Apicomplexa
Protozoa
Apicomplexa
Gregarinea
Sarcomastigophora
Coccidia
Piroplasma
Ciliophora
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 bestknown 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.
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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 hostparasite 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)
Oocysts (sporozoites)
sporogony
zygote
gametogony
gametes
merozoites
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 electrondense 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.