Structure of Plasmodium Merozoite
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Structure of Plasmodium Merozoite
The Plasmodium Merozoite is an ovoid cell and measures approximately 1.5 micron in length and 1 micron in width. The apical end of the Merozoite is a truncated cone-shaped projection demarcated by the polar rings. At the anterior end of the Merozoite are present three types of membrane-bound organelles, namely, Rhoptries (two prominent pear-shaped, 570 X330 nm), micronemes (ovoid bodies, 100 X40 nm), and dense granules (spheroid vesicles, 140 X120 nm). The contents of these organelles play a role in the binding and entry of the Merozoite into the host cells. Extracellular Merozoites are intrinsically short-lived and must rapidly invade a new host erythrocyte (Ref.1). The Merozoite is surrounded by a trilaminar pellicle that is composed of a plasma membrane and two closely aligned inner membranes. The plasma membrane measures about 7.5 nm in thickness. Just beneath this inner membrane complex is a row of subpellicular Microtubules, which originate from the polar ring of the Apical end and radiate posteriorly. The inner membrane complex and subpellicular Microtubules function as a cytoskeleton giving rigidity to the Merozoite and may be involved in invasion. MyosinA is found to be present in the apex of the mature Merozoites, and is involved in Merozoite motility during invasion. The outer membrane of the extracellular Merozoite is covered with a surface coat of about 20 nm in thickness, and plays an important role in the early stages of Merozoite invasion. A single mitochondrion is generally present at the posterior end of the Merozoite. Mitochondrion are generally acristae or with very few cristae. An additional structure, referred to as a spherical body, has also been identified. The plastid of P. falciparum (or ‘apicoplast’) is believed to be the evolutionary homologue of the plant chloroplast. The apicoplast is surrounded by four membranes and is likely to contain many prokaryote-type pathways. Golgi complexes are inconspicuous in the Merozoite. A single vesicular nucleus with a centrally located nucleolus is also present in Merozoite (Ref.2).

In general the Merozoites, once released into the blood, infect the red cells. However in the case of P. vivax and P.ovale, some Merozoites re-infect liver cells and lie dormant and are called hypnozoites. Identification and invasion of Merozoites in RBCs (Red Blood Cells) has long been a focus of research. The Merozoite first attaches to a RBC at any point on its surface, and then reorients to bring its apical end into contact with the RBC. The initial attachment stages are reversible, and Merozoites disassociates and attach to a new potential target cell. The subsequent steps are irreversible, and involve the formation of an electron-dense adhesion zone between the apical end of the Merozoite and the RBC. This zone then moves around the Merozoite toward its posterior end, with a concurrent invagination of the RBC membrane and entry of the Merozoite. The rhoptries play an important role in the process of infecting the RBC. The Merozoites attach to the surface of the red cell by a special binding receptor, such that rhoptries point towards the cell. The molecular adhesion details behind this tantalizing outline are sketchy. The MSPs (Merozoite Surface Proteins) together make up a structurally complex coat around the outer membrane of the Merozoite and have an important role in the initial reversible adhesive interaction between the Merozoite and the RBC. The P. vivax and Plasmodium knowlesi DBPs (Duffy Binding Proteins) and their P. falciparum orthologue, EBA175 (Erythrocyte Binding Antigen-175) also binds to well-defined glycoprotein motifs on the RBC membrane. AMA1 (Apical Membrane Antigen-1) and MAEBL, a chimeric protein containing features of both the DBP/EBA175 proteins and AMA1, also play a role in RBC invasion, but their precise functions are as yet unclear. Two P. falciparum orthologues of PvRBP2, Plasmodium falciparum RBP2 homologues a and b (PfRBP2-Ha and -Hb), together with the RBP2 of P. vivax and the P. yoelii 235-kDa proteins, constitute an important Plasmodium family that plays a pivotal role in the early phases of Merozoite invasion (Ref.3). Components of the low molecular mass Rhoptry Complex, the RAP (Rhoptry-Associated Proteins) 1, 2 and 3, also occur in Merozoites. The high molecular mass Rhoptry protein Complex (RhopH), together with RESA (Ring-infected Erythrocyte Surface Antigen), which is a component of dense granules, is transferred intact to new Erythrocytes at or after Invasion and may contribute to the host cell remodeling process (Ref.4).

Within the RBC, the Merozoite develops to form either an Erythrocytic stage (blood-stage) schizont (by the process of erythrocytic schizogony) or a spherical or banana shaped, uninucleate Gametocyte. The mature Erythrocytic stage Schizont contains 8 to 36 Merozoites, each 5 to 10 µm long, which are released into the blood when the schizont ruptures. These Merozoites proceed to infect another generation of erythrocytes. The time required for erythrocytic schizogony, which determines the interval between the release of successive generations of Merozoites varies with the species of Plasmodium and is responsible for the classic periodicity of fever in malaria (Ref.5).