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94-630: With the exception of some aquatic insects , the filaments and lamellae (folds) contain blood or coelomic fluid , from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in various groups of aquatic animals, including mollusks , crustaceans , insects, fish, and amphibians . Semiterrestrial marine animals such as crabs and mudskippers have gill chambers in which they store water, enabling them to use

188-500: A brain and a ventral nerve cord . Most insects reproduce by laying eggs . Insects breathe air through a system of paired openings along their sides, connected to small tubes that take air directly to the tissues. The blood therefore does not carry oxygen; it is only partly contained in vessels, and some circulates in an open hemocoel . Insect vision is mainly through their compound eyes , with additional small ocelli . Many insects can hear, using tympanal organs , which may be on

282-512: A fin ). Many microscopic aquatic animals, and some larger but inactive ones, can absorb sufficient oxygen through the entire surface of their bodies, and so can respire adequately without gills. However, more complex or more active aquatic organisms usually require a gill or gills. Many invertebrates, and even amphibians, use both the body surface and gills for gaseous exchange. Gills usually consist of thin filaments of tissue , lamellae (plates), branches, or slender, tufted processes that have

376-659: A neck mass or affecting nearby structures such as the superior vena cava ; detected because of screening in patients with myasthenia gravis, which has a strong association with thymomas and hyperplasia; and detected as an incidental finding on imaging such as chest x-rays . Hyperplasia and tumours originating from the thymus are associated with other autoimmune diseases – such as hypogammaglobulinemia , Graves disease , pure red cell aplasia , pernicious anaemia and dermatomyositis , likely because of defects in negative selection in proliferating T cells. Thymomas can be benign; benign but by virtue of expansion, invading beyond

470-634: A stem cell transplant . The thymus may contain cysts, usually less than 4 cm in diameter. Thymic cysts are usually detected incidentally and do not generally cause symptoms. Thymic cysts can occur along the neck or in the chest ( mediastinum ). Cysts usually just contain fluid and are lined by either many layers of flat cells or column-shaped cells . Despite this, the presence of a cyst can cause problems similar to those of thymomas, by compressing nearby structures, and some may contact internal walls ( septa ) and be difficult to distinguish from tumours. When cysts are found, investigation may include

564-418: A central medulla and an outer cortex, surrounded by a capsule. The thymus is made up of immature T cells called thymocytes , as well as lining cells called epithelial cells which help the thymocytes develop. T cells that successfully develop react appropriately with MHC immune receptors of the body (called positive selection ) and not against proteins of the body (called negative selection ). The thymus

658-409: A fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Fish gill slits may be the evolutionary ancestors of the thymus glands , parathyroid glands , as well as many other structures derived from the embryonic branchial pouches . The gills of fish form

752-610: A functional T cell receptor is formed, the thymocyte will begin to express simultaneously the cell surface proteins CD4 and CD8 . The survival and nature of the T cell then depends on its interaction with surrounding thymic epithelial cells. Here, the T cell receptor interacts with the MHC molecules on the surface of epithelial cells. A T cell with a receptor that doesn't react, or reacts weakly will die by apoptosis . A T cell that does react will survive and proliferate. A mature T cell expresses only CD4 or CD8, but not both. This depends on

846-531: A highly folded surface to increase surface area . The delicate nature of the gills is possible because the surrounding water provides support. The blood or other body fluid must be in intimate contact with the respiratory surface for ease of diffusion. A high surface area is crucial to the gas exchange of aquatic organisms, as water contains only a small fraction of the dissolved oxygen than air does, and it diffuses more slowly. A cubic meter of air contains about 275 grams of oxygen at STP . In fresh water ,

940-563: A hundred species, are marine. Insects such as snow scorpionflies flourish in cold habitats including the Arctic and at high altitude. Insects such as desert locusts , ants, beetles, and termites are adapted to some of the hottest and driest environments on earth, such as the Sonoran Desert . Insects form a clade , a natural group with a common ancestor, among the arthropods . A phylogenetic analysis by Kjer et al. (2016) places

1034-427: A mass of 20 to 50 grams by puberty. It then begins to decrease in size and activity in a process called thymic involution . After the first year of life the amount of T cells produced begins to fall. Fat and connective tissue fills a part of the thymic volume. During involution, the thymus decreases in size and activity. Fat cells are present at birth, but increase in size and number markedly after puberty, invading

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1128-421: A mature T cell needs to be able to bind to the MHC molecule ("positive selection"), and not to react against antigens that are actually from the tissues of body ("negative selection"). Positive selection occurs in the cortex and negative selection occurs in the medulla of the thymus. After this process T cells that have survived leave the thymus, regulated by sphingosine-1-phosphate . Further maturation occurs in

1222-625: A nearly immobile pupa . Insects that undergo three-stage metamorphosis lack a pupa, developing through a series of increasingly adult-like nymphal stages. The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonfly-like insects with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to have coevolved with flowering plants . Adult insects typically move about by walking and flying; some can swim. Insects are

1316-420: A number of slits connecting the pharynx to the outside of the animal on either side of the fish behind the head. Originally there were many slits, but during evolution, the number reduced, and modern fish mostly have five pairs, and never more than eight. Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six pairs with

1410-504: A problem with the development of the thymus gland, or a problem specific to thymocyte development. Immunodeficiency can be profound. Loss of the thymus at an early age through genetic mutation (as in DiGeorge syndrome , CHARGE syndrome , or a very rare "nude" thymus causing absence of hair and the thymus ) results in severe immunodeficiency and subsequent high susceptibility to infection by viruses, protozoa , and fungi . Nude mice with

1504-454: A rare subtype of Non-Hodgkins lymphoma , although by the activity of genes and occasionally microscopic shape, unusually they also have the characteristics of Hodgkins lymphomas . that occur most commonly in young and middle-aged, more prominent in females. Most often, when symptoms occur it is because of compression of structures near the thymus, such as the superior vena cava or the upper respiratory tract ; when lymph nodes are affected it

1598-478: A separate tube which has no respiratory tissue (the pharyngocutaneous duct) develops beneath the pharynx proper, expelling ingested debris by closing a valve at its anterior end. Lungfish larvae also have external gills , as does the primitive ray-finned fish Polypterus , though the latter has a structure different from amphibians. Tadpoles of amphibians have from three to five gill slits that do not contain actual gills. Usually no spiracle or true operculum

1692-419: A small mass of cells without any remaining gill-like structure. Marine teleosts also use their gills to excrete osmolytes (e.g. Na⁺, Cl). The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Seawater contains more osmolytes than the fish's internal fluids, so marine fishes naturally lose water through their gills via osmosis. To regain

1786-441: A sponge-like structure. During this stage, hematopoietic bone-marrow precursors migrate into the thymus. Normal development is dependent on the interaction between the epithelium and the hematopoietic thymocytes . Iodine is also necessary for thymus development and activity. The thymus continues to grow after birth reaching the relative maximum size by puberty. It is most active in fetal and neonatal life. It increases to

1880-451: A thin film of atmospheric oxygen in an area with small openings called spiracles that connect to the tracheal system. The plastron typically consists of dense patches of hydrophobic setae on the body, which prevent water entry into the spiracles, but may also involve scales or microscopic ridges projecting from the cuticle. The physical properties of the interface between the trapped air film and surrounding water allow gas exchange through

1974-428: A thymus, suggesting that this is an evolutionary process that has been conserved. The atrophy is due to the increased circulating level of sex hormones , and chemical or physical castration of an adult results in the thymus increasing in size and activity. Severe illness or human immunodeficiency virus infection may also result in involution. The thymus facilitates the maturation of T cells , an important part of

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2068-525: A variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate , or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light. Humans regard many insects as pests , especially those that damage crops, and attempt to control them using insecticides and other techniques. Others are parasitic , and may act as vectors of diseases . Insect pollinators are essential to

2162-412: A workup for tumours, which may include CT or MRI scan of the area the cyst is suspected to be in. Thymectomy is the surgical removal of the thymus. The usual reason for removal is to gain access to the heart for surgery to correct congenital heart defects in the neonatal period. Other indications for thymectomy include the removal of thymomas and the treatment of myasthenia gravis. In neonates

2256-440: Is a specialized primary lymphoid organ of the immune system . Within the thymus, thymus cell lymphocytes or T cells mature. T cells are critical to the adaptive immune system , where the body adapts to specific foreign invaders. The thymus is located in the upper front part of the chest, in the anterior superior mediastinum , behind the sternum , and in front of the heart . It is made up of two lobes, each consisting of

2350-442: Is located in the anterior mediastinum . It is made up of two lobes that meet in the upper midline, and stretch from below the thyroid in the neck to as low as the cartilage of the fourth rib. The lobes are covered by a capsule. The thymus lies behind the sternum, rests on the pericardium , and is separated from the aortic arch and great vessels by a layer of fascia . The left brachiocephalic vein may even be embedded within

2444-456: Is now an obsolete term. The importance of the thymus in the immune system was discovered in 1961 by Jacques Miller , by surgically removing the thymus from one-day-old mice, and observing the subsequent deficiency in a lymphocyte population, subsequently named T cells after the organ of their origin. Until the discovery of its immunological role, the thymus had been dismissed as a "evolutionary accident", without functional importance. The role

2538-430: Is often associated with thymic hyperplasia or thymoma, with antibodies produced probably because of T cells that develop abnormally. Myasthenia gravis most often develops between young and middle age, causing easy fatiguing of muscle movements. Investigations include demonstrating antibodies (such as against acetylcholine receptors or muscle-specific kinase ), and CT scan to detect thymoma or thymectomy. With regard to

2632-582: Is often in the mediastinum and neck groups. Such tumours are often detected with a biopsy that is subject to immunohistochemistry . This will show the presence of clusters of differentiation , cell surface proteins – namely CD30 , with CD19 , CD20 and CD22 , and with the absence of CD15 . Other markers may also be used to confirm the diagnosis. Treatment usually includes the typical regimens of CHOP or EPOCH or other regimens; regimens generally including cyclophosphamide , an anthracycline , prednisone , and other chemotherapeutics; and potentially also

2726-626: Is present, though many species have operculum-like structures. Instead of internal gills, they develop three feathery external gills that grow from the outer surface of the gill arches. Sometimes, adults retain these, but they usually disappear at metamorphosis . Examples of salamanders that retain their external gills upon reaching adulthood are the olm and the mudpuppy . Still, some extinct tetrapod groups did retain true gills. A study on Archegosaurus demonstrates that it had internal gills like true fish. Crustaceans , molluscs , and some aquatic insects have tufted gills or plate-like structures on

2820-405: Is renewed through the gills. In the larval dragonfly , the wall of the caudal end of the alimentary tract ( rectum ) is richly supplied with tracheae as a rectal gill, and water pumped into and out of the rectum provides oxygen to the closed tracheae. A plastron is a type of structural adaptation occurring among some aquatic arthropods (primarily insects), a form of inorganic gill which holds

2914-510: Is the largest and most active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to decrease in size and activity and the tissue of the thymus is gradually replaced by fatty tissue . Nevertheless, some T cell development continues throughout adult life. Abnormalities of the thymus can result in a decreased number of T cells and autoimmune diseases such as autoimmune polyendocrine syndrome type 1 and myasthenia gravis . These are often associated with cancer of

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3008-420: Is used by the diving bell spider , which maintains an underwater bubble that exchanges gas like a plastron. Other diving insects (such as backswimmers , and hydrophilid beetles ) may carry trapped air bubbles, but deplete the oxygen more quickly, and thus need constant replenishment. Insect Insects (from Latin insectum ) are hexapod invertebrates of the class Insecta . They are

3102-461: The Broadnose sevengill shark being the only cartilaginous fish exceeding this number. Adjacent slits are separated by a cartilaginous gill arch from which projects a cartilaginous gill ray . This gill ray is the support for the sheet-like interbranchial septum , which the individual lamellae of the gills lie on either side of. The base of the arch may also support gill rakers , projections into

3196-853: The Latin word insectum from in , "cut up", as insects appear to be cut into three parts. The Latin word was introduced by Pliny the Elder who calqued the Ancient Greek word ἔντομον éntomon "insect" (as in entomology ) from ἔντομος éntomos "cut in pieces"; this was Aristotle 's term for this class of life in his biology , also in reference to their notched bodies. The English word insect first appears in 1601 in Philemon Holland 's translation of Pliny. In common speech, insects and other terrestrial arthropods are often called bugs . Entomologists to some extent reserve

3290-2254: The Paraneoptera , and Kjer et al. 2016 for the Holometabola . The numbers of described extant species (boldface for groups with over 100,000 species) are from Stork 2018. Archaeognatha (hump-backed/jumping bristletails, 513 spp) [REDACTED] Zygentoma (silverfish, firebrats, fishmoths, 560 spp) [REDACTED] Odonata (dragonflies and damselflies, 5,899 spp) [REDACTED] Ephemeroptera (mayflies, 3,240 spp) [REDACTED] Zoraptera (angel insects, 37 spp) [REDACTED] Dermaptera (earwigs, 1,978 spp) [REDACTED] Plecoptera (stoneflies, 3,743 spp) [REDACTED] Orthoptera (grasshoppers, crickets, katydids, 23,855 spp) [REDACTED] Grylloblattodea (ice crawlers, 34 spp) [REDACTED] Mantophasmatodea (gladiators, 15 spp) [REDACTED] Phasmatodea (stick insects, 3,014 spp) [REDACTED] Embioptera (webspinners, 463 spp) [REDACTED] Mantodea (mantises, 2,400 spp) [REDACTED] Blattodea (cockroaches and termites, 7,314 spp) [REDACTED] Psocodea (book lice, barklice and sucking lice, 11,000 spp) [REDACTED] [REDACTED] Hemiptera (true bugs, 103,590 spp) [REDACTED] Thysanoptera (thrips, 5,864 spp) [REDACTED] Hymenoptera (sawflies, wasps, bees, ants, 116,861 spp) [REDACTED] Strepsiptera (twisted-wing flies, 609 spp) [REDACTED] Coleoptera (beetles, 386,500 spp) [REDACTED] Raphidioptera (snakeflies, 254 spp) [REDACTED] Neuroptera (lacewings, 5,868 spp) [REDACTED] Megaloptera (alderflies and dobsonflies, 354 spp) [REDACTED] Lepidoptera (butterflies and moths, 157,338 spp) [REDACTED] Trichoptera (caddisflies, 14,391 spp) [REDACTED] Diptera (true flies, 155,477 spp) [REDACTED] Mecoptera (scorpionflies, 757 spp) [REDACTED] Siphonaptera (fleas, 2,075 spp) [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] [REDACTED] Thymus gland The thymus ( pl. : thymuses or thymi )

3384-529: The Philadelphia translocation . Management can include multiple courses of chemotherapy , stem cell transplant , and management of associated problems, such as treatment of infections with antibiotics , and blood transfusions . Very high white cell counts may also require cytoreduction with apheresis . Tumours originating from the small population of B cells present in the thymus lead to primary mediastinal (thymic) large B cell lymphomas . These are

3478-451: The immune system providing cell-mediated immunity . T cells begin as hematopoietic precursors from the bone-marrow, and migrate to the thymus, where they are referred to as thymocytes . In the thymus they undergo a process of maturation, which involves ensuring the cells react against antigens ("positive selection"), but that they do not react against antigens found on body tissue ("negative selection"). Once mature, T cells emigrate from

3572-428: The sternum in the upper front part of the chest, stretching upwards towards the neck. In children, the thymus is pinkish-gray, soft, and lobulated on its surfaces. At birth, it is about 4–6 cm long, 2.5–5 cm wide, and about 1 cm thick. It increases in size until puberty, where it may have a size of about 40–50 g, following which it decreases in size in a process known as involution . The thymus

3666-623: The Hemiptera (true bugs), Lepidoptera (butterflies and moths), Diptera (true flies), Hymenoptera (wasps, ants, and bees), and Coleoptera (beetles), each with more than 100,000 described species. Insects are distributed over every continent and almost every terrestrial habitat. There are many more species in the tropics , especially in rainforests , than in temperate zones. The world's regions have received widely differing amounts of attention from entomologists. The British Isles have been thoroughly surveyed, so that Gullan and Cranston 2014 state that

3760-467: The T cells developed in the thymus are directed against the tissues of the body. This is because the malignant thymus is incapable of appropriately educating developing thymocytes to eliminate self-reactive T cells. The condition is virtually indistinguishable from graft versus host disease . Myasthenia gravis is an autoimmune disease most often due to antibodies that block acetylcholine receptors , involved in signalling between nerves and muscles . It

3854-452: The beating of cilia or other appendages, or by means of a pumping mechanism. In fish and some molluscs, the efficiency of the gills is greatly enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered. The gills of vertebrates typically develop in

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3948-503: The body, resulting in an enlarged liver , spleen , lymph nodes or other sites. Blood test might reveal a large amount of white blood cells or lymphoblasts , and deficiency in other cell lines – such as low platelets or anaemia . Immunophenotyping will reveal cells that are CD3 , a protein found on T cells, and help further distinguish the maturity of the T cells. Genetic analysis including karyotyping may reveal specific abnormalities that may influence prognosis or treatment, such as

4042-655: The body. Even though thymomas occur of epithelial cells, they can also contain thymocytes. Treatment of thymomas often requires surgery to remove the entire thymus. This may also result in temporary remission of any associated autoimmune conditions. Tumours originating from T cells of the thymus form a subset of acute lymphoblastic leukaemia (ALL). These are similar in symptoms, investigation approach and management to other forms of ALL. Symptoms that develop, like other forms of ALL, relate to deficiency of platelets , resulting in bruising or bleeding; immunosuppression resulting in infections; or infiltration by cells into parts of

4136-435: The capsule along septa . The cortex is mainly made up of thymocytes and epithelial cells. The thymocytes, immature T cells , are supported by a network of the finely-branched epithelial reticular cells , which is continuous with a similar network in the medulla. This network forms an adventitia to the blood vessels, which enter the cortex via septa near the junction with the medulla. Other cells are also present in

4230-554: The capsule of the thymus ("invasive thymoma"), or malignant (a carcinoma ). This classification is based on the appearance of the cells. A WHO classification also exists but is not used as part of standard clinical practice. Benign tumours confined to the thymus are most common; followed by locally invasive tumours, and then by carcinomas. There is variation in reporting, with some sources reporting malignant tumours as more common. Invasive tumours, although not technically malignant, can still spread ( metastasise ) to other areas of

4324-496: The commonly affected organs being hypothyroidism of the thyroid gland , Addison's disease of the adrenal glands , and candida infection of body surfaces including the inner lining of the mouth and of the nails due to dysfunction of TH17 cells , and symptoms often beginning in childhood. Many other autoimmune diseases may also occur. Treatment is directed at the affected organs. Thymoma-associated multiorgan autoimmunity can occur in people with thymoma. In this condition,

4418-520: The disease results from defects in the autoimmune regulator (AIRE) gene, which stimulates expression of self antigens in the epithelial cells within the medulla of the thymus. Because of defects in this condition, self antigens are not expressed, resulting in T cells that are not conditioned to tolerate tissues of the body, and may treat them as foreign, stimulating an immune response and resulting in autoimmunity. People with APECED develop an autoimmune disease that affects multiple endocrine tissues, with

4512-411: The dissolved oxygen content is approximately 8 cm/L compared to that of air which is 210 cm/L. Water is 777 times more dense than air and is 100 times more viscous. Oxygen has a diffusion rate in air 10,000 times greater than in water. The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life. Rather than using lungs, "[g]aseous exchange takes place across

4606-450: The dissolved oxygen when they are on land. Galen observed that fish had multitudes of openings ( foramina ), big enough to admit gases, but too fine to give passage to water. Pliny the Elder held that fish respired by their gills, but observed that Aristotle was of another opinion. The word branchia comes from the Greek βράγχια , "gills", plural of βράγχιον (in singular, meaning

4700-411: The gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally and

4794-408: The gland from the walls between the lobules first, then into the cortex and medulla. This process continues into old age, where whether with a microscope or with the human eye, the thymus may be difficult to detect, although typically weighs 5–15 grams. Additionally, there is an increasing body of evidence showing that age-related thymic involution is found in most, if not all, vertebrate species with

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4888-432: The human thymus. A second thymus in the neck has been reported sometimes to occur in the mouse As in humans, the guinea pig 's thymus naturally atrophies as the animal reaches adulthood, but the athymic hairless guinea pig (which arose from a spontaneous laboratory mutation) possesses no thymic tissue whatsoever, and the organ cavity is replaced with cystic spaces. [REDACTED] This article incorporates text in

4982-489: The insect can become oxygen-depleted if there is no water movement, so many such insects in still water actively direct a flow of water over their bodies. The inorganic gill mechanism allows aquatic arthropods with plastrons to remain constantly submerged. Examples include many beetles in the family Elmidae , aquatic weevils , and true bugs in the family Aphelocheiridae , as well as at least one species of ricinuleid arachnid and various mites. A somewhat similar mechanism

5076-608: The insects among the Hexapoda , six-legged animals with segmented bodies; their closest relatives are the Diplura (bristletails). Collembola (springtails) [REDACTED] Protura (coneheads) [REDACTED] Diplura (two-pronged bristletails) [REDACTED] Insecta (=Ectognatha) [REDACTED] The internal phylogeny is based on the works of Wipfler et al. 2019 for the Polyneoptera , Johnson et al. 2018 for

5170-407: The largest group within the arthropod phylum . Insects have a chitinous exoskeleton , a three-part body ( head , thorax and abdomen ), three pairs of jointed legs , compound eyes , and a pair of antennae . Insects are the most diverse group of animals, with more than a million described species ; they represent more than half of all animal species. The insect nervous system consists of

5264-406: The legs or other parts of the body. Their sense of smell is via receptors, usually on the antennae and the mouthparts. Nearly all insects hatch from eggs . Insect growth is constrained by the inelastic exoskeleton, so development involves a series of molts . The immature stages often differ from the adults in structure, habit and habitat. Groups that undergo four-stage metamorphosis often have

5358-429: The maturation of the hematopoietic progenitor cells , which are the precursors of both B and T cells. A number of genetic defects can cause SCID, including IL-2 receptor gene loss of function, and mutation resulting in deficiency of the enzyme adenine deaminase . Autoimmune polyendocrine syndrome type 1 , is a rare genetic autoimmune syndrome that results from a genetic defect of the thymus tissues. Specifically,

5452-428: The mouth and over the gills by rapidly swimming forward. In slow-moving or bottom-dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth. Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum , developed from

5546-478: The mouth by the beating of cilia. Respiration in the echinoderms (such as starfish and sea urchins ) is carried out using a very primitive version of gills called papulae . These thin protuberances on the surface of the body contain diverticula of the water vascular system . The gills of aquatic insects are tracheal , but the air tubes are sealed, commonly connected to thin external plates or tufted structures that allow diffusion. The oxygen in these tubes

5640-418: The mouth keep the water from escaping. The gill arches of bony fish typically have no septum, so the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack spiracles, the pseudobranch associated with them often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of

5734-402: The name "bugs" for a narrow category of " true bugs ", insects of the order Hemiptera , such as cicadas and shield bugs . Other terrestrial arthropods, such as centipedes , millipedes , woodlice , spiders , mites and scorpions , are sometimes confused with insects, since they have a jointed exoskeleton. Adult insects are the only arthropods that ever have wings, with up to two pairs on

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5828-415: The name for a "warty excrescence", possibly due to its resemblance to a bunch of thyme. Galen was the first to note that the size of the organ changed over the duration of a person's life. In the nineteenth century, a condition was identified as status thymicolymphaticus defined by an increase in lymphoid tissue and an enlarged thymus. It was thought to be a cause of sudden infant death syndrome but

5922-554: The only invertebrates that can achieve sustained powered flight; insect flight evolved just once. Many insects are at least partly aquatic , and have larvae with gills; in some species, the adults too are aquatic. Some species, such as water striders , can walk on the surface of water. Insects are mostly solitary, but some, such as bees , ants and termites , are social and live in large, well-organized colonies . Others, such as earwigs , provide maternal care, guarding their eggs and young. Insects can communicate with each other in

6016-511: The peripheral circulation. Some of this is because of hormones and cytokines secreted by cells within the thymus, including thymulin , thymopoietin , and thymosins . T cells have distinct T cell receptors. These distinct receptors are formed by process of V(D)J recombination gene rearrangement stimulated by RAG1 and RAG2 genes. This process is error-prone, and some thymocytes fail to make functional T-cell receptors, whereas other thymocytes make T-cell receptors that are autoreactive. If

6110-456: The pharyngeal cavity that help to prevent large pieces of debris from damaging the delicate gills. A smaller opening, the spiracle , lies in the back of the first gill slit . This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills. The spiracle is thought to be homologous to the ear opening in higher vertebrates . Most sharks rely on ram ventilation, forcing water into

6204-548: The relative size of the thymus obstructs surgical access to the heart and its surrounding vessels. Removal of the thymus in infancy results in often fatal immunodeficiency, because functional T cells have not developed. In older children and adults, which have a functioning lymphatic system with mature T cells also situated in other lymphoid organs, the effect is reduced, but includes failure to mount immune responses against new antigens, an increase in cancers, and an increase in all-cause mortality. When used as food for humans,

6298-434: The remains of the epithelial tubes, which grow out from the third pharyngeal pouches of the embryo to form the thymus. The arteries supplying the thymus are branches of the internal thoracic , and inferior thyroid arteries , with branches from the superior thyroid artery sometimes seen. The branches reach the thymus and travel with the septa of the capsule into the area between the cortex and medulla, where they enter

6392-535: The reproduction of many flowering plants and so to their ecosystems. Many insects are ecologically beneficial as predators of pest insects, while a few provide direct economic benefit. Two species in particular are economically important and were domesticated many centuries ago: silkworms for silk and honey bees for honey . Insects are consumed as food in 80% of the world's nations, by people in roughly 3000 ethnic groups. Human activities are having serious effects on insect biodiversity . The word insect comes from

6486-431: The same immunological function as in other vertebrates. Recently, in 2011, a discrete thymus-like lympho-epithelial structure, termed the thymoid , was discovered in the gills of larval lampreys . Hagfish possess a protothymus associated with the pharyngeal velar muscles, which is responsible for a variety of immune responses. The thymus is also present in most other vertebrates with similar structure and function as

6580-506: The septum of the gill arch in front of the first gill. In bony fish , the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside

6674-426: The spiracles, almost as if the insect were in atmospheric air. Carbon dioxide diffuses into the surrounding water due to its high solubility , while oxygen diffuses into the film as the concentration within the film has been reduced by respiration , and nitrogen also diffuses out as its tension has been increased. Oxygen diffuses into the air film at a higher rate than nitrogen diffuses out. However, water surrounding

6768-481: The strength of binding between the TCR and MHC class 1 or class 2. A T cell receptor that binds mostly to MHC class I tends to produce a mature "cytotoxic" CD8 positive T cell; a T cell receptor that binds mostly to MHC class II tends to produce a CD4 positive T cell. T cells that attack the body's own proteins are eliminated in the thymus, called "negative selection". Epithelial cells in the medulla and dendritic cells in

6862-405: The surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water." Usually water is moved across the gills in one direction by the current, by the motion of the animal through the water, by

6956-523: The surfaces of their bodies. Gills of various types and designs, simple or more elaborate, have evolved independently in the past, even among the same class of animals. The segments of polychaete worms bear parapodia many of which carry gills. Sponges lack specialised respiratory structures, and the whole of the animal acts as a gill as water is drawn through its spongy structure. Aquatic arthropods usually have gills which are in most cases modified appendages. In some crustaceans these are exposed directly to

7050-527: The thorax. Whether winged or not, adult insects can be distinguished by their three-part body plan, with head, thorax, and abdomen; they have three pairs of legs on the thorax. Estimates of the total number of insect species vary considerably, suggesting that there are perhaps some 5.5 million insect species in existence, of which about one million have been described and named. These constitute around half of all eukaryote species, including animals , plants , and fungi . The most diverse insect orders are

7144-452: The thymus arise from the vagus nerve and the cervical sympathetic chain . Branches from the phrenic nerves reach the capsule of the thymus, but do not enter into the thymus itself. The two lobes differ slightly in size, with the left lobe usually higher than the right. Thymic tissue may be found scattered on or around the gland, and occasionally within the thyroid. The thymus in children stretches variably upwards, at times to as high as

7238-427: The thymus express major proteins from elsewhere in the body. The gene that stimulates this is AIRE . Thymocytes that react strongly to self antigens do not survive, and die by apoptosis. Some CD4 positive T cells exposed to self antigens persist as T regulatory cells . As the thymus is where T cells develop, congenital problems with the development of the thymus can lead to immunodeficiency , whether because of

7332-483: The thymus itself; or alternatively directly enter the capsule. The veins of the thymus, the thymic veins , end in the left brachiocephalic vein , internal thoracic vein , and in the inferior thyroid veins . Sometimes the veins end directly in the superior vena cava. Lymphatic vessels travel only away from the thymus, accompanying the arteries and veins. These drain into the brachiocephalic, tracheobronchial and parasternal lymph nodes . The nerves supplying

7426-542: The thymus of animals is known as one of the kinds of sweetbread . The thymus was known to the ancient Greeks , and its name comes from the Greek word θυμός ( thumos ), meaning "anger", or in Ancient Greek, "heart, soul, desire, life", possibly because of its location in the chest, near where emotions are subjectively felt; or else the name comes from the herb thyme (also in Greek θύμος or θυμάρι ), which became

7520-566: The thymus played in ensuring mature T cells tolerated the tissues of the body was uncovered in 1962, with the finding that T cells of a transplanted thymus in mice demonstrated tolerance towards tissues of the donor mouse. B cells and T cells were identified as different types of lymphocytes in 1968, and the fact that T cells required maturation in the thymus was understood. The subtypes of T cells (CD8 and CD4) were identified by 1975. The way that these subclasses of T cells matured – positive selection of cells that functionally bound to MHC receptors –

7614-410: The thymus to provide vital functions in the immune system. Each T cell has a distinct T cell receptor , suited to a specific substance, called an antigen . Most T cell receptors bind to the major histocompatibility complex on cells of the body. The MHC presents an antigen to the T cell receptor, which becomes active if this matches the specific T cell receptor. In order to be properly functional,

7708-649: The thymus, and variable other associated problems, such as congenital heart disease , and abnormalities of mouth (such as cleft palate and cleft lip ), failure of development of the parathyroid glands , and the presence of a fistula between the trachea and the oesophagus . Very low numbers of circulating T cells are seen. The condition is diagnosed by fluorescent in situ hybridization and treated with thymus transplantation . Severe combined immunodeficiency (SCID) are group of rare congenital genetic diseases that can result in combined T, B, and NK cell deficiencies. These syndromes are caused by mutations that affect

7802-525: The thymus, including macrophages , dendritic cells , and a small amount of B cells , neutrophils and eosinophils . In the medulla, the network of epithelial cells is coarser than in the cortex, and the lymphoid cells are relatively fewer in number. Concentric, nest-like bodies called Hassall's corpuscles (also called thymic corpuscles ) are formed by aggregations of the medullary epithelial cells. These are concentric, layered whorls of epithelial cells that increase in number throughout life. They are

7896-522: The thymus, removal of the thymus, called thymectomy may be considered as a treatment, particularly if a thymoma is found. Other treatments include increasing the duration of acetylcholine action at nerve synapses by decreasing the rate of breakdown. This is done by acetylcholinesterase inhibitors such as pyridostigmine . Tumours originating from the thymic epithelial cells are called thymomas . They most often occur in adults older than 40. Tumours are generally detected when they cause symptoms, such as

7990-472: The thymus. In the neck, it lies on the front and sides of the trachea , behind the sternohyoid and sternothyroid muscles . The thymus consists of two lobes, merged in the middle, surrounded by a capsule that extends with blood vessels into the interior. The lobes consist of an outer cortex rich with cells and an inner less dense medulla . The lobes are divided into smaller lobules 0.5-2 mm diameter, between which extrude radiating insertions from

8084-411: The thyroid gland. The thymocytes and the epithelium of the thymus have different developmental origins. The epithelium of the thymus develops first, appearing as two outgrowths, one on either side, of the third pharyngeal pouch . It sometimes also involves the fourth pharyngeal pouch. These extend outward and backward into the surrounding mesoderm and neural crest -derived mesenchyme in front of

8178-477: The tissue of the thymus, called thymoma , or tissues arising from immature lymphocytes such as T cells, called lymphoma . Removal of the thymus is called thymectomy . Although the thymus has been identified as a part of the body since the time of the Ancient Greeks , it is only since the 1960s that the function of the thymus in the immune system has become clearer. The thymus is an organ that sits behind

8272-525: The total of around 22,500 species is probably within 5% of the actual number there; they comment that Canada's list of 30,000 described species is surely over half of the actual total. They add that the 3000 species of the American Arctic must be broadly accurate. In contrast, a large majority of the insect species of the tropics and the southern hemisphere are probably undescribed. Some 30–40,000 species inhabit freshwater ; very few insects, perhaps

8366-432: The ventral aorta . Here the thymocytes and epithelium meet and join with connective tissue. The pharyngeal opening of each diverticulum is soon obliterated, but the neck of the flask persists for some time as a cellular cord. By further proliferation of the cells lining the flask, buds of cells are formed, which become surrounded and isolated by the invading mesoderm. The epithelium forms fine lobules, and develops into

8460-404: The very rare "nude" deficiency as a result of FOXN1 mutation are a strain of research mice as a model of T cell deficiency. The most common congenital cause of thymus-related immune deficiency results from the deletion of the 22nd chromosome , called DiGeorge syndrome . This results in a failure of development of the third and fourth pharyngeal pouches, resulting in failure of development of

8554-401: The walls of the pharynx , along a series of gill slits opening to the exterior. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other. The gills are composed of comb-like filaments, the gill lamellae , which help increase their surface area for oxygen exchange. When

8648-565: The water, marine fishes drink large amounts of sea water while simultaneously expending energy to excrete salt through the Na/K-ATPase ionocytes (formerly known as mitochondrion-rich cells and chloride cells ). Conversely, fresh water contains less osmolytes than the fish's internal fluids. Therefore, freshwater fishes must utilize their gill ionocytes to attain ions from their environment to maintain optimal blood osmolarity. Lampreys and hagfish do not have gill slits as such. Instead,

8742-419: The water, while in others, they are protected inside a gill chamber . Horseshoe crabs have book gills which are external flaps, each with many thin leaf-like membranes. Many marine invertebrates such as bivalve molluscs are filter feeders . A current of water is maintained through the gills for gas exchange, and food particles are filtered out at the same time. These may be trapped in mucus and moved to

8836-512: Was known by the 1990s. The important role of the AIRE gene, and the role of negative selection in preventing autoreactive T cells from maturing, was understood by 1994. Recently, advances in immunology have allowed the function of the thymus in T-cell maturation to be more fully understood. The thymus is present in all jawed vertebrates , where it undergoes the same shrinkage with age and plays

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