Class Note for ANT 475 at UA- Human Adaptability(2)
Class Note for ANT 475 at UA- Human Adaptability(2)
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Date Created: 02/06/15
Skin Color Human Pigmentation and Adaptation Skin The body s largest organ Functions in many ways 7 Thermoregulation 7 Protection from physical and chemical injury 7 Protection from invasion by microorganisms 7 Manufactures essential nutrient Skin Color As one of the most conspicuous human polytypic variations skin color has probably attracted more scholarly attention than any other aspect of human variability Skin color has served as a primary feature in most systems of racial classi cation Measurement of Skin Color I By the latter half of the nineteenth century While anthropologists still had no clear idea of the underlying causes of pigmentation they began to devise measurement techniques to use skin color in racial classi cation Broca established a 34 tone scale Which Was simpli ed by his student Topinard These techniques Were used into the 20L11 century until the introduction of the re ectance spectrophotometer in the early 1950s Re ectance Spectrophotometer A Re ectance Spectrophotometer shines a light of a speci c wave length using a lter and measures the intensity of light re ected by the skin 7 The technique involves alcohol Wash of the skin on the inner upper arm allow time for local circulation to retum to normal shine light and measure re ectance Re ectance Spectrophotometry Darknr ig39uer FElCCHlugn ol l mu rallecled Biological Determinants of Skin Color The pigments Carotene Hemoglobin and Melanin are involved in skin color Carotene the least common skin pigment results in a yellowing of skin 7 Results primarily from the overconsumption of carotene containing foods like carrots 7 This pigment is signi cant almost exclusively in pathological or abnormal skin coloration Hemoglobin Hemoglobin is the complex molecule responsible for transport of oxygen throughout our b odies 7 It is the primary protein constituent of Red Blood Cells Oxygenated hemoglobin has a reddish hue 7 Produces a pinkish tint to lightly pigmented skin Deoxygenated hemoglobin has a purplish color 7 Produces the bluish tint to lightly pigmented skin that is characteristic of oxygen deprivation and suffocation Melanin The primary determinant of variability in human skin color is the amount density and distribution of the pigment melanin Melanin has a dark brownpurpleblack color that is intensi ed by denser compaction of the melanin granules in the cells of the upper layers of the skin Structure of the Epidermis Malpuglnan law basal layer shaler basalel MEAN NUMBER OF FOREARM MELANOCYTES Melanocytes and Skin Color 3000 3 O taco I o c g 2000 395 m 396 w c g lt i s s 5 3 pi 1500 E 5 5 v 5 lt g E m 1000 E E 8 z a o E ea E 5 500 3 O i i i 4 i I 2 a 4 a s 7 a e 10 n 12 POPULATION L f th E 39d 39 Layers of the Epidermis ayers 0 6 p1 cums I The stratum basale consists ofcolumnar cells the The Stratum granulosum COHSiStS Of several keratinocy tes with about 10 of the cells layers of attened polyhedral cells with comprising melanocytes their long axis parallel to the skin surface 7 This is the germinal level of the skin which gives rise to 7 cytoplasm contains k atohyhn the outer layers of cells and the melanin granules that pignem them 7 As the cells increase in size they die out Thisis alsowhere 90 ofVltamlnD syntheslstakesplace o The stratum corneum is composed Of a I The stratum spinusum consists of several layers varying number Of layers of dead 0f megmaI POIYhedIal cans keraunocytes keratinized cells fused to one another except attened on the edges for the outer edge where aking takes place Melanocytes synthesize melanin Which is combined into Melanin Metabolism granules and injected into the surrounding keratinocytes Stillman ariaAemv Tyrosimse ryrnsinasc mei n ONES i i Tyrnsm ana Ddgaqmnnnc ii 1 77gt contributes to cupulymcnzalaun 567Dihyrlrnxyindnle Lwrndnpzchrmne r 39 a 39 srsr Cydemyldnpa Zrczrhnxylic acid quot SrSrCystEinyldnpzr l BEnInthiz nc 1 55 Diliydxnxyindnly l l MES lmermcdiaricd i ll swimms i l I c l y xx Melanocyle from Emalapn Fhael anin Robins 1991111 DOPA path tn melanin In dark skin melanosomes accumulate ma mm in P39hydmxyphznylwnw We llydxnxyphmylpymvic Acid eumelanm and remam as Single pamcles 5 Tyrnsine u gt rhymu39nc wh1le 1n 11ghter skins they contam mcreas mg g amounts of phaeomelanin and cluster in W 39gt WWW claim membrane bound organelles lysosomes milmm no Fhenylpynlviczcid dihydxn henylalanine mmquot Wham dimquot V anaminc m H Neurnmdanin reduces E Nandremlmz Adamhm nxidatjve damage via the 1 inactilmtilm nr damaging 39ml tf free radicals Mcisnccimu Nciimnclanin aim plays aw quotmm l amn a mic in irnn chclau39nn Melanin Metabolism 39 39 T ndnase Tyrosinase Phenylalanlne Metabollsm rynsinequot gt uni uni2mm if contributes to Plelotropy A situation whereby seyeral seemingly unrelated WPDIYme phenotypic etrects are In uenced by the acuon ofa single gene sy mhydm yindm Lmndngzchrmg s Cysta39nyldnpz ordcthretirdatiantsnui body small head an manta retirditicntnem Q l between mm iigrt hair and skin a Tyrnsinase de ciency causes alhimsm 39 DquotP c on have repmdumve implications includng enhanced or reduced fertility depending annitingprererences i e Frhydxnxyphenylpyruvic nxidase de clmcy causes tyrusinusis i 55Dihyiirmryinduly lndeisedleyels otcirculitingtyrosne mdtyrosne metabolites i phenylalanine hydrmrylaseinsuaiciencycausesphyenlketcinuna FKU Lmhm y udd I Srsrwgzinyldnpzr l quinone i Doesndtilmormillymcreisesynthessotmelcmn i Benzrtthia e rte ciem enzymes in the liiiliuanmi ot39ryrodue on this pathway causes i muggy lntermcdi39arieg hereditary cretamsm i 5mmquot II i simuntaicdinedenciencydseise hutncttreitahlehydet l t t 0 i Lift Melanln Synthesm Tyrosmase 1nh1b1tlon I The metabolic pathway to melanin is extremely up Tyrosinage activity is complicated involving several intermediate steps inhibited by anumber of e Staits with the amino acid tyrosine oxidized by the compounds including reduced coppercontaining enzyme tyrosinase to glutathione see G6P D dihydroxyphenylalanine dopa and then to dopaquinone 55H notes WSW med 7 1n the skin the enzyme glutathione re ductase GR is responsible for producing GSH A mutataon to the gene for the enzyme tyrosinase thatproduces a protein with decreasedfunctzonallty will resultin areduced productaon ofmelanm e in the extreme this prciduces a genetac furm ufalblmsm Q a E E e Africans tendto hayelower levels of GSH and GR Glutathi une Reductase mm 5555 Ofthe 3 GR yanants Africans tend to have the least ef nmt Oxidized f Glutathiune m e LawexGSIHevelsludta greater neimin synthess Melanln Synthesm 2 Genetlcs of Skln color 7 Dopaqulnone undergoes a series ofnonenzymatlc reactions andreanangements forming the ditrerent molecules that are co polymerized to make up one ofthe Skin color is a Polygemc trait meaning multiple types Ofmelamn genetic loci are involved in determining skin color Eumelanln is the dark brownpurpleblack compoundfoundln 7 Multiple genes wmking together produce Momma 5 ha distribution in a Bell Shape curve of degrees oflight Phaeomelanln is the yellowrtorreddishrbrown pigment which is to dam present in red hairresulung from the addition ofthe amino acid Wm 7 Early models suggested 2 or 4 major genes Recent work suggests many genes working togetherin yery Both fomls Ofmelamn Wwa 1111 other Proteins 10 complex additive and nonraddltlve combinations fonn the melanosome that Is distributed from the The nonrenzymatlc conversion of dopaqulnone into eumelanin and phaeomelanin and their combination into melanosomes is melanocyte to surrounding cells affected by several genetic loci Hun I I 4 WIan HUM nit 2 ultlnlin wlm wmuwnInwmmnrwlm Mumva Eusmg Ewdunli uni mm mnnmr nmi Mm me pumm D Hmlinn MilLuuuwlllhmnml 1 Orva39 llll lr IlIlIlIll unlxlhnl m mum 6 muurnum llilmllpuUlM U c ii pIU39 Imlum mm gtl39 UA Ill ml mm A m lent JE aEu mun imu JIEHuer V in u immumn 39 mm 39mum iim7mn 5quot minmmmwmnamnmhvmnmmn A 4 U mum Vtmluyrm nun 77i iyniy7i mm WM BMW s mm muLmuygnmmm Imllwr r 7 r 397 f r Mmmmmn r F wanimam 1m WWW 7 m L umu Lzu um 39 unmmu AIM EWB BHIquot umw n W Phonmelonowmz Eumunncsomu Domains nu mamnoqml Melanocyte Tanning Tanning is a twostage acclimatizational response of the skin to increasing levels of UV exposure 7 Immediate tanning is the transient brownish tan following exposure to UVA and visible light Reaches a maximum within 12 hours a er exposure 7 Fades between 324 hours a er exposure No new melanosomes formed so the likely mechanism is the photooxidation of existing melanin or other epidermal elements More Tanning 7 Delayed tanning is the durable browning caused by repeated exposure primarily to UV B but UVA and visible light also play a role Gradual process of skin darkening starting 4872 hours after irradiation 7 Reaches a maximum 19 days after an exposure 7 Requires 9V2 months for skin to retum to original melanin content Melanocytes enlarge increase dendrite density and experience other internal changes Melanosomes increase in number and melanization UV Tyrosinase interaction Dopa UV radiation Ultravmlet radiation reduces the levels of GSH GSH Tyrosinase Reduced 7 Lessens the inhibition Glutathione of tyrosinase activity 0 55 GSH 8 7 Increases the synthes1s available to g f 1 inhibit tyrosinase g 0 me am Tyrosine cuVquot 5 7 Darkens the skin in GSSG res onse to ex osure Oxidized p p Glutathione Distribution of Skin Color V agg r NM W numl 1n7pvncmlanwtzi a mu Framdamccliaelw a Emu ll W x mm Vlluieimavei ie was a are um Malev um ma matataavmhhh Distribution of Skin Color 2 nhhi 1 1 ii 2 c m Dismmza 1min Equnmr degrees at Inmudu Distribution of Skin Color 3 The clinal nature of skin color distribution suggests an association with environmental factors varying with latitude 7 Ultraviolet Radiation in particular the quantity of UV rays striking the surface of the earth from the sun 7 Temperature JustSo Stories about Skin Color 0 Noah s curse on Ham s son Canaan 0 Response to the heat of the sun 0 Bile theory 0 Aerobic theory 0 Optic theory 0 Pigmentation as pathology of adverse environment 0 Result of disease 0 Vitamin D Folic Acid Cold tolerance Noah s curse on Ham s son Canaan 0 An early explanation of dark skin comes from the biblical story ofNoah s curse Genesis Chapter 9 King James Version 24 AndNoah awoke orn his wine and lmew what his younger son had done unto him 25 And he said Cursed be Canaan aservant ofservants shall he be unto his brethren 26 And he said Blessed be the LORD God ofShem and Canaan shall be his servant 27 God shall enlarge Japheth and he shall dwell in the tents of Shem and Canaan shall be his servant 7 Nowhere does this mention darkening of the skin although other sections of the bible notably in Job and Isaiah associate curses with darkness A Biblical Account of Dark Skin Color 0 Lamentations chapters 4 and 5 refer to skin color becoming dark as a result of famine 7 Ch 4 v 8 Their visage is blacker than a coal alternate translation darker than blackness they are not known in the stream their skin cleaveth to their bones it is withered it is become like a stick 7 Ch 5 v10 Our skin was black like an ovenbecause of the terrible famine alternate translations terrors or storms Voyages of Discovery 0 By the time the slave trade was actively operating on the West African coast in the mid to late 16 11 century curiosity about the cause of dark skin color was growing 0 The accepted environmental explanation was that the action of the sun s heat was the cause of the differences in the complexion of Europeans and Africans 17th Century Developments I Accumulating evidence made it clear that the heat of the sun explanation was not satisfactory r The skin color ofthe aborigines ofNonh Americain similar climates to Europeans and Africans ms neither black nor white but olive r Africans were found to vary in color from black to yellovlf according to sources ofthe day 7 Africans forced into European settings were showing no lightening ofskin and those Europeans living in Africa were not appreciably darkening I The view emerged that the A ican s blackness was innate and permanent Thomas Browne 1605 7 1682 I This English physician published his view in his 1646 Preudodoxia epidemica r Ifthe fervour ofthe Sun or intemperate heat of clime did solely occasion this complexion surely amigration or change thereormight cause asensible ifnot atotal mutation which notwithstanding experience will not admit Despite theirtransplantation there remains among their descendants a strong shadow oftheir onginals and ifthey preserve their copulations entire they still maintain their complexion Likewise fair or white people translated in hotter Countries receive not impressions amounting to this complexion 7 He also dismissed the Ham s curse explanation as a foolish tale Biological Differences I Attempts made to resolve Whether differences in skin color was due to innate differences in biological structure or transient response to the sun s heat 7 Santorio Santorio 15611636 1614 D2 Staml medzcma skin s complexion ms determined by the presence orblack bile Revival ofanotion from the ninth century Arabian physician Ali alTabari in his medical compendium Paradise of Wzsdum 7 Supported by Italian anatomist Malpighi 1687 I Determined that dermis and stratum comeum is colorless in both Balcks and Whites he agreed with Santorius that the blackness ofAfricans must originate in the underlying mucous and reticular bodyicolored by bile Early Eighteenth century anatomists reported the presence of black bile in the Malpighian layer To Bile or Not To Bile I Scholars attempting to avoid the polygenistic implications of the bile theory clung to the Work of the French surgeon Littre Who failed to nd a black gelatinous bile in the malpighian layer of Africans I Monogenists began advocating a more complex view of environmental causes that harks back to Hippocrates Airs Wafers Placer focusing on the differential qualities of the atmosphere 7 This aerobic theory was championed by Du Bos in 1719 and picked up by others including Blumenbach in his Varieties ofMan in 1795 Aerobic and Optic Theories I Le Cat 1765 discerned black deposits he called 2ethiops in nerve tissues of animals 7 Without having any idea about the origin or role of these aethiops he ms convinced that there ms a relationship with the environment and he conjectured that Negroes had more ofthe structures than Europeans did making them the basis ofthe dark complexion of the Africans I Patot 1733 claimed that complexion differences Were based on the ability of the human skin to transmit lightgthe optic theory of skin color A New World Perspective I John Mitchell 1744 published the rst major scienti c study of skin color in the New World 7 Compared skin ofcolonial Whites and African slaves in Virginia Coneludedthere was no anatomical basis forthe bile theory only structural ditrerenee was thieker skin among the Africans 7 Applied Newtonian riptres to suggest that skin eulrir was based unthickness ufthe skin and its ability to transmit light 7 As thiekness increased the skin appeared darker Arriean s thiek skin prohibited the transmission or any Eulur through them He also eonieeturedthat the original skin eolor ofman was neither blaek nor white but something in between with Europeans and Arrieans represented divergent extremes eaused by the degenerative in uenees ofthe environment Degeneration of the Primordial Type Buffon argued that skin color differences were re ective of the geographical degeneration of the primordial type 7 He suggests restoration of the degenerate races to the purity and vigor of the original type would require the transplantation of the these people to a more temperate zone plus a change of diet and a long span of time Pigmentation as Pathology Lafitau 1724 argues that dark skin color is a congenital malformation7already present in African fetuses Rush 1795 claimed the skin color ofthe Negro was derived from leprosy 7 He argues that Africans sntrer from a congenital disease so rni1d that excess pigment ms the on1y symptom Albinism and other depigmentation conditions among Africans were seen as reversion to the original complexion An Universal Freckle I Samuel Stanhope Smith 1810 used the depigmentation of Henry Moss an African American from Virginia to suggest that pigmentation was nothing more than an universal freckle occasioned by environmental exposure to sunlight He argued that climate affected skin color 7 Evidence the darkening and 1ightening ofcomplexion with the seasons 7 Cold air chafes the countenance and increases the ruddiness ofthe complexion Physiology of Pigmentation Late in the 19 11 century the process of melanization in plants was discovered to be dependent on the enzyme tyrosinase German histochernist Bruno Bloch demonstrated in 1927 that this same pathway was involved in pigmentation of human skin in vim By 1950 the cells Where these reactions took place the melanocytes were identi ed and it was found that these cells originate embryonically in the neuml crest By 1955 the UV protective effects of melanin in the skin were demonstmted 20th Century JustSo Stories The discovery of the interaction between melanin and UV led to anumber of explanations of skin color The association between vitamin D synthesis UV mdiation and skin pigmentation was suggested in the 1930s but it was largely ignored until revived by Loomis in 1967 e Loornis proposed that the rate ofvitamin D synthesis is governed by pigmentation and keratinization which a ect the amount of UV penetrating to the stratum gramlumm making skin co1or responsive to UV 1eve1s Selection Favoring Dark Skin Selection favoring high levels of melanin pigmentation in areas of high Ultraviolet UV radiation may involve several selective agents 7 Sunburn can cause skin lesions and infections preventing some degree of heat loss 7 Also predisposes to skin cancer High1y pigrn ented skin provides partial protection so se1ection would favor dark skins in high UV areas Skin Cancer 7 Skin cancer is found to be prevalent among light skinned individuals in tropical latitudes 7 In Nigeria and Tanzania no albino over the age of 20 years was found to be free of malignant or premalignant skin lesions 7 In Tanzania chronic skin damage was found in every albino infant by the end of the rst year of life This high rate and early evidence ofskin damage suggests that cancermaiy have been a strong selective pressure in tropical areas Folic Acid De ciency 7 UV light causes denaturization a chemical breakdown of Folic Acid circulating in the blood This can induce adeficiency even ifthe diet supplies adequate folic acid 7 Derieieney symptoms inelude anemia infertility and birth defects espeeiallyneural tube defects High melanin content in the epidermis can protect circulating Folic Acid thereby selecting for dark skin in low latitude areas Immune Suppression 7 Ultraviolet light is known to suppress immune function It has been hypothesized that increasedmelanization could protect the immune system by shielding the blood bome components ofthe system from UV A recent study demonstrated that ditrerences in skin color were not associated with di erences in UV induced immune de cits Selection for Depigmentation Selection favoring low levels of melanin pigmentation in ecosystems where there are low levels of UV radiation primarily in the high latitudes may also involve different factors 7 The regulation of Vitamin D synthesis 7 Frost bite sensitivity and cold tolerance Regulation of Vitamin D 7 Regulation of Vitamin D synthesis Vitamin D in the body is derived primarily from the skin and secondarily from the diet Vitamin D is synthesized in skin by the action of UVB 7 Precursormolecule is 77DHC or77dehydrocholesterol which occurs in the strata granulosum andbasale ofthe epidermis 7 UV7B exposure eauses aphotoehemieal nonenzymaue eonversion of77DHC into previtamin D High melanin eontent in skin reduces U39V7B exposure and cuts photoehemieal eonversion Regulation of Vitamin D 2 7 Previtamin D is transformedinto vitamin D by a temperature dependentproeess over 273 days 7 vitamin D then ditruses into the blood vessels ofthe dermis 7 The liver andkidney rurthertransrorm the vitamin D into 1257dihydroxyvitamin D which is the most aeuve form of the vitamin The function ofVitamin D is to actively cause calcium absorption across the vwll ofthe small intestine into the blood stream Regulation of Vitamin D 3 Calcium is used for bone and tooth development as well as for nervous and muscle action a The skeleton serves as a calcium reservoir 7 1r calcium ieveisin intracellular uid drops hormones are reieasedto eause resorption ofboneplacing calcium into eireuiauon Regulation of Vitamin D 4 39 A de ciency of Vitamin D in infants and children causes Rickets in adults a deficiency causes osteomalacia r Rickets refers to a defect in the calci cation ofgrowing bone so that the bones are structurally weak and unable to withsth mechanical pressure Symptoms ineiude museie Weakness deformity ofthe long bones ineiudng bowedlegs knucklerlike proieetaons along the rib eage raehitae rosary deformities ofthe pelvis that are often permanent e Lunghune derurmity impairs iueumutiun e Pelvic disturuun can make ehiidh mung danga39uusnputenually hlling mutherand baby Prior to widespread Vitamin D supplementation in the 1930s Black women in the U 5 showed nearly 8 times greater peivie deformity than White women Rickets Regulation of Vitamin D 5 Assuming that our ancestors had dark skin in the tropics as hominids moved into higher latitudes there would have been substantial selection favoring 1ower me1anin content in the skin to improve Vitamin D synthesis Potential for synthesis of previtamin D3 in lightly pigmented human skin based on annual average UV if x7 a e JV ll a 4 Net V r znnei deizimeiiv i1iiineieiiiiv quotandquot mrmghm an on iiermeinr mew r iiie Malayur Areas in which UV is not sufficient over the year for lightly moderately and highly melanized skin on averaged over the entire year for moderatel pigmented human skin 9th gmented human sk Predicted shading of skin colors for indigenous humans Gradation of skin colors for known indigenous human populations and biosynthesis ofVitamin D I k e Mncan l o nastnralrsts Alternatlves to skln productlon of a 09 I wllhltlnh w reliance nn Vltam 1n D for Calc1um Absorptlon E quotash Hulk 0 an 39 Lactose appears to promote the absorption of 70 39 calcium E 39 w 50 7 Human milk is 50 higher in lactose than cow s milk 3 infants show better calcium absorption when nursing E 5 7 Milk drinking increases calcium and lactose intake 5 40 Lamsand may lessen selection on UVVitamin D E 30 42222 39 Fish consumption increases intake of vitamin D 3 20 E 3 quotquot3 quotquot 7 Fish eating would decrease selection for light skin color 53 resnurces 5 1391 D 20 10 SD LATITUDE DEGREES N or S Frost Bite and Cold Tolerance 7 Frost bite sensitivity and cold tolerance As our ancestors moved into higher latitudes they also would have been subj ect to colder temperatures There is a great deal of anecdotal evidence and some medical record evidence most from the Korean War suggesting that individuals with heavily pigmented skin are more susceptible to frostbite Animal studies demonstrate that melanocytes are more easily destroyed by freezing than the rest of the skin cells Frost Bite and Cold Tolerance 2 Experiments with guinea pigs cooling both dark and light skinned areas on the same animal showed dark skin more susceptible to frost bite Frost bite cripples hands and feet causing survival problems and secondary infections including gangrene may be fatal Sexual Selection Mating preference for skin color was promoted by Darwin as being one of the primary reasons for differences in skin color between the races of man Recently Aoki 2002 has argued that this is the primary reason for the distribution of aboriginal skin color Sexual selection and skin color Aoki argues that selection is unidirectional favoring dark skin to prevent sun burn skin cancer folate denaturation etc 7 This selection wouldbe relaxed at higherlantudes with lower UV stress He suggests that there is anear universal preference for light skin in human mate selection 7 As selection lessened at higherlantudes this intersexual selection forlighter skin color wouldbecome a stronger force driving skin eolorgenes in the direction ofless pigmentation The clinal distribution ofskin color results from the balance between natural and sexual selection References Aoki K 2002 Sexual selection as a eause ofhuman skin eolourvarratron Darwin39s hypothesis revisited Ann Hum Biol 29 5894508 Beall CM and Steegmann T Jr 2000 Human adaptauon to elrmate temperature ultraviolet radiation and alutude in sunson s Bogin B HussrAsbmore R O39Rourke D eds Human Biology An Evoluuonary andBiocultural Perspeetrve New York Wileeriss Pp 163224 Blangero J and WilliamsrBlangero s 1992 Quantitauve genetre analysis of skin re ectance amultrvarrate approaeh Human Biology 64 35439 Jablonsky NG and G Chaplin 2000 Evolution ofhuman skin color Journal ofHuman Evoluuon 39 577106 Jeevan A and Kripke ML 1993 Ozone depleuon andthe immune system The Lancet3421159r1160 Joblrng MA Hurles ME andTylerrSmith c 2004 Human Evolutionary Geneues origins Peoples Disease NY NY Garland smith Montellano BR Cruz 1993 Melanin Afrocenmcity and pseudoserenee Yearbook of Physreal Anthropology 36 3358 Robins AH 1991 Biologeal perspeeuves on human pigmentation Cambridge Cambridge University Press Sturm RA 1998 Human pigmentation genes and their response to solar UV radrauon Mutat Res Rev 422 6916 Spencer r 1997 Skin color in r Spencer ed History ofPhysical Anthropology Vol 2 Pp 9457955
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