. The thallus of Dictyotales grows by the activity of one or more apical cells showing variable degree of polarity. These cells do not exhibit any type of apical growth. In Fucales the vegetative thallus develops by means of an active apical meristem, which includes a large apical cell. This cell does not show polar organization or apical growth. However, in germinating zygotes of Fucales a polar axis is established and during the first stages of development they show a typical tip growth. In the present paper, the available information on the structure and division pattern of apical cells is presented. Their morphogenetic role is discussed, in relation to polarity, cytoskeleton organization, and apical dominance.