A rhoptry is a specialized

secretory Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classical mec ...

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

. They are club-shaped

s connected by thin necks to the extreme apical pole of the parasite. These

s, like

microneme Micronemes are secretory organelles, possessed by parasitic apicomplexans. Micronemes are located on the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electron microscopy they have an electron-dense ma ...

s, are characteristic of the motile stages of

Apicomplexa The Apicomplexa (also called Apicomplexia; single: apicomplexan) are organisms of a large phylum of mainly parasitic alveolates. Most possess a unique form of organelle structure that comprises a type of non-photosynthetic plastid called an ap ...

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

ns. They can vary in number and shape and contain numerous

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s that are released during the process of host penetration. The proteins they contain are important in the interaction between the host and the parasite, including the formation of the parasitophorous vacuole (PV).

Characteristics

Rhoptries are one of the three characteristic secretory

s present in all

along with

s and dense granules. Rhoptries and

s are localized at the apical complex of the

n organism, which suggests common ancestry of the members of the phylum and the evolution process they have experienced. The name rhoptry indicates its shape as it comes from the Greek word for “club-shaped.” These large membrane-bound

organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

are electron-dense and highly acidic and have similar high density across those in Apicomplexan species. There is a variation in the number of rhoptries present in different species and during different developmental stages. For example, the

tachyzoite 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 ...

stage of ''

Toxoplasma gondii ''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...

,'' which is found during the acute phase of

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 month ...

, has 10 to 12 rhoptries, while the bradyzoite stage observed during the chronic phase of the infection has one to three rhoptries. ''

Plasmodium falciparum ''Plasmodium falciparum'' is a Unicellular organism, unicellular protozoan parasite of humans and is the deadliest species of ''Plasmodium'' that causes malaria in humans. The parasite is transmitted through the bite of a female ''Anopheles'' mos ...

'' merozoites have two, the sporozoites have two to four, and the noninvasive ookinetes have none. Meanwhile, ''

Cryptosporidium ''Cryptosporidium'', sometimes called crypto, is an apicomplexan genus of alveolates which are parasitism, parasites that can cause a respiratory and gastrointestinal illness (cryptosporidiosis) that primarily involves watery diarrhea (inte ...

'' sporozoites only have a single rhoptry.

Structure and content

Rhoptry mainly comprises two regions: the rhoptry neck and the rhoptry bulb. Those two regions physically divide the parasites and have different features and materials. The rhoptry neck is an electron-dense duct that narrowly extends at the

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

tip and contains the rhoptry neck proteins (RONs), which are named after where they localize in the parasite. On the other hand, the rhoptry bulb is a larger, bulbous base that is electron-lucent and contains the rhoptry bulb proteins (ROPs) and membranous materials. So far, eight rhoptry bulb proteins, ROP1 through ROP8, have been identified in '' T. gondii''. Those two classes of proteins, RONs and ROPs, follow the typical

secretory pathway Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell (biology), cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. Th ...

from the

endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryote, eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for ...

to the Golgi, then finally, where they are normally stored, the rhoptry. They have critical functions in the host invasion and replication within the host of

n parasites. During the host invasion process, the proteins are secreted at different times at which they each function. RONs are exocytosed first because they contribute during the invasion. ROPs follow afterward and perform a post-invasion role.

Synthesis

''De novo'' assembly of rhoptries occurs during cell replication. They are first synthesized as pre-rhoptries, which are spherical-shaped, trans-Golgi-derived vesicles. Yet, how these immature rhoptries are formed is still unknown. Pre-rhoptries elongate and mature into the functional rhoptries just before

cytokinesis Cytokinesis () is the part of the cell division process and part of mitosis 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 ...

, which then move to the apexes of the parasites to localize to their normal position—the apical complexes.

Functions

The three unique secretory

—

s, rhoptries, and dense granules—release their contents by

exocytosis Exocytosis is a term for the active transport process that transports large molecules from cell to the extracellular area. Hormones, proteins and neurotransmitters are examples of large molecules that can be transported out of the cell. Exocytosis ...

at different stages of the host invasion as the process is regulated in time and space.

Microneme Micronemes are secretory organelles, possessed by parasitic apicomplexans. Micronemes are located on the apical third of the protozoan body. They are surrounded by a typical unit membrane. On electron microscopy they have an electron-dense ma ...

contents are secreted first to the apical end of the parasite when the parasite attaches to the host cell, followed by rhoptry as invasion proceeds, and then dense granules near post-invasion. The micronemal proteins secreted to the parasite’s surface direct the rhoptry proteins to the host cell by forming complexes together. The rhoptry proteins then localize to different locations within the host cell, including the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

, the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

, the nucleus, the parasitophorous vacuole membrane (PVM), and the PV lumen. The primary functions of rhoptries are to assist host invasion and to exploit host cellular functions for enhanced

parasitism Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...

. Still, the specific roles differ depending on where they localize within the host upon direct injection into the host

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

and on the host

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

. During the initial stage of host invasion, rhoptry contents help the parasite attach to the host, and the rhoptry membranous material forms the PVM around the parasite entering the host cell to establish its protective

intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...

protective compartment for successful development by inducing

invagination Invagination is the process of a surface folding in on itself to form a cavity, pouch or tube. In developmental biology, invagination of Epithelium, epithelial sheets occurs in many contexts during Animal embryonic development, embryonic developme ...

of its

. In ''

Plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a Hematophagy, blood-feeding insect host (biology), host which then inj ...

'', some rhoptry proteins localize to the PVM and promote the formation of the

vacuole A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...

n parasites also utilize rhoptries to divert the host cell’s

immune response An immune response is a physiological reaction which occurs within an organism in the context of inflammation for the purpose of defending against exogenous factors. These include a wide variety of different toxins, viruses, intra- and extracellula ...

. The host can come to favor the parasitic invasion if the rhoptry proteins manipulate the host’s actin cytoskeleton. Furthermore, rhoptry proteins in ''

'' can mistraffic the host’s immune factors for its

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...

. Another function of rhoptry proteins is nutrient import during the lytic cycle of

References

{{Protist Organelles