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Protozoa Apicomplexa Sarcomastigophoraciliophora Apicomplexa Protozoa Apicomplexa Sarcomastigophora Ciliophora Gregarinea Coccidia Piroplasma Introduction to the Apicomplexa: Parasitic, pathogenic protists In traditional protist taxonomy most parasitic protists were placed in the class Sporozoa. This group has since been found to include protists from a number of unrelated lineages, and has been dropped from current usage. However, many of the protists in the old Sporozoa share certain structural features, in particular an apical complex of microtubules within the cell. These protists have now been grouped in the Apicomplexa, probably the largest and best- known taxon of parasitic protists. There are about 4,000 known species, but this is almost certainly a gross underestimate of the actual number. This group is a very important: Apicomplexans infect both invertebrates and vertebrates; they may be relatively benign or may cause serious illnesses. Species in the genus Plasmodium cause malaria in humans and other animals. Other apicomplexans cause serious illnesses, such as coccidiosis and toxoplasmosis, in humans and domestic animals. • Diverse group • Three classes in this taxon: Gregarinia, Coccidia, Hematozoa. • All are Parasitic - endoparasitic • Most have complex life cycles; many have a spore stage. • associations range from being almost avirulent to very virulent pathogens • No obvious external organelles involved in locomotion • No cilia or flagella, except for gametes • Asexual and sexual reproduction. • Include parasites with huge impact on human and animal health. Classification of Apicomplexa Structure of apical complex Location of various stages within host(s) Structure of tissue cyst Oocyst size and shape Number of sporocysts and sporozoites Molecular data APICOMPLEXA The Apicomplexas form a unique group because all known members are parasitic. The common feature of all members is the presence of an apical complex in one or more stages of the life cycle. Although the exact components of the apical complex varies among members. The apical complex is found in stages such as the sporozoite and merozoite that must enter host cells. The apical complex consists of one or two electron dense polar rings at the anterior end of the cell, a conoid (seen in some coccidia) that lies within the polar ring, and two or more rhoptries located within the polar ring extending posteriorly from the plasma membrane. Micronemes lie parallel to the rhoptries and often merge with them at the apex of the cell. Rhoptries and micronemes probably are secretory organelles and facilitate penetration into host cell. Rhoptries and Micronemes are secretory: important in invasion of host cells Microtubules: support-these disappear after parasite is established in the host cell. Apicomplexa Unique cellular or anatomical features Apical complex. Consists of: Polar rings - one or more electron dense structures Conoid - hollow truncated cone composed of a number of spirally coiled microtubules. Rhoptries - electron dense long-necked bags which may function in cell penetration and nutrient transport. Micronemes - short, rod-like organelles located in large numbers in the cytoplasm that are crucial to the attachment of the parasite to the host cell during invasion. Subpellicular microtubules - organelles that run backward from the polar rings. Dense granules - spherical organelles containing proteins that are released after invasion to function in calcium binding and formation of cysts. No flagella or cilia except for flagellated microgametes in some groups. At some point during their life cycle, members of the apicomplexa either invade or attach to host cells. The apical organelles play a role in these host- parasite interactions. 1- the apical complex Function...??? - attachment and penetration of cell Apicomplexans have complex life cycles, and there is much variation among different apicomplexan groups. Both asexual and sexual reproduction are involved, although some apicomplexans skip one or the other stage. The basic life cycle starts when an infective stage, or sporozoite, enters a host cell, and divides repeatedly to form numerous merozoites. Some of the merozoites transform into sexually reproductive cells, or gamonts. Gamonts join together in pairs and form a gamontocyst. Within the gamontocyst, the gamonts divide to form numerous gametes. Pairs of gametes then fuse to form zygotes, which give rise by meiosis to new sporozoites, and the cycle begins again. Apicomplexans are transmitted to new hosts in various ways; some, like the malaria parasite, are transmitted by infected mosquitoes, while others may be transmitted in the feces of an infected host, or when a predator eats infected prey Typical Sporozoite: Some facts: •Rhoptries, micronems and conoid form the 'apical complex' which is important for invading the host cell. •Number of organelles of the apical complex differ from species to species •Refractile bodies contain lipid materials probably for incorporation in the parasitophorous vacuole The apicomplexa generally have complex life cycles that are characterized by three distinct processes: sporogony, merogony and gametogony. Sporogony occurs immediately after a sexual phase and consists of an asexual reproduction that culminates in the production of sporozoites. Sporozoites develop into forms that undergo another asexual replication known as merogony (also called schizogony). Quite often there are multiple rounds of merogony and sometimes these multiple rounds involve a switch in host or cell type. Similarly, sporogony and gametogony can involve different hosts or cell types. As an alternative to asexual replication merozoites can become gametes through a process variously called gametogony. As in other types of sexual reproduction, the gametes fuse to form a zygote which differentiates into a form yielding sporozoites. The sporozoites and merozoites are 'invasive' forms and possess the apical organelles. Although most apicomplexa exhibit this overall general life cycle the details vary greatly between species. Merozoite Trophozoite resulting from schizogony (merogony) - asexual Oocyst Encapsulated form resulting from gamete union. The diploid zygote undergoes sporogony to produce haploid sporozoites, generally inside sporocysts. The oocyst which may or may not be contained within a thick, resistant spore wall. Sporozoite Trophozoite (usually inside a sporocyst) contained within the oocyst. Sporocyst Cystic product of sporogony. The zygote undergoes sporogony to form sporozoites inside sporocysts, which are usually inside an oocyst. trophozoites Merogony (schizogony) merozoites Oocysts (sporozoites) sporogony zygote gametogony gametes Plasmodium Early malarial merozoite infection of red blood cells, crescent shapes. Malarial sporozoites in mosquito salivary gland smear. The means by which this group of organisms acquires energy/nutrition -Along the side of the organisms are micropores / cytostome. -The edges of the micropore / cytostome have 2 concentric electron- dense rings immediately below the cell membrane -As host cytoplasm or other food within a parisitophorous vacuole is pulled through these rings the cell membrane invaginates and pinches of to form a food vacuole. -Apicomplexans parasitize the metabolic machinery of the host cells. -ATP is required -Nutrients are selectively imported The impact of Apicomplexa on other groups of organisms. Malaria, caused by infections of the Plasmodium parasite, is one of the most serious diseases in the world. Sporozoite-merozoite-gamete Much research on the development of a vaccine. The gene encoding the sporozoite antigen has been cloned but it is not certain how effective a vaccine against sporozoites might be. When a mosquito inserts its proboscus into a human, it injects about a thousand sporozoites. They travel to the liver within a few minutes, where they are no longer exposed to antibodies circulating in the blood. Merozoites- inside red blood cells, rupture, invade new cells Gametes- present in RBC and in blood What stage do you target- sporozoites, merozoites or gametocytes? Plasmodium sp. - Gametocytes de P. falciparum and P. vivax. .
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