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59-481: See text . Digenea (Gr. Dis – double, Genos – race) is a class of trematodes in the Platyhelminthes phylum , consisting of parasitic flatworms (known as flukes ) with a syncytial tegument and, usually, two suckers , one ventral and one oral. Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes of vertebrates . Once thought to be related to

118-407: A snail ) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium , which must locate and penetrate the body wall of the snail host. After post-ingestion hatching or penetration of

177-495: A top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with

236-608: A uroproct . Digeneans are also capable of direct nutrient uptake through the tegument by pinocytosis and phagocytosis by the syncitium . Most adult digeneans occur in the vertebrate alimentary canal or its associated organs , where they most often graze on contents of the lumen (e.g., food ingested by the host, bile, mucus), but they may also feed across the mucosal wall (e.g., submucosa , host blood). The blood flukes, such as schistosomes, spirorchiids and sanguinicolids, feed exclusively on blood. Asexual stages in mollusc intermediate hosts feed mostly by direct absorption, although

295-429: A canal called Laurer's Canal , which leads from the oviduct to the dorsal surface of the body. The function of this canal is debated, but it may be used for insemination in some species or for disposal of waste products from reproduction in other species. Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down

354-417: A decline in reproductive performance leading to menopause . This decline is tied to a decline in the number of ovarian follicles. Although about 1 million oocytes are present at birth in the human ovary, only about 500 (about 0.05%) of these ovulate, and the rest do not ovulate. The decline in ovarian reserve appears to occur at a constantly increasing rate with age, and leads to nearly complete exhaustion of

413-445: A disorders of the reproductive system . If the egg fails to release from the follicle in the ovary an ovarian cyst may form. Small ovarian cysts are common in healthy women. Some women have more follicles than usual ( polycystic ovary syndrome ), which inhibits the follicles to grow normally and this will cause cycle irregularities. the ovaries. Cryopreservation of ovarian tissue, often called ovarian tissue cryopreservation ,

472-434: A distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in the animal kingdom are Linnaeus's classes similar to

531-587: A long groove along one side of the body in which the female is clasped. Females reach sexual maturity after they have been united with a male. After mating the two remain locked together for the rest of their lives. They can live for several years and produce many thousands of eggs. The four species of schistosomes that infect humans are members of the genus Schistosoma . The seven major species of non-schistosomes that infect humans are listed below. People become infected after ingesting metacercarial cysts on plants or in undercooked animal flesh. Most species inhabit

590-425: A role in the menstrual cycle and fertility . The ovary progresses through many stages beginning in the prenatal period through menopause . Each ovary is whitish in color and located alongside the lateral wall of the uterus in a region called the ovarian fossa . The ovarian fossa is the region that is bounded by the external iliac artery and in front of the ureter and the internal iliac artery . This area

649-455: A stratified epithelium. The ovary also contains blood vessels and lymphatics . At puberty , the ovary begins to secrete increasing levels of hormones. Secondary sex characteristics begin to develop in response to the hormones. The ovary changes structure and function beginning at puberty. Since the ovaries are able to regulate hormones, they also play an important role in pregnancy and fertility . When egg cells (oocytes) are released from

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708-419: A terminal or subterminal mouth, a muscular pharynx that provides the force for ingesting food, and a forked, blind digestive system consisting of two tubular sacs called caeca (sing. caecum ). In some species the two gut caeca join posteriorly to make a ring-shaped gut or cyclocoel . In others the caeca may fuse with the body wall posteriorly to make one or more anuses , or with the excretory vesicle to form

767-461: A variety of physical and chemical signals, such as gut pH levels, digestive enzymes , temperature , etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats. The evolutionary origins of the Digenea have been debated for some time, but there appears general agreement that the proto-digenean was a parasite of a mollusc, possibly of

826-451: A very different way in most invertebrates than they do in vertebrates, and are not truly homologous. Many of the features found in human ovaries are common to all vertebrates, including the presence of follicular cells, tunica albuginea, and so on. However, many species produce a far greater number of eggs during their lifetime than do humans, so that, in fish and amphibians, there may be hundreds, or even millions of fertile eggs present in

885-412: A woman's ovaries as "female testes". Birds have only one functional ovary (the left), while the other remains vestigial. In mammals including humans, the female ovary is homologous to the male testicle , in that they are both gonads and endocrine glands . Ovaries of some kind are found in the female reproductive system of many invertebrates that employ sexual reproduction . However, they develop in

944-435: Is about 4 cm x 3 cm x 2 cm in size. The ovaries are surrounded by a capsule, and have an outer cortex and an inner medulla. The capsule is of dense connective tissue and is known as the tunica albuginea . Usually, ovulation occurs in one of the two ovaries releasing an egg each menstrual cycle . The side of the ovary closest to the fallopian tube is connected to it by infundibulopelvic ligament , and

1003-400: Is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle. A typical digenean trematode life cycle is as follows. Eggs leave the vertebrate host in faeces and use various strategies to infect the first intermediate host , in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually

1062-475: Is found in other members of the Neodermata ; a group of platyhelminths comprising the Digenea, Aspidogastrea , Monogenea and Cestoda . Digeneans possess a vermiform , unsegmented body-plan and have a solid parenchyma with no body cavity ( coelom ) as in all platyhelminths. There are typically two suckers , an anterior oral sucker surrounding the mouth , and a ventral sucker sometimes termed

1121-594: Is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by the same process until the modern bewildering diversity of life cycle patterns developed. Digenea includes about 80 families. They are listed below, organised by order. Digenea Only about 12 of the 6,000 known species are infectious to humans, but some of these species are important diseases afflicting over 200 million people. The species that infect humans can be divided into groups,

1180-415: Is not the case in all species. In most birds and in platypuses , the right ovary never matures, so that only the left is functional. (Exceptions include the kiwi and some, but not all raptors , in which both ovaries persist. ) In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish , and some teleosts, there is only one ovary, formed by the fusion of the paired organs in

1239-408: Is of interest to women who want to preserve their reproductive function beyond the natural limit, or whose reproductive potential is threatened by cancer therapy, for example in hematologic malignancies or breast cancer. The procedure is to take a part of the ovary and carry out slow freezing before storing it in liquid nitrogen whilst therapy is undertaken. Tissue can then be thawed and implanted near

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1298-412: Is responsible for the appearance of secondary sex characteristics for females at puberty and for the maturation and maintenance of the reproductive organs in their mature functional state. Progesterone prepares the uterus for pregnancy, and the mammary glands for lactation. Progesterone functions with estrogen by promoting menstrual cycle changes in the endometrium . As women age, they experience

1357-498: Is the general process in eukaryotic organisms by which germ cells are formed, and it is likely an adaptation for removing DNA damages, especially double-strand breaks, from germ line DNA (see Meiosis and Origin and function of meiosis ). Homologous recombinational repair is especially promoted during meiosis. Titus et al. also found that expression of 4 key genes necessary for homologous recombinational repair of DNA double-strand breaks (BRCA1, MRE11, RAD51 and ATM) decline with age in

1416-401: Is the general rule among the Digenea. Usually two testes are present, but some flukes can have more than 100. Also present are vasa efferentia , a vas deferens , seminal vesicle , ejaculatory duct and a cirrus (analogous to a penis) usually (but not always) enclosed in a cirrus sac. The cirrus may or may not be covered in proteinaceous spines. The exact conformation of these organs within

1475-651: The Monogenea , it is now recognised that they are closest to the Aspidogastrea and that the Monogenea are more closely allied with the Cestoda . Around 6,000 species have been described to date. Characteristic features of the Digenea include a syncytial tegument; that is, a tegument where the junctions between cells are broken down and a single continuous cytoplasm surrounds the entire animal. A similar tegument

1534-400: The acetabulum , on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure. A monostome is a worm with one sucker (oral). Flukes with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes . Distomes are flukes with an oral sucker and a ventral sucker, but

1593-461: The cercaria (pl. cercariae). Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish). The life cycles of some digeneans include only two hosts,

1652-423: The developmental biology of the asexual stages remains a problem. Electron microscopic studies have shown that the light microscopically visible germ balls consist of mitotically dividing cells which give rise to embryos and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes is not proven, the exact definition remains doubtful. Protandry

1711-402: The germinal epithelium . The outer layer is the ovarian cortex , consisting of ovarian follicles and stroma in between them. Included in the follicles are the cumulus oophorus , membrana granulosa (and the granulosa cells inside it), corona radiata , zona pellucida , and primary oocyte . Theca of follicle , antrum and liquor folliculi are also contained in the follicle. Also in

1770-408: The redia stage found in some groups does have a mouth, pharynx and simple gut and may actively consume host tissue or even other parasites. Encysted metacercarial stages and free-living cercarial stages do not feed. Paired ganglia at the anterior end of the body serve as the brain . From this nerves extend anteriorly and posteriorly. Sensory receptors are, for the most part, lacking among

1829-685: The DNA repair gene BRCA1 undergo menopause prematurely, suggesting that naturally occurring DNA damages in oocytes are repaired less efficiently in these women, and this inefficiency leads to early reproductive failure. The BRCA1 protein plays a key role in a type of DNA repair termed homologous recombinational repair that is the only known cellular process that can accurately repair DNA double-strand breaks. Titus et al. showed that DNA double-strand breaks accumulate with age in humans and mice in primordial follicles. Primordial follicles contain oocytes that are at an intermediate (prophase I) stage of meiosis. Meiosis

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1888-427: The adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larva recognising and locating its host. There is a bewildering array of variation on the complex digenean life cycle, and plasticity in this trait is probably a key to the group's success. In general,

1947-469: The central part of the ovary is a hollow, lymph -filled space. The ovary of teleosts is also often hollow, but in this case, the eggs are shed into the cavity, which opens into the oviduct . Certain nematodes of the genus Philometra are parasitic in the ovary of marine fishes and can be spectacular, with females as long as 40 cm (16 in), coiled in the ovary of a fish half this length. Although most female vertebrates have two ovaries, this

2006-450: The classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to

2065-437: The cortex is the corpus luteum derived from the follicles. The innermost layer is the ovarian medulla . It can be hard to distinguish between the cortex and medulla, but follicles are usually not found in the medulla. Follicular cells are flat epithelial cells that originate from surface epithelium covering the ovary. They are surrounded by granulosa cells that have changed from flat to cuboidal and proliferated to produce

2124-400: The early nineteenth century. Ovary The ovary (from Latin ōvārium  'egg') is a gonad in the female reproductive system that produces ova ; when released, an ovum travels through the fallopian tube / oviduct into the uterus . There is an ovary on the left and the right side of the body. The ovaries are endocrine glands , secreting various hormones that play

2183-406: The embryo. At maturity, ovaries secrete estrogen , androgen , inhibin , and progestogen . In women before menopause, 50% of testosterone is produced by the ovaries and released directly into the blood stream. The other 50% of testosterone in the blood stream is made from conversion of the adrenal pre-androgens ( DHEA and androstenedione) to testosterone in other parts of the body. Estrogen

2242-423: The fallopian tube, a variety of feedback mechanisms stimulate the endocrine system, which cause hormone levels to change. These feedback mechanisms are controlled by the hypothalamus and pituitary glands. Messages or signals from the hypothalamus are sent to the pituitary gland. In turn, the pituitary gland releases hormones to the ovaries. From this signaling, the ovaries release their own hormones. The ovaries are

2301-535: The fallopian, either orthotopic (on the natural location) or heterotopic (on the abdominal wall), where it starts to produce new eggs, allowing normal conception to take place. A study of 60 procedures concluded that ovarian tissue harvesting appears to be safe. The ovarian tissue may also be transplanted into mice that are immunocompromised ( SCID mice ) to avoid graft rejection , and tissue can be harvested later when mature follicles have developed. In former centuries, medical authors, for example Galen , referred to

2360-462: The final host. Fertilisation is internal, with sperm being transferred via the cirrus to the Laurer's Canal or genital aperture . A key group of digeneans which are dioecious are the schistosomes . Asexual reproduction in the first larval stage is ubiquitous. While the sexual formation of the digenean eggs and asexual reproduction in the first larval stage (miracidium) is widely reported,

2419-402: The human gastrointestinal tract, where they shed eggs along with host feces. Paragonimus westermani , which colonizes the lungs, can also pass its eggs in saliva . These flukes generally cause mild pathology in humans, but more serious effects may also occur. Class (biology) The class as a distinct rank of biological classification having its own distinctive name – and not just called

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2478-503: The level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders. The class was considered the highest level of the taxonomic hierarchy until George Cuvier 's embranchements , first called Phyla by Ernst Haeckel , were introduced in

2537-442: The life cycles may have two, three, or four obligate (necessary) hosts, sometimes with transport or paratenic hosts in between. The three-host life cycle is probably the most common. In almost all species, the first host in the life cycle is a mollusc . This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently. The alternation of sexual and asexual generations

2596-458: The male terminal genitalia is taxonomically important at the familial and generic levels. Usually there is a single ovary with an oviduct , a seminal receptacle , a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlis’ gland , which is believed to lubricate the uterus for egg passage. In addition, some digeneans possess

2655-482: The mantle cavity. Evidence for this comes from the ubiquity of molluscs as first intermediate hosts for digeneans, and the fact that most aspidogastreans (the sister group to the Digenea) also have mollusc associations. It is thought that the early trematodes (the collective name for digeneans and aspidogastreans) likely evolved from rhabdocoel turbellarians that colonised the open mantle cavity of early molluscs. It

2714-484: The oocytes of humans and mice. They hypothesized that DNA double-strand break repair is vital for the maintenance of oocyte reserve and that a decline in efficiency of repair with age plays a key role in ovarian aging. A study identified 290 genetic determinants of ovarian ageing, also found that DNA damage response processes are implicated and suggests that possible effects of extending fertility in women would improve bone health, reduce risk of type 2 diabetes and increase

2773-423: The other side points downwards attached to the uterus via the ovarian ligament . Other structures and tissues of the ovaries include the hilum . The ovaries lie within the peritoneal cavity, on either side of the uterus, to which they are attached via a fibrous cord called the ovarian ligament . The ovaries are uncovered in the peritoneal cavity but are tethered to the body wall via the suspensory ligament of

2832-446: The ovary , which is a posterior extension of the broad ligament of the uterus. The part of the broad ligament of the uterus that covers the ovary is known as the mesovarium . The ovarian pedicle is made up part of the fallopian tube , mesovarium , ovarian ligament, and ovarian blood vessels. The surface of the ovaries is covered with a membrane consisting of a lining of simple cuboidal -to-columnar shaped mesothelium , called

2891-469: The ovary at any given time. In these species, fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in some elasmobranch fish; in other species, the remnants of the follicle are quickly resorbed by the ovary. In birds, reptiles, and monotremes , the egg is relatively large, filling the follicle, and distorting the shape of the ovary at maturity. Amphibians and reptiles have no ovarian medulla;

2950-585: The reserve by about age 52. As ovarian reserve and fertility decline with age, there is also a parallel increase in pregnancy failure and meiotic errors resulting in chromosomally abnormal conceptions. The ovarian reserve and fertility perform optimally around 20–30 years of age. Around 45 years of age, the menstrual cycle begins to change and the follicle pool decreases significantly. The events that lead to ovarian aging remain unclear. The variability of aging could include environmental factors, lifestyle habits or genetic factors. Women with an inherited mutation in

3009-504: The risk of hormone-sensitive cancers. A variety of testing methods can be used in order to determine fertility based on maternal age. Many of these tests measure levels of hormones FSH, and GnrH. Methods such as measuring AMH ( anti-Müllerian hormone ) levels, and AFC (antral follicule count) can predict ovarian aging. AMH levels serve as an indicator of ovarian aging since the quality of ovarian follicles can be determined. Ovarian diseases can be classified as endocrine disorders or as

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3068-405: The schistosomes and the non-schistosomes. The Schistosomes occur in the circulatory system of the definitive host. Humans become infected after free-swimming cercaria liberated from infected snails penetrate the skin. These dioecious worms are long and thin, ranging in size from 10 to 30 mm in length to 0.2 to 1.0 mm in diameter. Adult males are shorter and thicker than females, and have

3127-469: The second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host . Two-host life cycles can be primary (there never was a third host) as in the Bivesiculidae , or secondary (there was at one time in evolutionary history a third host but it has been lost). In three-host life cycles, cercariae develop in

3186-411: The second intermediate host into a resting stage, the metacercaria , which is usually encysted in a cyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host . Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host's gut in response to

3245-407: The site of production and periodical release of egg cells , the female gametes. In the ovaries, the developing egg cells (or oocytes ) mature in the fluid-filled follicles . Typically, only one oocyte develops at a time, but others can also mature simultaneously. Follicles are composed of different types and number of cells according to the stage of their maturation , and their size is indicative of

3304-412: The snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst . The mother sporocyst undergoes a round of internal asexual reproduction , giving rise to either rediae (sing. redia) or daughter sporocysts . The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage,

3363-441: The stage of oocyte development. When an oocyte completes its maturation in the ovary, a surge of luteinizing hormone is secreted by the pituitary gland , which stimulates the release of the oocyte through the rupture of the follicle, a process called ovulation . The follicle remains functional and reorganizes into a corpus luteum , which secretes progesterone in order to prepare the uterus for an eventual implantation of

3422-495: The uterus. The uterus typically opens into a common genital atrium that also received the distal male copulatory organ (cirrus) before immediately opening onto the outer surface of the worm. The distal part of the uterus may be expanded into a metraterm, set off from the proximal uterus by a muscular sphincter, or it may be lined with spines, as in the Monorchiidae and some other families. As adults, most digeneans possess

3481-437: The ventral sucker is somewhere other than posterior. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy. The vast majority of digeneans are hermaphrodites . This is likely to be an adaptation to low abundance within hosts, allowing the life cycle to continue when only one individual successfully infects

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