Surface properties of extracellular malaria parasites: morphological and cytochemical study

Morphological and cytochemical surface characteristics of isolated malaria parasites (Plasmodium berghei) and host erythrocytes were compared by electron microscopy by using thin section and carbon replica techniques. Erythrocytes were uniform in shape and had fine, granular surfaces. In contrast, free parasites exhibited a variety of sizes, shapes, and surface textures. Fine surface stippling was a common topographical feature of isolated parasites. Small, infective forms often had patterned surfaces resulting from the protuberance of an underlying thick intermediate layer. Results of cytochemical analysis using a sialophilic colloidal iron stain indicated that the malaria parasite's surface lacked exposed sialic acid groups which would normally give rise to a net negative surface charge common to erythrocytes. Biochemical assay demonstrated that malaria parasites contained about one-half the amount of sialic acid per unit weight as did control red cell extracts. Similarly, external acidic mucopolysaccharide coats of free parasites, as revealed by ruthenium red staining were extremely thin as compared with the thick glycocalyx layer of red cells. Lipid plaques at the surface of parasites and red cells were localized by lipophilic iron colloid staining. Although the gross patchwork distribution of plaques was somewhat similar for the two cell types, the parasites were stained more intensely and had a closer-knit patchwork pattern than those exhibited by the erythrocytes. Such findings indicate that there are slight differences in the arrangement of phospholipids at the surfaces of limiting membranes of host cells and parasites. The significance of the above cytochemical surface properties of the malaria parasite (which are seemingly akin to those of intracellular organelles is discussed in relation to certain host-parasite interactions, such as parasite adhesion to target cells and enhanced clearance of extracellular parasites.