Lecture_Cestodas.pdf
- 1. General Characteristics of the Cestoda
- 2. Cestoda • The Cestoda, or tapeworms, being a subclass of parasitic flatworms, possess all the characteristics of the phylum Platyhelminthes. • The most striking difference between members of this subclass and the class Trematoda is that tapeworms lack a mouth and digestive tract.
- 3. Cestoda • The Cestoda are the most highly specialized flatworm parasites known. Adults of this subclass are endoparasitic in the alimentary tract and associated ducts of various vertebrates, including humans; • the larvae, on the other hand, infect both vertebrates and invertebrates. • The life cycle requires one or two intermediate hosts, in each of which the tapeworm undergoes a specific developmental phase
- 5. MORPHOLOGY • The body of the typical adult eucestode consists of three distinct regions: • scolex, • neck, • strobila
- 6. MORPHOLOGY • The scolex, located at the anterior end, is the attachment terminal, the morphology and dimensions of which are key features in identification of these worms • The neck, an unsegmented, poorly differentiated region immediately posterior to the scolex, is generally the narrowest part of the worm
- 7. MORPHOLOGY • As new proglottids are formed in the neck region, they push the older ones progressively posteriad, creating a chain of proglottids, the strobila, that constitutes the third body region
- 8. MORPHOLOGY • The asexual process of forming segments is termed strobilation. • As each proglottid is shifted posteriad, its sexual reproductive system matures progressively; hence, the anteriormost proglottids have the least developed reproductive systems, while the more posteriorly the proglottids are located, the higher their level of development • This progressive maturity of the reproductive systems permits a loose subdivision of the strobila into regions of immature, mature, and gravid proglottids
- 9. MORPHOLOGY • In apolytic species, gravid proglottids detach from the strobila and exit the body of the host with fecal wastes. • In anapolytic species, eggs are released through a uterine (or genital) pore directly into the host’s intestine and, subsequently, also are discharged to the exterior in feces. • Most anapolytic tapeworms produce protective, tanned eggshells.
- 10. MORPHOLOGY Tegument • Tegument (helminth) is a term in helminthology for the outer body covering of members of the phylum Platyhelminthes. The name is derived from a Latin word tegumentum or tegere, meaning "to cover". It is characteristic of flatworms including the broad groups of tapeworms and flukes. • The surface of the tapeworm tegument bears specialized microvilli, known as microthrices (singular, microthrix), which project from the outer, limiting membrane of the tegument.
- 12. MORPHOLOGY Parenchyma • The space enclosed by the tegument except for the portion occupied by reproductive organs, osmoregulatory structures, muscle fibers, and nervous tissued is filled with a spongy tissue known as the parenchyma • In live tapeworms, fluid fills the spaces between parenchymal cells. Parenchymal cells are the primary sites for synthesis and storage of glycogen. • In live tapeworms, fluid fills the spaces between parenchymal cells. Parenchymal cells are the primary sites for synthesis and storage of glycogen. •
- 13. MORPHOLOGY Scolex • To facilitate attachment to the host’s intestinal wall, tapeworms utilize several types of structures on their scolices, the most common of which are suckers. • Muscles in the scolex make possible the holdfast action of this organ. The musculature of the scolex consists of sets of crisscrossing fibers attached to the inner surfaces of the suckers, enabling them to contract
- 14. MORPHOLOGY Scolex • Scolices of tapeworms that infect humans are categorized as either acetabulate (a) or bothriate (b), depending on the type of sucker
- 15. Nervous System The cestode nervous system is relatively complex. The “brain,” located in the scolex, is a rectangular or circular arrangement of nerve tissue varying in complexity from a simple ganglion to a combination of several ganglia and commissures
- 16. Reproductive Systems The general pattern of the reproductive system of cestodes resembles that of digenetic trematodes except for the cul-de-sac uterus in some forms (cyclophyllideans), the presence of a separate vaginal canal, and often a laterally situated genital pore
- 17. Reproductive Systems Male System • The male reproductive system consists of from one to many testes embedded in the medullary parenchyma of each proglottid. • Emanating from each testis is a single vas efferens; in cases of multiple testes, the vasa efferentia unite to form a common vas deferens, which is usually coiled. • The distal portion of the vas deferens is modified as a muscular cirrus, usually enclosed within a cirrus sac. • In some species, the cirrus is equipped with spines that hold the organ in place during copulation. The cirrus everts through the male genital pore, which then opens into the common genital atrium.
- 18. Reproductive Systems Male System • In most species there is an enlarged area of the vas deferens, the seminal vesicle, for storage of sperm. • When located within the cirrus sac, it is designated an internal seminal vesicle; • located outside the sac, it is termed an external seminal vesicle. • Some species possess both.
- 19. Reproductive Systems Female System • Ova are produced in a single, sometimes bilobed ovary. • Following fertilization in the proximal portion of the oviduct, the resulting zygote passes into a region of the oviduct, the ootype, equipped with structures involved in eggshell formation similar to those found in digeneans. • A Mehlis’ gland surrounds the ootype and secretes into it material essential to formation of the eggshell; a single, common vitelline duct enters the oviduct in the vicinity of the ootype.
- 20. Reproductive Systems Female System • The common vitelline duct is formed by the union of many primary vitelline ducts arising from vitelline glands, which vary in size and location according to species. • Vitelline glands (collectively designated as the vitellaria) may form a compact body or consist of numerous follicles scattered throughout the medullary region of the parenchyma. • With few exceptions, secretions of the vitelline glands contain shell precursors and provide nourishment for the developing larva.
- 21. Reproductive Systems Female System • The vagina, a tubular organ that joins the oviduct at the level of Mehlis’ gland, provides a passage for sperm between the genital atrium and the oviduct. • Fertilization occurs in the region where the vagina and oviduct join. Sperm are stored in an enlargement of the vagina known as the seminal receptacle.
- 22. Reproductive Systems Female System • The oviduct continues as the uterus, which, in some tapeworms such as the anapolytic members of the order Pseudophyllidea, opens to the outside of the proglottid through a uterine pore. • Eggs are produced continuously and are expelled through this opening. In other species, including members of the order Cyclophyllidea, the uterus is a blind sac in which developing eggs accumulate; the uterus becomes distended with eggs, filling the medullary region of the proglottid.
- 23. Reproductive Systems Female System • The gravid proglottid eventually becomes detached from the strobila and is discharged from the host. In some tapeworms (Dipylidium, etc.), there is a modification of uterus-egg interaction in which a much-reduced uterus, upon receiving a specific number of eggs, begins pinching off egg capsules, which eventually fill the medullary region of the gravid proglottid. • During copulation, the cirrus of one proglottid may be inserted in the vagina of another proglottid of the same, or another, worm
- 24. The Eggs • The morphology of tapeworm eggs is important for species identification. The following is a brief general description of the basic parts of a typical egg
- 25. The Eggs • The oncosphere, containing three pairs of hooks, is encased in an inner envelope that is surrounded by another membranous structure, the embryophore. • A cellular zone known as the outer envelope lies between the embryophore and the shell (or capsule), usually the outermost covering of the egg.
- 26. The Eggs Tapeworm eggs exhibit certain variations within this basic pattern and are classified into four types: 1. pseudophyllidean (a), 2. dipylidean (b) 3. taenioid (c) 4. stilesian
- 27. The Eggs Tapeworm eggs exhibit certain variations within this basic pattern and are classified into four types: 1. pseudophyllidean (a), 2. dipylidean (b) 3. taenioid (c) 4. stilesian
- 28. The Eggs The three types of egg-forming tapeworms that infect humans. • (a) Pseudophyllidean, e.g., in Diphyllobothrium latum. • (b) Dipylidean, e.g., in Hymenolepis nana. • (c) Taenioid, e.g., in Taenia solium.
- 29. LIFE CYCLE PATTERNS Tapeworms that infect humans display one of two basic life cycle patterns, • one typical of members of the order Pseudophyllidea, • the other of members of the order Cyclophyllidea
- 30. LIFE CYCLE PATTERNS Tapeworms that infect humans display one of two basic life cycle patterns, • one typical of members of the order Pseudophyllidea, • the other of members of the order Cyclophyllidea