A fine structure analysis is presented of the sequential development of erythrocytic stages of Plasmodium fallax, P. lophurae and P. cathemerium.
Merozoites of these plasmodia showed a highly complex structure including many organelles, such as conoid, paired organelles, dense bodies, spherical body, cytostome, pellicular complex, nucleus, mitochondrion, ribosomes and endoplasmic reticulum. The conoid is a cone-shaped structure which is located anteriorly and may possibly function as a perforator when the merozoite invades a host cell. The paired organelles are two osmophilic structures of tear-drop shape, located near the conoid which may secrete proteolytic enzymes through the conoid. The dense bodies are scattered around the paired organelles and it is presumed that these bodies are functionally related to the paired organelles. A spherical body is situated posteriorly and is always associated with the mitochondrion in such a way that it may conceivably be a source of energy to the mitochondrion. The cytostome is a pellicular depression which is situated midway between the anterior and posterior ends of the parasites and is the place through which a developing trophozoite ingests red cell cytoplasm.
The pellicular complex is composed of three distinct layers: a thin outer membrane, a thick interrupted inner membrane, and a layer of microtubules interior to both membrane systems. The inner interrupted membrane has a pattern resembling the mesh of chicken wire and may give rigidity to the merozoite. Microtubular structures which radiate posteriorly from a conoid probably function in the motility of the parasite.
A nucleus is centrally located and its nucleoplasm is electron translucent with scattered particles. A mitochondrion is always situated posteriorly and is a crescent-shaped structure with a microtubular cristae, characteristic of protozoa. The endoplasmic reticulum is well developed and is associated with ribosomes.
The capacity of erythrocytic merozoites to survive briefly in the free form, their motility and ability to enter a new host cell is probably related to certain organelles such as conoid, paired organelles, dense bodies and pellicular complex. When a merozoite infects a new host cell, it rounds up and loses much of its organelles, retaining only a nucleus, mitochondrion, endoplasmic reticulum, ribosomes, cytostome and a spherical body. As the parasite grows, the spherical body enlarges and becomes vacuolated and the mitochondrion increases in number.
Through a series of successive nuclear divisions and a simultaneous increase in cytoplasmic components, a uninuclear trophozoite grows into a schizont. The nuclear division of the erythrocytic stages of avian malarial parasites is more than simple nuclear fission and requires the development of spindle fibers and probably chromosomal structures and the local disruption of nuclear membrane. After the completion of a few nuclear divisions, merozoites start to form. Various organelles are drawn from the mother schizont into the budding merozoites and finally 10 to 15 merozoites are formed from an original single parasite, leaving a residual body with malarial pigments behind.