Cestodes: general characterization of cestodes

Cestodoses are diseases caused by pathogens belonging to the class Cestoidea.

Of medical importance are mainly representatives of two orders: tapeworms - Pseudophyllidea and tapeworms - Cyclophyllidea, which belong to the subclass of true tapeworms (Eucestoda).

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Structure of cestodes

The body of cestodes (from the Greek cestos - belt, ribbon) is usually ribbon-shaped, flattened in the dorsoventral direction, consists of a head (scolex), neck and strobila, divided into segments (proglottids). The length of the entire cestode, depending on the species, can vary from a few millimeters to 10 m or more, and the number of proglottids - from one to several thousand. In tapeworms, the scolex is more or less rounded, has four suckers with muscular walls. At the top of the scolex is a muscular outgrowth - a proboscis, bearing weapons in the form of one or more rows of hooks. The number, size, shape and location of the hooks are important for determining the species of tapeworms. In tapeworms, the scolex is elongated, equipped with two suction pits (bothria). Behind the scolex is a narrow, short, unsegmented section of the body - the neck, which serves as a growth zone. Young segments bud off from it, as a result of which the older ones gradually move towards the back of the strobila.

The body of cestodes is covered with a skin-muscle layer (skin-muscle sac), consisting of a cuticle and subcuticle. The cuticle is a dense non-cellular formation on the surface of epithelial tissue cells. It consists of three layers: the outer one, containing keratin, the middle one - cytoplasmic, rich in proteins and lipids, and the inner one - fibrous or basal. Keratin, together with mineral substances and proteins, gives the cuticle mechanical strength; lipids contribute to its water resistance. Due to the resistance of the cuticle to the action of the host's enzymes and the release of substances through it that neutralize the effect of enzymes, cestodes can exist in the aggressive environment of the intestines of humans and vertebrates. The cuticle is covered with villus-like outgrowths - microtrichia, which come into close contact with the microvilli of the intestinal mucosa, which helps to increase the efficiency of absorption of nutrients. The subcuticle contains a layer of submerged epithelial cells, as well as an outer annular and inner longitudinal layer of smooth muscle fibers.

Inside, the body of cestodes is filled with parenchyma, consisting of large irregularly shaped cells, the processes of which are intertwined with each other. In the superficial layers of the parenchyma are single-celled skin glands, as well as reserves of nutrients - proteins, lipids and glycogen. The latter is of great importance in the processes of anaerobic respiration. Here also lie the "calcareous bodies" containing phosphates and carbonates of calcium and magnesium, with the participation of which the buffer properties of the environment are regulated.

The excretory, nervous and reproductive systems are located in the deeper layers of the parenchyma. The digestive, respiratory and circulatory systems are absent. Nutrition is provided through the body's integuments.

Excretory systemThe cestodes are constructed according to the protonephridial type. They consist of numerous cells with a "flickering flame" and thin canals, which, connecting with each other, flow into large lateral longitudinal excretory canals. These canals in each segment are connected with each other by a posterior transverse canal. When the segment breaks off, the lateral excretory canals open outward on the surface of the break.

The nervous system consists of longitudinal nerve trunks, the largest of which are lateral. In the scolex, they are connected by transverse commissures, which are connected to a rather complex head ganglion. The sense organs are not developed.

Reproductive systemalmost all cestodes are hermaphroditic. Most of them have very complex genitals. The hermaphroditic genital apparatus is repeated in each proglottid. The first segments that bud off from the neck do not yet have a genital apparatus. As the strobila grows and the segments move away from the neck, the organs of the male genital system are formed in them, consisting in most species of numerous testes that look like vesicles scattered in the parenchyma of the segment. From them, the seminal tubules extend, flowing into the vas deferens, ending in the copulatory organ (cirrus), which is located in the genital bursa (bursa cirri). The genital bursa opens, as a rule, on the lateral (sometimes on the ventral) side of the segment on the genital tubercle in a special depression called the genital cloaca.

Later, a more complex female reproductive system appears. The female genital opening is located in the genital cloaca next to the male. It leads into a narrow vaginal canal, which at the inner end forms an expansion - the spermatheca - and opens into a special chamber - the ootype. The ducts of the ovaries (oviducts), vitelline glands and Mehlis's corpuscles also flow into the ootype. Egg cells enter the ootype from the ovaries through the oviduct, and spermatozoa accumulated in the spermatheca after copulation penetrate through the vagina. Fertilization of the eggs and formation of eggs occurs in the ootype. They are formed from the nutrient material coming from the vitelline glands, and their membranes are created from the secretions of the Mehlis gland. The formed eggs move into the uterus, which is beginning to develop. As eggs enter it, the uterus increases in size and occupies an ever larger part of the segment volume, and the hermaphroditic reproductive apparatus gradually reduces. The terminal segments of the strobila are completely occupied by the uterus, filled with a huge number of eggs.

Segments containing developed sexual organs are called hermaphroditic, and those filled with only the uterus are called mature. In tapeworms, the mature uterus is closed. It has no communication with the genital tract or the external environment. Eggs leave it only when the terminal proglottids separate, which is accompanied by the destruction of the tissues of the segment and the uterine wall.

Tapeworms have an open uterus, through its external opening the eggs enter the host's intestines, and are then excreted into the external environment with feces. Tapeworm eggs have a lid similar to trematode eggs.

The eggs of tapeworms are quite uniform in their structure, so it is often impossible to determine their species by microscopy. Mature eggs are oval or spherical in shape and are covered with an extremely delicate transparent outer shell, through which the larva inside, the oncosphere, is clearly visible. It is surrounded by a thick, radially striated inner shell, the embryophore, which performs the main protective function. The oncosphere has six embryonic hooks, driven by muscle cells. With the help of the hooks and the secretion of glandular cells, the larva penetrates the host tissues during migration. Oncospheres are often colorless, less often colored yellow or yellowish-brown. When examining feces, oncospheres are found covered only by the embryophore, since the outer shell quickly deteriorates.

Development cycle of cestodes

All cestodes are biohelminths; the postembryonic development of most of their species occurs with a double (in tapeworms) or triple (in tapeworms) change of hosts.

In the intestine of the final host, when two or more worms are present, mutual fertilization between different individuals occurs. If only one cestode is parasitic, fertilization can occur between its different proglottids; self-fertilization of the same proglottid is possible. In tapeworms, the formation of the oncosphere ends in the uterus; in tapeworms, it occurs in the external environment (usually in water). When a mature tapeworm egg falls into water, the lid opens and a coracidium emerges from it - a spherical, free-swimming larva covered with a layer of ciliated cells and armed with six hooks.

Further development of the larvae continues in intermediate hosts.

Oncospheres that enter the gastrointestinal tract of the intermediate host with food or water are released from the embryophore, penetrate the intestinal wall and migrate, entering various internal organs with the blood, where, depending on the type of cestode, they develop into the corresponding type of larva - larvae (from the Latin larva - larva and the Greek kystis - bladder). Some of these larvae (coenurs, echinococci, alveococci) can reproduce asexually in the body of the intermediate host.

The main types of larvae are:

1. Cysticercus - a small vesicular formation filled with liquid and containing a scolex with fixation organs immersed inside. When entering the body of the final host, the scolex moves out of the larval bladder similar to how a finger of a glove turns inside out. Cysticercus is the most common of the larvacysts found in the tissues of vertebrates.
2. Cysticercoid consists of a swollen bladder-like part with a scolex and neck immersed in it and a tail appendage (cercomere) on which there are three pairs of embryonic hooks. Cysticercoid usually develops in the body of invertebrate intermediate hosts: crustaceans, mites, insects.
3. Coenur (Coenums) is a vesicular larvocyst with multiple immersed scolexes, each of which subsequently gives rise to a separate strobila. Thus, a large number of parasites develop from one oncosphere (asexual reproduction by budding). Coenur is characteristic of the genus Malticeps, and is found in sheep and some rodents.
4. The larvocyst of the cystic echinococcus (Echinococcus granulosus) is the most complex cestodes larva. It is a single-chambered bladder filled with liquid. Its internal germinal membrane can produce brood capsules with the simultaneous formation of embryonic scolexes (protoscolexes) and secondary and then tertiary bladders, due to which the process of asexual reproduction acquires special intensity. In the body of the intermediate host, the echinococcus takes on various modifications. It parasitizes mammals.
5. The larvocyst of the alveococcus (Echinococcus multilocularis) is a conglomerate of a large number of small, irregularly shaped vesicles, from the outer surface of which daughter vesicles bud off. Protoscolices develop in the vesicles. The larvocyst tends to grow into adjacent tissues.

In lower cestodes (tapeworms), the larvae that parasitize in intermediate hosts are elongated, resembling worms in shape. Their main forms.

1. Procercoid is the larval stage of tapeworms, formed in the first intermediate host (crustacean) from the coracidium. Its length is about 0.5 mm. At the front end there is a depression (primary bothria). The rear end of the body (cercomere) is separated by a constriction and equipped with chitinous hooks.
2. Plerocercoid - the larval stage of tapeworms, developing from the procercoid in the second intermediate host (fish). In some species of tapeworms, it can reach several tens of centimeters in length. Bothria are present at the anterior end of the body.

Definitive hosts become infected by feeding on intermediate hosts infested with plerocercoids.

Thus, the development of tapeworms consists of five phases:

1. an egg in which embryogenesis occurs in water;
2. a coracidium that hatches from an egg and leads a free life;
3. a procercoid that develops from a coracidium in the body of copepods;
4. plerocercoid, developing from a procercoid in fish;
5. an adult cestode (marita) that develops from a plerocercoid in the intestine of warm-blooded animals.

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