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Apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...
ns, a group of intracellular parasites, have
life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...
stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an
apicomplexa The Apicomplexa (also called Apicomplexia) are a large phylum of parasitic alveolates. Most of them possess a unique form of organelle that comprises a type of non-photosynthetic plastid called an apicoplast, and an apical complex structure. Th ...
n organism is typified by a ''cellular variety'' with a distinct
morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...
and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...
. Not all apicomplexa develop all the following cellular varieties and division methods. This presentation is intended as an outline of a hypothetical generalised apicomplexan organism.


Methods of asexual replication

Apicomplexans (sporozoans) replicate via ways of
multiple fission Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how ...
(also known as schizogony). These ways include , and , although the latter is sometimes referred to as schizogony, despite its general meaning. Merogony is an asexually reproductive process of apicomplexa. After infecting a host cell, a trophozoite ( see glossary below) increases in size while repeatedly replicating its nucleus and other organelles. During this process, the organism is known as a or .
Cytokinesis Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and mei ...
next subdivides the
multinucleated Multinucleate cells (also known as multinucleated or polynuclear cells) are eukaryotic cells that have more than one nucleus per cell, i.e., multiple nuclei share one common cytoplasm. Mitosis in multinucleate cells can occur either in a coordinat ...
schizont into numerous identical daughter cells called merozoites ( see glossary below), which are released into the blood when the host cell ruptures. Organisms whose life cycles rely on this process include '' Theileria'', '' Babesia'', '' Plasmodium'', and '' Toxoplasma gondii.'' Sporogony is a type of sexual and asexual reproduction. It involves karyogamy, the formation of a
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
, which is followed by
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
and
multiple fission Fission, in biology, is the division of a single entity into two or more parts and the regeneration of those parts to separate entities resembling the original. The object experiencing fission is usually a cell, but the term may also refer to how ...
. This results in the production of sporozoites. Other forms of replication include and . Endodyogeny is a process of asexual reproduction, favoured by parasites such as '' Toxoplasma gondii''. It involves an unusual process in which two daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation. Endopolygeny is the division into several organisms at once by internal
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...
.


Glossary of cell types


Infectious stages

A (ancient Greek ', seed + ', animal) is the cell form that infects new hosts. In '' Plasmodium'', for instance, the sporozoites are cells that develop in the mosquito's salivary glands, leave the mosquito during a blood meal, and enter
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it ...
cells (
hepatocyte A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass. These cells are involved in: * Protein synthesis * Protein storage * Transformation of carbohydrates * Synthesis of cholesterol, ...
s), where they multiply. Cells infected with sporozoites eventually burst, releasing merozoites into the bloodstream. Sporozoites are motile and they move by gliding. A (G. ', part f a series+', animal) is the result of
merogony Apicomplexans, a group of intracellular parasites, have life cycle stages that allow them to survive the wide variety of environments they are exposed to during their complex life cycle. Each stage in the life cycle of an apicomplexan organism i ...
that takes place within a host cell. During this stage, the parasite infects the host's cells and then replicates its own nucleus and induces cell segmentation in a form of asexual reproduction. In
coccidiosis Coccidiosis is a parasitic disease of the intestinal tract of animals caused by coccidian protozoa. The disease spreads from one animal to another by contact with infected feces or ingestion of infected tissue. Diarrhea, which may become bloody i ...
, merozoites form the first phase of the internal life cycle of coccidian. In the case of '' Plasmodium'', merozoites infect
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s and then rapidly reproduce asexually. The red blood cell host is destroyed by this process, which releases many new merozoites that go on to find new blood-borne hosts. Merozoites are motile. Before schizogony, the merozoite is also known as the schizozoite. A ( G. ', partner + ', cell) is a name given to a parasite's
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
-forming cells. A male gametocyte divides to give many flagellated microgametes, whereas the female gametocyte differentiates to a macrogamete. An (G. ', egg + ', motile) is a fertilised
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
capable of moving spontaneously. It penetrates epithelial cells lining the midgut of
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
es to form a thick-walled structure known as an oocyst under the mosquito's outer gut lining. Ookinetes are motile and they move by gliding. A (G. ', nourishment + ', animal) is the activated, intracellular feeding stage in the apicomplexan life cycle. After gorging itself on its host, the trophozoite undergoes schizogony and develops into a schizont, later releasing merozoites. A hypnozoite (G. ', sleep + ', animal) is a quiescent parasite stage that is best known for its "... probable association with latency and relapse in human malarial infections caused by ''Plasmodium ovale'' and ''P. vivax''". Hypnozoites are directly sporozoite-derived. A (G. ', slow + ', animal) is a sessile, slow-growing form of
zoonotic A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s such as '' Toxoplasma gondii'', among others responsible for parasitic infections. In chronic (latent)
toxoplasmosis Toxoplasmosis is a parasitic disease caused by '' Toxoplasma gondii'', an apicomplexan. Infections with toxoplasmosis are associated with a variety of neuropsychiatric and behavioral conditions. Occasionally, people may have a few weeks or mont ...
, bradyzoites microscopically present as clusters enclosed by an irregular crescent-shaped wall (
cyst A cyst is a closed sac, having a distinct envelope and division compared with the nearby tissue. Hence, it is a cluster of cells that have grouped together to form a sac (like the manner in which water molecules group together to form a bubble) ...
s) in infected muscle and brain tissues. Also known as a bradyzoic merozoite. A (G. ', fast + ', animal), contrasting with a bradyzoite, is a form typified by rapid growth and replication. Tachyzoites are the motile forms of those coccidians which form tissue
pseudocyst Pseudocysts are like cysts, but lack epithelial or endothelial cells. Initial management consists of general supportive care. Symptoms and complications caused by pseudocysts require surgery. Computed tomography (CT) scans are used for initial ima ...
s, such as '' Toxoplasma'' and ''
Sarcocystis ''Sarcocystis'' is a genus of protozoan parasites, with many species infecting mammals, reptiles and birds. Its name is dervived from Greek ''sarx'' = flesh and ''kystis'' = bladder. The lifecycle of a typical member of this genus involves two ...
''. Typically infecting cellular
vacuole A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic m ...
s, tachyzoites divide by endodyogeny and endopolygeny. Also known as a tachyzoic merozoite (same journal reference as for "bradyzoic merozoite", above). An (G. ', egg + ', bladder) is a hardy, thick-walled spore, able to survive for lengthy periods outside a host. The
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...
develops within the spore, which acts to protect it during transfer to new hosts. Organisms that create oocysts include ''
Eimeria ''Eimeria'' is a genus of apicomplexan parasites that includes various species capable of causing the disease coccidiosis in animals such as cattle, poultry and smaller ruminants including sheep and goats. ''Eimeria'' species are considered to be ...
'', ''
Isospora ''Isospora'' is a genus of internal parasites in the subclass Coccidia. It is responsible for the condition isosporiasis, which causes acute, non-bloody diarrhoea in immunocompromised individuals. Taxonomy At least 248 species were originall ...
'', '' Cryptosporidium'', and '' Toxoplasma''. File:Babesia life cycle human en.svg, Lifecycle of the '' Babesia'' parasite File:Eimeria life cycle usda.jpg, Lifecycle of the ''
Eimeria ''Eimeria'' is a genus of apicomplexan parasites that includes various species capable of causing the disease coccidiosis in animals such as cattle, poultry and smaller ruminants including sheep and goats. ''Eimeria'' species are considered to be ...
'' parasite File:Toxoplasmosis life cycle en.svg, Lifecycle of the '' Toxoplasma'' parasite


See also

*
Trematode lifecycle stages Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing sec ...


References

{{reflist Apicomplexa Reproduction