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Home , Human skin color, Skin

Differences in are among the most con- spicuous of physical variations. From time im- memorial people have been curious about them. It is only in the last 30 years that we have made consider- able progress in understanding the nature of skin - mentation, but the knowledge we have acquired has unfortunately not been transmitted to most teachers. The subject has been ignored by most text- books, hence, for the most part, high school and col- lege students are not exposed to the teaching of this importanttopic. This articleis an attemptto fill the gap between researchers and educators by summarizing Biology of Skin what has been published only in scientific journals. Differences in skin color are largely due to the amount and distribution of the , a very durablebrown- pigment, widely distributed Colo Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021 both in plants and animals. In , as in other mam- mals, it is synthesized and secreted by special cells, called , which are found between and often below the basal cells of the (fig. 1A). The striking differences observable in skin pigmenta- tion are not due to differences in number of melano- Alain Corcos cytes per unit of skin area, as one might expect, but rather to differences in the level of activity of the melanocytes present. The number of melanocytes per unit of surface area in any part of the body is, in fact, roughly the same among all individuals, regardless of the color of their skin (Toda et al., 1973). Differences in activityof melanocytes is attributableto two factors, one genetic and one environmental ( radiation). Viewed from the perspective of photobiology, skin pigmentationis for practicalpurposes divisibleinto two components: 1) Constitutive skin color which designates the geneticallydetermined levels of melanin pigmen- tation developed in the absence of exposure to , and 2) Facultative(inducible) skin color-"tan"-that characterizesthe increase in melanin pigmenta- tion induced by ultravioletradiation over consti- tutive levels. Facultative and constitutive melanin pigmentation stem from the same process: synthesis of pigment granules, called , that travel from the lower to the upper epidermis. People vary in their con- stitutive skin color, but whatever their innate pigmen- tation, most people are able to increase the melanin Alain Corcos is a professor in the Departmentof Natural Science, content of their skin by exposure to the . This is MichiganState University, East Lansing 48824. He was bom in Paris, fortunate, since melanin has an important biological Franceand came to the United States afterWorld War II. He earned B.S., M.S. and Ph.D. degrees in botany and plant from function: protection against skin damage from the Michigan State University. He has taught at the University of harsh ultraviolet rays of the sun. California-SantaBarbara and the Oregon School of Education,and has been a researchassociate in the Institute for CancerResearch, Universityof MichiganMedical School. He has taught a course for Biology of Melanin, Melanocytes, and non-scientists on the biological concept of race for more than ten Melanosomes years.Dr. Corcosis a memberof AAAS, GeneticsSociety of America, and the AmericanGenetics Association, among other organizations. Melanin is a complex biological substance which is

90 THE AMERICAN BIOLOGY TEACHER, VOLUME 45, NO. 2, FEBRUARY 1983 formed from the colorless amino acid through by hormones. It would also explain the fact that they a series of at least seven steps, with several in- have dendrites, threadlikeextensions characteristicof termediate compounds (Lerner1961). A simplified il- cells worming their way between epidermal lustration of how a colorless product turns into a col- cells. Melanocytesare associatedwith a group of cells, ored one will aid in understanding this process. the , into which they secrete melano- Though plants do not produce melanin, they produce somes (fig. 1B). a similarsubstance. If cut open and left exposed to the These melanosomes go through various stages of air, an apple becomes on the exposed surfaces. development within the melanocytes as described by The formation of the brown pigment in the apple is Toda and Fitzpatrick(1971) (fig. 1B): similar to the formation of melanin in the bodies of Stage I. The is a spherical vesicle human beings. Cells of an apple contain a colorless without recognizable structure. product, phenol, which in the presence of in StageII. The melanosome is an oval organelle with the air turns into a colored substance called quinone. distinctive internal structure. There are two steps in this process. First, the phenol StageIII. The melanosomecontains a certainamount is oxidized into a diphenol, then the diphenol is ox- of melanin. idized into a quinone. In man, tyrosine is oxidized in- StageIV. The melanosome is fully melanized.

to a quinone called dopa-quinone. The tyro- How the melanosomes travel from below the epi- Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021 sinase acts as a catalystin the reaction.In furthersteps, to the upper layers of the epidermis is not en- dopa-quinone is changed into dopa-melanin. Finally, tirely known. It is known, however, that the melano- units of dopa-melanin combine into a chain and unite somes travel to the tips of the dendrites which are en- with to make melanin, a very stable end pro- gulfed by the keratinocytesin contact with them. The duct. If the enzyme is not availablebecause melanosomes migratewith the epidermalcells toward of a genetic defect, melanin cannot be formed, the surfaceof the skin. Once inside those cells, melanin resulting in an albino individual. granules tend to collect around the nucleus, forming The amount of melanin produced in an individual a shroud over it. Such an orientationof melanin sug- is under the direct influence of the gests that it is there to protectthe genetic materialfrom stimulating hormone (MSH) produced by the pituita- damage by ultraviolet radiation. ry . The release of MSH from the pituitarygland is controlled by two hormones, hydrocortisone and Melanosome and Human Diversity adrenocorticotropichormone (ACTH).The hydrocor- Before irradiation with ultraviolet , the tisone, secreted by the outer part of the adrenalgland, melanocytesof Europeans() containa few stage inhibits the release of MSH while the ACTH acts to IV melanosomes, but there may be a number of stage increase the release. The balanced action of the two II and III melanosomes present. In individuals with hormones results in the individual's appropriate very light pale skin there are a few in any stage of degree of constitutive skin color. If are given development. In the skin of -headed Europeans, of their large injections MSH, skin color will begin to the melanocytes are small with few dendrites (Szabo darkenwithin 24 hours. Daily doses lead to more dark- et al., 1972)-a possible reason why such people are until skin ening the injections are discontinued. The more susceptible to . The skin of Asians, returns to its normal within color three to five weeks AmericanIndians, and EastIndians contain moderate- after the last injection. People with respond ly melanized stage III and IV melanosomes. On the more to MSH than with skin. markedly people light otherhand, the melanocytesof Africansand Australian The of skin in in darkening pregnancyresults partfrom aboriginesare usuallyfilled with stage IV melanosomes the of the and enlargement pituitary the consequent and with only a few stage II and III melanosomes. release of extra MSH (Lerner 1961). Afterexposure to ultravioletradiation, these "racial" in Melanin is synthesized specialized organelles differencesdecrease considerably. The skin of non-red- melanosornes within called the cytoplasm of the headed Europeanscontains melanosomes in all stages inside the skin. The melanocytes deep melanocytes of development. There is an increase in the relative originateas partof the embryonicnervous system from proportion of stage II and III in the melanocytes of the ridges of the spinal cord (Quevedo 1973). (More Africansand Australiansand an increase in activityof precisely, when the neuralfolds fuse at the dorsalmid- stage III and IV melanosomes in the skin of Asians, line of the spinal cord, part of the embryonic cells of American Indians, and East Indians. the are trapped outside between the But the most strikingultrastructural "racial" differ- neural tube and the epidermal . Some of ence exists in the distributionand size of melanosomes these cells, then, migrate to form the eventual melan- within the keratinocytes. Melanosomes can occur as ocytes.) This relationshipto nerve cells helps in under- single units surrounded by a limiting membrane or standing why skin pigmentationis so much influenced they can be aggregated, packaged two to four parti-

SKIN COLOR 91 DARK LIGHT OR SKIN TANNED SKIN

EPIDERMIS \\ Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021

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FIGURE1. Schematicdiagram: A. section. B. Melanocyteunit producing melanosomes of increasingpigmentation (1-4) that are transferredto epidermal cells. C. Melanosomes aggregated in organelles (5) and isolated (6). [After Quevedo et al., 1975]

92 THE AMERICAN BIOLOGY TEACHER, VOLUME 45, NO. 2, FEBRUARY 1983 cles within a limiting membrane (fig. 1C). This phe- volved in skin pigmentation. Though European- nomenon of aggregation appears to be dependent Africanhybrids tend to be closerto the Europeans(Bar- upon two main factors, size and level of UV irradia- nicot 1958; Harrison and Owen 1964), European and tion. Small melanosomes tend to aggregate, but large Indian (hybrids)tend to be very close to the midpoint ones do not (Toda et al., 1972). In dark-pigmented value (Tiwari1963). The small number of offspring of Africansand Australians, the melanosomes are most- Papauan-Europeanmarriages recorded (Walsh and ly single, whereas in Europeans and Asians they are Price 1963)also seem to be approximatelymidway be- aggregated.Furthermore, the melanosomesof Africans tween the two ancestral groups. and Australians are significantly larger than those of The near absence of in skin pigmenta- Europeans and Asians. Preliminarystudies indicate tion is at variance with what we know about enzyme that hybrids between Europeans and Africans have activity. Heterozygosity for most enzyme variations mixed types of melanosomes (Szabo et al., 1972). On results in enzyme activity closer to the most active the other , packagingof melanosomescan change variant. Since tyrosinase is the major enzyme in- in skin exposed to UV radiation(Toda and Fitzpatrick volved, one would expect that the (or ) re- 1971) (fig. 1C). This suggests that the packaging of sponsible for tyrosinase activity would show melanosomes is environmental as well as genetic in dominancewith no additiveeffect. On the other hand, nature. As the melanosomestravel up through the epi- tyrosinase inhibitorsare known to be present in light- Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021 dermis, they undergo degradation, tending to break skinned individuals (Kinglemanand Van Scott 1961), into amorphous melanin particles. This is particularly and the genes responsible for these inhibitors would true for those melanosomes transportedin complexes. be expected to show dominance for color. These ultrastructuralstudies suggest that the dif- It is possible that balanced dominance between in- ferences between dark and light pigmented skin are hibitorsand producersto tyrosinaseis a betterassump- a question of degree, not of absoluteness. That these tion than non-dominance.Variations in skin color and differencesare not clear- furthersubstantiated by estimates of underlying genetic control are probably the fact that Europeansare a very heterogenous group influenced by the size, melanization, and dispersion with regard to skin pigmentation. On one end of the of the melanosomes. Moreover, differences in degree are the almost unpigmented light blonde Nordic of degradation of melanosomes may serve to widen people with very few melanosomesin the basal epider- the variation in skin color and lead to exaggerated mal cells before irradiation,but with melanocytes that estimates of genetic differences. Quevedo (1973) has can produce pigment after UV stimulation. On the suggested that the already modest number of genes other end of the scale are the darklypigmented Medi- which account for differences in skin pigmentation terranean people with melanosome complexes well may still be too great. developed even before irradiation. Skin Color, Geography,and NaturalSelection Genetics of Skin Color Skin color and latitudeare seemingly correlated.The Skin color differences are apparently due to the amounts of pigment tend to diminish from tropical variationin the number of melanosomes, their distri- zones to temperatezones and to increase again in the bution, and their degree of melanization. Obviously arcticpopulations. Although sharp breaks in this pat- these differencesmust be in part genetic. Unfortunate- tern occur, the distributioncan be broadly character- ly we know little about the genetics in skin color, ized as clinal, and correspond to naturallevels of the especially at the biochemical level. incidence of ultraviolet radiation. Most of the earlygenetic studies involved differences If the evidence of man's origins near the equator in between Africans and Europeans (Davenport and Africais accepted,we are confrontedwith the possibili- Danielson 1913;Herskovits 1926, 1930;Barnicot 1958; ty that our remote ancestors may have been, on the Stern 1953, 1970; Harrison and Owen 1964; Harrison whole, darkly pigmented as is the case with popula- et al. 1967). Other studies considered differences be- tions still native to that region. If so, the lesser tween Europeansand Vietnamese(Olivier 1967), Euro- pigmented groups now occupying more temperate peans and American Indians (Conway and Baker zones in the world must have developed from these 1972), Europeans and Australian aborigines (Walsh more pigmented groups. This suggests a geographic and Price 1963);Europeans and Indians (Tiwari1963; factor of some sort at work related in some way to Kalla 1969). Though researchers have used different levels of UV radiation. methods to measure pigmentation and studied dif- Total solar radiation varies little in relationship to ferentpeople, they all concludedthat only a few genes, latitude, while that of the shorter wavelengths of the three to four pairs, are involved in the inheritance of spectrum(in the ultravioletrange) change rapidlywith skin color, and their effects seem to be equal and ad- an increase in latitude. It is thisKpart of the solar radia- ditive. Relatively little dominance seems to be in- tion that causes sunburn and stimulates the synthesis

SKIN COLOR 93 of D needed in formation.It is assumed, women. In those days, this was often fatal for the and logicallyso, that melaninprotects against sunburn, mother as well as for the child. since dark-skinnedpeople are less susceptibleto it than Another factor that might have played a role in the light-skinned people. But it is doubtful that freedom selection for light skin color in northern latitudes is from sunburn was a factorin of constitutive frost . It has been known since World WarI that skin color. Blum (1961) stresses, "Sunburn is essen- "black" soldiers were more prone to frost injury than tially an acute effect, which while it may be briefly de- "." This was clearly established by careful epi- bilitatingin severe cases involving a considerablearea demiologicalstudies during the Koreanconflict (Army of the body does not cause prolonged, systemic dam- Medical Research Laboratory1953). Recently, Post, age." The sunburning portion of sunlight seems to be Daniels, and Binford (1975) tested the idea that pig- the principalcause of cancerof human skin. However, mentation may be a factor in cold injury. They took it is also doubtful that skin pigmentation resulted as black and white piebald guinea , anesthetized an adaptation to sunlight for this reason. Many skin them, removed their by plucking, and froze pig- cancers are not malignant, appearing late in life, well mented and non-pigmented skin of this animal. The after reproduction. only differences between black and white skin in the A more likely factorin the evolution of variableskin animal is the presence of melanin. Any difference dis- pigmentation the is formation of which is covered would have to be attributedto this pigment Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021 essential for proper metabolism. Unlike other and could not be attributedto individualvariation since , it is not present in significantamounts in the both pigmented and non-pigmented areasoccur in the normal diet; it occurs in the liver oils of bony , same animal. Pigmented cells were damaged more and in very small amounts in a few foodstuffs in the seriously than non-pigmented cells indicating that summer. Almost none is present in foodstuffs in the melanin might be a significant factor in frost injury. winter. In man, vitamin D is synthesized by action of Pigmentationwas thought for a while to be beneficial UV on a sterol compound, 7 dehydrocholesterol,in the in a tropical climate, because melanin would prevent lower layers of the epidermis. Variations in the con- the formationof too much vitamin D, which would in centration of melanin and its resulting filtering of the turn cause calcificationof . However, there is no sun's rays have a very importantconsequence in alter- known case of vitamin D toxicity due to prolonged ex- ing the skin's abilityto synthesize vitaminD. The func- posure to the sun. This has been explained recently tion of vitamin D is essentiallya regulatorof the body's by Holick, MacLoughlin, and Dopplet (1981). They use of calcium and phosphorus. It controls found that if the skin is exposed to sunlight for a long of calciumthrough the intestine, regulatesrenal excre- time, previtamin D3, a precursor of vitamin D, is tion, and aids in skeletal mineralization. These func- changed into two non-functionalcompounds that are tions are so criticalthat the lack of vitamin D causes eliminated. bone defects, in children, and osteomalacia in Brandaand Eaton (1978)explained why human pop- adults. ulations native to areas of intense sunlight tend to be It is probablethat decreasedmelanization was a strict heavily melanized. Dark pigmentation may protect requirementfor increased efficiency of vitamin D for- against photolysis of crucial light sensitive vitamins mation in the northern latitudes, for in northern and other products by ultraviolet radiation. To test climates where sunlight is limited during the winter their hypothesis, they used (a derivative of the months, and where the body is generallyfully clothed vitamin folic acid) as a model. They found that ex- against the cold, vitamin D synthesis will depend on posure to UV does decrease the amount of folate in the exposure of a small area of skin to relatively small the . Since deficiency of folate, which occurs in amounts of UV light. Eskimos are an exception. many marginallynourished , causes severe Though they are darklypigmented, they have no prob- and infertility,it is possible that prevention of lems synthesizing vitamin D because there is an ade- ultravioletphotolysis of folate and other light sensitive quate amount of the vitamin in their diet. The im- by a dark skin may be one of the reasons for portance of vitamin D synthesis by ultraviolet radia- the maintenanceof this characteristicin human groups tion in the northern climates is illustrated by the fact indigenous to regions of intense solar radiation. that in the 19th century in industrial cities of , where smoke obscured sunlight, there was a prev- Conclusion alence of rickets among the children. Before the ad- vent of vitamin D-enriched milk, vitamin D deficien- Pigmentary variation is among the most conspicu- ous of human differences cy was more prevalent among American than and has raised speculation as to where it was among Americanwhites (Loomis1970). Though and why these differences originated. It is only in the victims usually survived, distortion of the pelvis the last 25 years that a large amount of in- formation on the frequently caused difficult childbirth in affected biology of pigmentation has ac- cumulated. Only recently speculation has given way

94 THE AMERICAN BIOLOGY TEACHER, VOLUME 45, NO. 2, FEBRUARY 1983 to researchin explainingwhy some human groups are . 1930. The anthropometry of the American more pigmented than others. Melanin pigmentation Negro. ColumbiaUniversity Contributions in Anthropology. can be regarded as a four-step process: production of II: 1:28. HOLICK, M.F., MACLOUGHLIN,J.A., and DOPPLET, melanosomes, melanization, and transfer of these S.H. 1981. Regulation of cutaneous Previtamin D3 melanosomes from melanocytesto epidermalcells, and photosynthesis in man. Skin pigment is not an essential degradation of the melanosomes on their way to the regulator. Science211:590-592. upper epidermis. These four steps appear to be influ- KALLA,A.K. 1969. Affinities in skin pigmentationof some enced by a few genetic factors as well as by a primary Indian populations. HumanHeredity 19:499-505. , ultraviolet radiation. In most KINGLEMAN,T.P., and VAN SCOTT,E.J. 1961. Tyrosinase activity in melanocytes of human albinos. Journal of Inves- people, the melanin content of the skin increases to tigativeDermatology 37:73-76. protect them against the harsh ultraviolet rays of the LERNER,A. 1961. Hormones and skin color. Scientific sun. This increase is familiarlyknown as tanning. The American205:98-108. way in which melanin increases during tanning is not LOOMIS,W.F. 1970. Rickets. ScientificAmerican 223:76-91. fundamentallydifferent from the way it is formed con- OLIVIER,G. 1967. Les characteresdescriptifs des Franco- Vietnamiens. Bulletinet memoirede la societed'anthropologie stitutively. There is an increase in the activity of de Paris (12 series). Jan.-Mai. 1-15. melanocytes involving a greater amount of tyrosinase POST, P.W., DANIELS,F., and BINFORD,Jr., R.T. 1975. and a new production of melanosomes. In addition, Cold injuryand the evolution of white skin. HumanBiology Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/90/40177/4447637.pdf by guest on 03 October 2021 some limited division of melanocytes may also con- 47:65-80. tribute to the increase of melanin observed. QUEVEDO,W.C., FITZPATRICK,T.B., PATHAK,M.A., and JIMBOW,K. 1975. Role of light in Since the mechanisms for skin color formation(con- variation. American Journal of Physical Anthropology stitutive and inducible) are similarin individuals vary- 43:393-408. ing widely in depth of pigmentation, and are inter- I 1973. Genetic control of melanin metabolism related, and since the genes for these mechanisms are within the melanin unit. Journalof InvestigativeDermatology very few, the importanceof skin color variationin the 60(6):407-417. STERN,C. 1953. Model estimates of the frequencyof white total picture of human diversity is much, much less and near segregates in the AmericanNegro. Acta Genetica than society has made it. StaticaMedica 4:281-297. . 1970. Model estimates of the number of gene References pairs involved in pigmentation variabilityof the Negro- ARMYMEDICAL RESEARCH LABORATORY. 1953. Cold American. HumanHeredity 20:165-168. injury-Korea 1951-1952,Report 113. Fort Knox, Ky. SZABO, G., GERALD,A.B., PATHAK, M.A., and FITZ- BARNICOT, N.A. 1958. Reflectometry of the skin in PATRICK,T.B. 1972.The ultrastructuresof racialcolor dif- SouthernNigerians and in some mulattoes.Human Biology ferences in man. In Riley, V. (ed.) Pigmentation:Its genesis 30:15-160. andbiological control. New York:Appleton-Century Crofts. BLUM,H.J. 1961. Does the melanin pigment of human skin TIWARI,S.C. 1963.Studies of crossingbetween Indiansand have adaptive value? An essay in human ecology and the Europeans. Annalsof HumanGenetics 26:219-227. evolution of race. QuarterlyReview of Biology36:50-63. TODA, K., and FITZPATRICK,T.B. 1971. The origin of BRANDA, R., and EATON, J. 1978. Skin color and melanosomes. In Kawamura,T., Fitzpatrick,R., and Se- photolysis: An evolutionary hypothesis. Science jel, M. (eds.) Biologyof normaland abnormalmelanocytes. 201:625-626. Tokyo: University of Tokyo Press. CONWAY, D., and BAKER,P.T. 1972. Skin reflectanceof TODA, K., PATHAK,M., PARRISH,J.A., FITZPATRICK, Quecha Indians:The effects of genetic admixture,sex and T.B., and QUEVEDO,Jr., W.C. 1972. Alterationof racial age. AmericanJournal of PhysicalAnthropology 36:267-282. differencesin melanosomedistribution in human epiderm- DAVENPORT,C.B., and DANIELSON,F.H. 1913. Skin col- is after exposure to ultravioletlight. Nature(New Biology) or in Negro-white crosses. CarnegieInstitute of Washington 236:143-145. Publication188. TODA, K., PATHAK,M., FITZPATRICK,T.B., QUEVEDO, HARRISON,G.A., and OWEN, I.T.T. 1964. Studies on the W.C., MORIKAWA,J., and WAKAZAMA,J. 1973. Skin inheritance of human skin color. AnnalsHuman Genetics, color: Its ultrastructureand its determiningmechanisms. London28:27-37. In Riley, V. (ed.) Mechanismsin pigmentation,Vol. 1, Pig- mentcell. , OWEN, I.T.T., DaROCHA, F.I., and SAL- ZANO, F.M. 1967. Skin color in Southern Brazilpopula- WALSH,R.J., and PRICE,A.V.G. 1963. Studies in melanin tions. HumanBiology 39:21-31. pigmentationof the skin of racialcrosses in Port Moresby. HERSKOVITS,M.J. 1926. Socialselection in a mixed popula- Oceania33:287-292. tion. ProceedingsNational Academy of Sciences U.S.A. 12:587-593.

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