Many say the human malaria parasites, five species of the genus Plasmodium, are host to an inherently complex and complicated life cycle. When reading a description of the various stages of the protozoan’s life, this would certainly appear the truth—each form is visually different; home to detailed mechanisms of transformation; subject to alien terminologies, words ending in –cyte, processes like schizogony.
Merozoites of Plasmodium infecting red blood cells. Image courtesy of National Geographic.
As an example, find below a description of the lives of Plasmodium, found in a 2010 review focusing on the history of how the parasite, its transmission via the mosquito vector, and its pathogenesis were discovered.
Infection begins when (1) sporozoites, the infective stages, are injected by a mosquito and are carried around the body until they invade liver hepatocytes where (2) they undergo a phase of asexual multiplication (exoerythrocytic schizogony) resulting in the production of many uninucleate merozoites. These merozoites flood out into the blood and invade red blood cells where (3) they initiate a second phase of asexual multiplication (erythrocytic schizogony) resulting in the production of about 8-16 merozoites which invade new red blood cells. This process is repeated almost indefinitely and is responsible for the disease, malaria. As the infection progresses, some young merozoites develop into male and female gametocytes that circulate in the peripheral blood until they are (4) taken up by a female anopheline mosquito when it feeds. Within the mosquito (5) the gametocytes mature into male and female gametes, fertilization occurs and a motile zygote (ookinete) is formed within the lumen of the mosquito gut, the beginning of a process known as sporogony. The ookinete penetrates the gut wall and becomes a conspicuous oocyst within which another phase of multiplication occurs resulting in the formation of sporozoites that migrate to the salivary glands of a mosquito and are injected when the mosquito feeds on a new host.
The process becomes somewhat clearer with the aid of the following simple cyclical diagram.
Life cycle of the Plasmodium parasite
But even then, there’s still mystique to the organism. We now know that sporozoites are the infective stage of Plasmodium, that they are injected into the human body by the mosquito’s proboscis, and that they become merozoites through exoerythrocytic schizogony in the liver, specifically in the hepatocyte cells. We now know that these same merozoites invade red blood cells, the erythrocytes, and undergo another process of multiplication known as erythrocytic schizogony. We now know that the replication of merozoites continues in the erythrocytes, and that some of these develop into male and female gametocytes. We now know that these move throughout the bloodstream until they are taken up by another feeding mosquito, and that within the vector the gametocytes develop again into gametes, fertilize, and undergo sporogony, the process of ookinete development and the eventual production of new sporozoites, completing the circle. But words like exoerythrocytic, ookinete, and schizogony are rather abstract—much in like the anthropological critique of popular reliance on statistics, the abstraction often obscures rather than illuminates form. As statistics seem to hide the individual, a voice, a face, a story, the technical description of Plasmodium‘s journey and fate leaves our imagination empty as to what a sporozoite actually looks like, if merozoites are larger or smaller than their life-history precursor, how the gametocytes move in the blood. In short, we can’t really picture Plasmodium. And if we can’t picture the organism, then all these processes, which are so detailed and meticulous in containing the What’s of each stage, become rather shallow. If we are told how photosynthesis works, but haven’t seen a leaf much less a chloroplast, knowing the process isn’t of much use. Even without considering this protozoa, it isn’t hard to imagine the conundrum of how this visualizing and understanding applies to microorganisms.
Luckily, we have researchers not only hard at work, but also committed to—in contrast to the popular saying—seeing the trees, not just the forest. The following video comes from DNAtube, a fantastic scientific video site, where you can find detailed visuals in motion of a range of biological processes and phenomena. Although the narrator of the video comments that the life cycle of Plasmodium is “very complex,” the visualization asserts the opposite: the life of the organism is not inherently simple—it is complex is nature—but it can be displayed and explained in simple form, perhaps even in ways that exhibit beauty of sorts.