Mapping of highly ordered membrane domains in the plasma membrane of the ciliate Cyclidium glaucoma

Abstract

The entire cortex of Cyclidium glaucoma has been analysed in a correlated freeze-fracturing and ultrathin-sectioning study. Three types of highly ordered domains of intramembranous particles were found in the plasma membrane; owing to the many landmarks in the cortex of C. glaucoma it was possible to map their location in relation to the precisely known position of every single cilium. Eight particle plates made of parallel tripartite particle ridges have been found in precisely determined locations at the anterior pole of the cell and in the buccal cavity. It is suggested that these membrane specializations represent mechanosensitive sites. An array of three parallel rows of particles, termed a ‘simple pellicular rug’, forms a second type of membrane domain, which is closely associated with somatic dikinetids found exclusively in the anterior half of the cell. A third extremely complex array, termed a ‘complex pellicular rug’, is associated with the monokinetids in the posterior half of the cell. There is no gradient in the distribution of these domains but a sharp boundary at the level of the future fission zone. The function of the pellicular rugs is unknown, but their specific distribution will be helpful in further studies on their assembly prior to or during cell division. The intracellular diversification of the cell surface is further augmented by the disclosure of three different aspects of the proximal part of the ciliary membrane. Beyond the double-stranded ciliary necklace, which is a common feature of all cilia, only the motile somatic cilia of C. glaucoma have typical ciliary plaques made of three rows of particles, while all oral cilia have modified plaques, which consist of four rows of particles. the single stiff caudal cilium has no plaques at all. The functional significance of the topographic differences in the plaque areas is discussed. Obviously, the plasma membrane and at least parts of the ciliary membrane form a non-fluid mosaic membrane.