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Medical Biology Protozoa

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Medical Biology Dr: Fadia Al-Khayat Protozoa Protozoa are unicellular animals in which the various activities like metabolism, locomotion…..are carried out by different organelles of the cell .Also they have a well defined nucleus and don’t have a rigid cell wall which gave them a variation in size and shape. Structure of protozoa 1- nucleus: usually only one is present 2- Cytoplasm:which differentiated into a- outer cytoplasm(ectoplasm) b- inner cytoplasm(endoplasm) Nutrition of protozoa 1-Holozoic protozoa: utilize preformed food material derived from living animals or plants. 2-Saprozoic protozoa:absorb nutrients. means that certain protozoa absorb predigested or soluble nutrients through their cell membrane 3-Holophytic protozoa:like plants(chlorophyll). Locomotion of protozoa Protozoa can be move by: pseudopodia, flagella, cilia and gliding Reproduction of protozoa 1-Sexual reproduction a-Conjugation: two organisms exchange nuclear material and then separate. b-Syngamy:two gametes fuse together to form zygotes(microgametes+macrogametes) 2- Asexual reproduction a- Binary fission: the nucleus divide then the cytoplasm(1,2,4,8.16….) b- Multiple fission:the nucleus divide several times then the cytoplasm does. c-Budding:two or many daughter forms are produced from the parent cell. d-Sporogoy:occurs after the sexual reproduct(syngame),a process of multiple fission ,number of sporozoites are formed within the wall of a cyst. Four main groups of protozoa are recognized on the basis of their locomotion using specialized subcellular and cytoskeletal features Amoebae use pseudopodia (singular: pseudopodium) to creep or crawl over solid substrates. Pseudopodia (or ‘false feet’) are temporary thread- like or balloon-like extensions of the cell membrane into which the protoplasm streams. Similar amoeboid motion has been observed in cells of many life-forms, especially phagocytic cells (e.g. human macrophages). Flagellates use elongate flagella (singular: flagellum) which undulate to propel the cell through liquid environments. Flagella are ‘hair-like’ extensions of the cell membrane constructed of basal body , hook and filament with an inner core of microtubules arranged in a specific 2+9 configuration (2 single central microtubules surrounded by 9 peripheral doublets). This configuration is conserved throughout eukaryotic biology, many organisms produce flagellated cells (e.g. human spermatozoa. Ciliates use numerous small cilia (singular: cilium) which undulate in waves allowing cells to swim in fluids. Cilia are ‘hair-like’ extensions of the cell membrane similar in construction to flagella but shorter and present in much larger numbers. Ciliated cells are found in specialized tissues and organs in many other higher life-forms (e.g. human bronchial epithelial cells). Sporozoa (‘spore-formers’) were originally recognized not on the basis of their locomotion, but because they all formed non-motile spores as transmission stages. Recent studies, however, have shown that many pre- spore stages move using tiny undulating ridges or waves in the cell membrane imparting a forward gliding motion, but the actual mechanisms involved are not yet known. Human amoebas Free-living protozoan groups that inhabit soils and natural waters are extremely diverse, not only in their structure but also in the manner in which they feed, reproduce, and move. The amoeboid group includes hundreds of different organisms, ranging in size from about .25 to 2.5 mm. Amoebas are considered the most primitive animals and are classified in the kingdom Protista. All amoeboid organisms have thin cell membranes, a semirigid layer of ectoplasm, a granular, jellylike endoplasm, and an oval nucleus Amoebas are identified by their ability to form temporary cytoplasmic extensions called pseudopodia, or false feet, by means of which they move about. This type of movement, called amoeboid movement,. Some species live on aquatic plants and some in moist ground; others are parasitic in animals and humans. Amoeba contains food vacuoles, food is taken in and material excreted at any point on the cell surface. During feeding, extensions of cytoplasm flow around food particles, surrounding them and forming a vacuole into which enzymes are secreted to digest the particles . contractile vacuoles involved in osmoregulation pumps excess water out of the cell. Reproduction is asexual (binary fission). Three groups of amoebas are concerned with human health.these groups are: 1-Amoebas live in the alimentary canal and able to invade tissue, such as Entamoeba histolytica. 2-Amoebas live in the alimantry canal and unable to invade tissue,but dwell in the canal lumen such as Entamoeba coli, Entamoeba gingivalis and Dientamoeba fraglis. 3-Amoebas live freely in water and can infect human brain while swimming in contaminated water such as Naegleria fowleri. Entamoeba histolytica Entamoeba histolytica is a protozoan parasite responsible for a disease called amoebiasis. It occurs usually in the large intestine and causes internal inflammation as its name suggests (histo = tissue, lytic = destroying). 50 million people are infected worldwide, mostly in tropical countries in areas of poor sanitation. In industrialized countries most of the infected patients are immigrants, institutionalized people and those who have recently visited developing countries. Inside humans Entamoeba histolytica lives and multiplies as a trophozoite. Trophozoites are oblong and about 15–20 µm in length. In order to infect other humans they encyst and exit the body. Life cycle Entamoeba histolytica does not require any intermediate host. Mature cysts (spherical, 12–15 µm in diameter) are passed in the feces of an infected human. Another human can get infected by ingesting them in fecally contaminated water, food or hands. If the cysts survive the acidic stomach, they transform back into trophozoites in the small intestine. Trophozoites migrate to the large intestine where they live and multiply by binary fission. Both cysts and trophozoites are sometimes present in the feces. Cysts are usually found in firm stool, whereas trophozoites are found in loose stool. Only cysts can survive longer periods (up to many weeks outside the host) and infect other humans. If trophozoites are ingested, they are killed by the gastric acid of the stomach. Mode of infection: 1- contaminated food and water containing the infective stage (mature cyst) 2- carriers(asymptomatic) 3- Homosexual men Symptoms/Pathogenicity Infections of E. histolytica vary in intensity from asymptomatic to severe or fatal invasions. Asymptomatic infections are responsible for the spread of the parasite with numerous cysts being passed in normal stools. Diarrheic stools primarily contain trophozoites which cannot persist in the environment. • Infections of E. histolytica vary in intensity from asymptomatic to severe or fatal invasions. • Intestinal amoebiasis:The trophozoites come in contact with mucosa,secrete proteolytic enzymes ,which enable them to penetrate the epithelium and begin moving deeper.The intestinal lesion spreads in the intestinal wall as a duct like ulcer.Through these ulcers,blood escapes to the intestine lumen to demonstrate the important sign of amoebiasis. Trophozoite carried out by blood and lymph to form secondary lesion through out the body Extracellular amoebiasis:Invasive forms cause secondary lesion mainly in liver(hepatic or liver amoebiasis),in lung(pulmonary or lung amoebiasis),in brain,spleen. Invasive forms of the disease lead to amoebic dysentery in which the trophozoites invade the intestinal wall, leading to the formation of amoebic ulcers. This results in severe diarrhea with blood and mucus present. In such cases it is important to identify E. histolytica in the stools to differentiate among other causes of dysentery. If trophozoites penetrate the intestinal wall, serious problems can occur, including liver abcesses, or spread to the lungs and brain, usually resulting in death. Most Entamoeba histolytica infections are asymptomatic and trophozoites remain in the intestinal lumen feeding on surrounding nutrients. About 10–20 % of the infections develop into amoebiasis which causes 70 000 deaths each year. Minor infections (luminal amoebiasis) can cause symptoms that include: gas (flatulence) intermittent constipation loose stools stomach ache stomach cramping. Severe infections inflame the mucosa of the large intestine causing amoebic dysentery. The parasites can also penetrate the intestinal wall and travel to organs such as the liver via bloodstream causing extraintestinal amoebiasis. Symptoms of these more severe infections include: anemia appendicitis (inflammation of the appendix) bloody diarrhea fatigue fever gas (flatulence) genital and skin lesions intermittent constipation liver abscesses (can lead to death, if not treated) malnutrition peritonitis (inflammation of the peritoneum which is the thin membrane that lines the abdominal wall) stomach cramping toxic megacolon (dilated colon) weight loss. Prevention To prevent spreading the infection to others, one should take care of personal hygiene. Always wash your hands with soap and water after using the toilet and before eating or preparing food. Amoebiasis is common in developing countries. Some good practices, when visiting areas of poor sanitation: Wash your hands often. Avoid eating raw food. Avoid eating raw vegetables or fruit that you did not wash and peel yourself. Avoid consuming milk or other dairy products that have not been pasteurized.
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