1、Chapter 20 - Class Cestoidea: Form, Function, and Classification,GeneralComplete absence of an alimentary canal: no mouth, no gut, no anus; all nutrients are acquired through a specialized tegumentEndoparasitic with sexually mature worms living in the alimentary tract and associated ducts of all cla
2、sses of vertebratesLarval stages infect both vertebrates and invertebratesThe life cycle require one or two intermediate hosts, in each of which the tapeworm undergoes a specific phase of development,General MorphologyThe body of the typical cestode consists of 3 distinct regions: scolex, neck, and
3、strobila,Form and Function: The ScolexScolex (pl. scolices) is located at the anterior end and functions as an attachment structureIn some groups, the holdfast function of the scolex is lost early in life, and the anterior end of the stroblia becomes distorted into a pseudoscolex to function as a ho
4、ldfast,To facilitate attachment to the hosts intestinal wall, tapeworms utilize several types of structures on their scolices, the most common of which are suckersMuscles in the scolex make possible the holdfast action of this organ,Form and Function: The Scolex cont.The scolices of tapeworms are ty
5、pically categorized as either acetabulate or bothriate, depending on the type of sucker presentAn acetabulate scolex is characterized by the presence of 4 muscular cups sunk into the equatorial surface of the scolex; cups are radially arranged equidistant from each other,In addition to muscular cups
6、, there may be accessory holdfast structures, such as hooks to help anchor the scolex to the hosts intestinal wallIn this case, the scolex is called an armed scolexThese hooks are usually grouped at the apical end of the scolex on a protrusible rostellum,rostellum,A bothriate scolex is characterized
7、 by the presence of 2, or rarely 4 to 6, longitudinally arranged, shallow depressions called bothria (sing. bothrium),The Scolex cont.,Various types of glandular secretions are associate with the scolex of many tapewormsThe function of these secretions has not been firmly established, although it ha
8、s been speculated that they are proteolytic, adhesive, and/or stimulatory, depending on the species,Form and Function: The NeckThe neck is an unsegmented, poorly differentiated region immediately posterior to the scolexIt contains stem cells that are apparently responsible for giving rise to the str
9、obila (= a series of proglottids)Note:In those species without a neck, similar kinds of cells in the posterior part of the scolex give rise to new proglottids,Each proglottid contains a set of reproductive organs of both sexesAs each proglottid is shifted posteriad, its sexual reproductive system ma
10、tures progressivelyThe most anterior proglottids have the least developed reproductive systems; more posteriorly located the proglottids are more developedThe stroblia can be loosely subdivided into 3 regions: immature, mature, and gravid proglottids,Form and Function: The StrobilaAs new proglottids
11、 are formed from the neck region, they push the older ones progressively posteriad, creating a chain of proglottids - the strobilaThe asexual process of forming segments is termed strobilation,In some groups, the gravid proglottids detach from the other proglottids and are released with the hosts fe
12、ces - apolysisThe eggshells of these species are not well protectedSome groups are referred to as anapolytic species - eggs are released through a uterine or genital pore directly into the hosts intestine and, subsequently, also are discharged to the exterior in fecesAnapolytic tapeworms produce pro
13、tective, tanned eggshells,Form and Function: The Strobila cont. Reproductive organs in immature proglottids are visible but nonfunctional, while those of mature proglottids are fully functionalGravid proglottids are filled with eggs,The cyton region contains Golgi complexes, mitochondria, rough ER,
14、and other organelles involved in protein synthesis and packagingUnderlying the distal cytoplasm are 2 layers of muscles - tegumental musculatureIt consists of an outer layer with its contractile fibrils oriented in a circular pattern and an inner layer with contractile fibrils oriented longitudinall
15、y,TegumentA syncytial epithelium, with distal and proximal cytoplasmic regionsThe distal cytoplasm is replete with mitochondria, usually aligned in a broad, basal band, as well as several types of vesicles and scattered membranes; glycogen granules are also presentThe vesicles arise in the nucleated
16、, proximal cytoplasm, or cyton, sunk deep in the parenchyma,These tips provide resistance to the peristaltic movement of the hosts intestineThey also agitate intestinal fluids in the immediate microhabitat, increasing accessibility of nutrient materials as well as flushing away waste products,Tegume
17、nt cont.The surface of the tapeworm tegument bears specialized microvilli known as microthrices (singl. microthrix) that project from the outer, limiting membrane of the tegumentEach microthrix includes an electron dense, apical tip separated from the more basal region by a multilaminar plane,Tegume
18、nt cont.Covering the entire surface of the tegument is a layer of carbohydrate containing macromolecules - the glycocalyx - that serves several important purposes:,- protecting the parasite from host digestive enzymes- enhancing nutrient absorption- maintaining the parasites surface membrane,Parench
19、ymaThe space enclosed by the tegument - except for the portion occupied by reproductive organs, osmoregulatory structures, muscle fibers and nervous tissue - is filled with a spongy tissue known as parenchyma In live tapeworms, fluid fills the spaces between the parenchyma cellsParenchyma cells are
20、the primary sites for synthesis and storage of glycogenThere is speculation that a single population of cells, the myoblasts, gives rise to both the parenchyma and the musculature of most tapeworms,Calcareous CorpusclesLarge numbers of concretions known as calcareous corpuscles occur in the parenchy
21、ma of numerous cestode species These spherical bodies, which are most noticeable in larval forms, consist of organic and inorganic componentsThe organic portion consists of DNA, RNA, proteins, glycogen, mucopolysaccharides, and alkaline phosphataseThe inorganic portion consists of calcium, magnesium
22、, phosphorous, and traces of metalsPossible functions of the corpuscles include:- buffers against anaerobically produced acids- reservoirs for inorganic ions required during development- enzyme activators- a form of excretory product of metabolism,Nervous SystemThe “brain” is a rectangular or circul
23、ar nervous tissue varying in complexity from a simple ganglion to a combination of several ganglia and commissuresSeveral pairs of longitudinal nerve cords extend posteriorly from this “brain” along the length of the strobila, lateral to the osmoregulatory canals,The cords are connected in each prog
24、lottid by cross connectivesSmall motor nerves emanating from the cords and cross-connectives innervate the reproductive organs and musculature, while small sensory nerves supplying the tegument merge with the cords and connectives,All 4 canals lie just inside the medullary margin of the parenchyma,
25、and a single transverse canal connects the ventral canals at the posterior end of each proglottidThe ventral canals carry fluid away from the scolex, the dorsal canals toward itIn some tapeworms, the 4 longitudinal canals are linked within the scolex by either a network of canals or a single ring of
26、 vesiclesIn others, the dorsal and ventral canals on each side are linked by a simple connection in the region of the scolex, with no apparent exchange between the 2 sides,OsmoregulationConsists of 2 components: the collecting canals and the flame cellsFour laterally aligned collecting canals (2 dor
27、sal and 2 ventral) extend the entire length of the strobila,Osmoregulation cont.In the terminal proglottid of young worms, there is an excretory vesicle into which the ventral canals empty; in older tapeworms, the posterior ends of the ventral canals open independently to the exterior,Flame cells ar
28、e associated with the ventral canals; fluid connected by the flame cells passes through secondary tubules into the main canalsAnalysis of fluid within the osmoregulatory system has revealed that its consists primarily of glucose, soluable proteins, lactic acid, urea, and ammonia,Reproductive Systems
29、: GeneralTapeworms are, for the most part, monoecious, with each proglottid containing one complete set of both male and female reproductive organs (there are notable exceptions, e.g. 2 sets of genitalia per segment)Most cestodes are protandrous (male system develops first)On rare occasion the femal
30、e system is known to develop first and this condition is referred to as protogynyFor the most part, the reproductive systems of cestodes resemble those of trematodes except:- for the cul-de-sac uterus in some forms- the presence of a separate vaginal canal- and often a laterally situated genital por
31、e,Male Reproductive SystemConsists of one to many testes embedded in the medullary parenchyma of each proglottidEmanating from each testis is a single vas efferens,In cases of multiple testes, the vas efferentia unite to form a common vas deferens, which is usually coiledThe distal portion of the va
32、s deferens is modified as a muscular cirrus, usually enclosed within a cirrus sac,Male Reproductive System cont.In some species, the cirrus is equipped with spines that hold the organ in place during copulationThe cirrus everts through the male genital pore, which in turn, opens into the common geni
33、tal atrium,In most species there is an enlarged area of the vas deferens, the seminal vesicle, for the storage of sperm When located within the cirrus sac, it is designated an internal seminal vesicle; when outside the sac, it is termed an external seminal vesicle,Female Reproductive SystemOva are p
34、roduced in a single, sometimes bi-lobed ovaryFollowing fertilization, the resulting zygote passes into a region of the oviduct, the ootype, equipped with structures involved in eggshell formation,Mehlis gland surrounds the ootype and secretes into it material essential to the formation of the egg sh
35、ellA single common vitelline duct enters the oviduct in the vicinity of the ootypeCommon vitelline duct is formed by the union of many primary vitelline ducts arising from vitelline glands,Female Reproductive System cont.Vitelline glands (=vitellaria) may form a compact body or consist of numerous f
36、ollicles scattered throughout the medullary parenchyma,The vagina carries sperm from the genital atrium to the oviduct, and fertilization occurs in the region where the vagina and oviduct joinSperm is stored in an enlargement of the vagina known as the seminal receptacle,Female Reproductive System c
37、ont.The oviduct continues as the uterus, which in some tapeworms (e.g. O. Pseudophyllidea) opens to the outside of the proglottid through a uterine pore; eggs are expelled through this openingIn other species (e.g. O. Cyclophyllidea) the uterus is a blind sac in which developing eggs accumulateThe u
38、terus becomes distended with eggs, filling the medullary region of the proglottidAnd this gravid proglottid later becomes detached from the strobila and is discharged from the host,The EggThe oncosphere (larvae within the egg), containing 3 pair of hooks, is encased in an inner envelope that in turn
39、 is surrounded by another membranous structure, the embryophore,Tapeworms eggs exhibit certain variations on this basic theme and can be classified into 3 types: 1) Pseudophyllidean, 2) Dipylidium, 3) Taenioid,A cellular zone known as the outer envelope lies between the embryophore and the shell (ca
40、psule), usually the outer most covering of the egg,Pseudophylidean Egg (e.g. Diphyllobothrium) The fully developed egg has a thick, quinone-tanned shell, usually with a lid-like operculum at one endNumerous vitelline cells are associated with the zygote, providing stored food for subsequent developm
41、entThe zygote develops into an oncosphere, which is covered by a ciliated embryophore that enables it to swim upon hatchingThis form of organism is called a coracidium (pl. coracidia),Dipylidean Egg (e.g. Dipylidium and Hymenolepis) Possesses a thin shell, a thin nonciliated embryophore, and a relat
42、ively thick outer envelope,Taenioid Egg (e.g. Taenia and Echinococcus) The shell and outer envelope are lacking, and the thick, nonciliated embryophore constitutes the outermost covering,Life Cycle PatternsFor our purposes, we will break down the life cycle of tapeworms into 2 basic patterns: one ty
43、pical of the members of order Pseudophyllidea; the other of members of the order Cyclophyllidea,Pseudophyllidean Pattern Eggs containing coracidia leave the host with the feces to waterThe coracidium escapes from the eggshell through the operculum and swims by means of its ciliated embryophoreCoraci
44、dium is ingested by the first intermediate host (an aquatic arthropod) within which the embryo sheds its ciliated embryophore and metamorphoses into a procercoid in the hosts hemocoelDuring development, the oncosphere hooks are retained in a tail-like structure called the cercomerWhen the first inte
45、rmediate host is ingested by a second intermediate host (usually a fish), the procercoid migrates via the peritoneal cavity to various parts of the body, primarily the musculatureHere it grows and develops into a plerocercoid that shows the beginning of strobilation and a self-formed adult scolexThe
46、 plerocercoid is infective to the definitive hostWhen ingested, it attaches to the wall of the small intestine, where strobilation occurs,Cyclophyllidean PatternThe oncosphere (also called a hexacanth) lacks a ciliated embryophore and must remain passive until the egg is ingested by a vertebrate or
47、invertebrate hostIn species that normally utilize a invertebrate host (e.g. an arthropod) the oncosphere upon hatching in the digestive tract, employs its hooks and its penetration glands to enter the hemocoel, where it metamorphoses into a cysticercoidThis form is solid-bodied and possesses a fully
48、 developed acetabulate scolexIt is surrounded by several layers of cystic tissue and has a prominent cercomer containing hooksThe cystic tissue and cercomer are digested away in the digestive tract of the definitive host, freeing the scolex and neck to begin strobilation,Cyclophyllidean Pattern cont
49、.In species that utilize vertebrate intermediate hosts, the oncosphere, after ingestion, penetrates the intestinal lining and enters a venuleIt is carried by the blood to any of several areas of the body where it develops into a cysticercus with an acetabulate scolex invaginated into a fluid-filled
50、vesicle or bladder - bladderworm Two other forms that follow this developmental pattern are the coenurus and the hydatid cystsIn the former, the wall of the bladder develops several invaginated scolicesIn the latter, secondary cysts are formed as invaginations on the wallsThese second generation cys
51、ts are called brood capsules since they, in turn, give rise to scolices, each of which, when ingested by a suitable definitive host, can develop into an adult wormIn some tapeworms, certain immature stages (e.g. hydatid cyst, cysticercus, pleurocercoid, etc.) are capable of developing in extraintestinal tissues of humans,
