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Biology of Skin Color

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Biology of Skin Color Differences in skin color are among the most con- spicuous of human 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 pig- mentation, but the knowledge we have acquired has unfortunately not been transmitted to most teachers. The subject has been ignored by most biology 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 pigment melanin, a very durablebrown-black 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 man, as in other mam- mals, it is synthesized and secreted by special cells, called melanocytes, which are found between and often below the basal cells of the epidermis (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 (ultraviolet 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 sunlight, 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 melanosomes, 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 sun. 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 genetics 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 tyrosine 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- nerve 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 keratinocytes, 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 brown 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 melanosome is a spherical vesicle human beings. Cells of an apple contain a colorless without recognizable structure. product, phenol, which in the presence of oxygen 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 enzyme 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, dermis 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 proteins to make melanin, a very stable end pro- gulfed by the keratinocytesin contact with them. The duct. If the enzyme tyrosinase 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 melanocyte gests that it is there to protectthe genetic materialfrom stimulating hormone (MSH) produced by the pituita- damage by ultraviolet radiation. ry gland. 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 light, the tisone, secreted by the outer part of the adrenalgland, melanocytesof Europeans(whites) 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 humans are given development. In the skin of red-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 sunburn. 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 dark skin 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 part of 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 nervous system are trapped outside between the But the most strikingultrastructural "racial" differ- neural tube and the epidermal ectoderm. 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 _X FIGURE1. Schematicdiagram: A. Human skin section. B. Melanocyteunit producing melanosomes of increasingpigmentation (1-4) that are transferredto epidermal cells. C. Melanosomes aggregated
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