Table Of ContentBLACK SKIN
STRUCTURE AND FUNCTION
William Montagna
Oregon Regional Primate Research Center
Beaverton, Oregon
Giuseppe Prota
Department of Organic and Biological Chemistry
University of Naples
Naples, Italy
John A. Kenney, Jr.
Department of Dermatology
Howard University College of Medicine
Washington, D.C.
Academic Press, Inc.
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Library of Congress Cataloging-in Publication Data
Black skin / edited by William Montagna, J. A. Kenney, Giuseppe Prota.
p. cm.
Includes bibliographical references and index.
ISBN 0-12-505260-X
1. Skin—Physiology. 2. Blacks—Physiology. 3. Color of man.
4. Malanin. 5. Melanocytes. I. Montagna, Williams. II. Kenney,
J. A. (John A.) III. Prota, Giuseppe.
QP88.5.B62 1993
612.7'927—dc20 93-14784
CIP
PRINTED IN THE UNITED STATES OF AMERICA
93 94 95 96 97 98 BB 9 8 7 6 5 4 3 2 1
This book is dedicated to the late Wilbur McNulty, M.D.,
a dear friend and counselor who judged people for
what they were regardless of skin color.
Preface
Classifications of humankind have been based primarily on skin color.
Such grouping has had, and still has, more than any other racial trait, an
enormous impact on social relationships. However, in an essay on human
skin color, Slack (1844) pointed out that if all the people in the world were
arranged in a straight line according to the color of their skin, beginning with
the whitest and ending with the blackest, it would be impossible to see where
the white race ended and the black race began. And yet European (Christian)
art and literature have reduced the gamut of human skin colors to a black-
white dualism with the underlying notion that “Whites” are biologically and
intellectually superior to “Blacks.”
For many years even scientific research was not spared from racial
prejudices, and the answer to such a basic question as why black skin is black
xi
xii Preface
and, conversely, why white skin is white was left largely to anatomists and
anthropologists. During the last two decades, however, progress in the
chemistry of melanin pigmentation has led us to a better understanding of the
regulatory mechanisms that underlie skin color differences among the hu
man races. Although many things still remain to be explained, it now seems
clear that skin pigmentation is the result of a complex interplay between
enzymes and cofactors that appear to act extracellularly and to control the
amount of melanin formed. It is because of these epigenetic factors, and
especially the redox state of the glutathione system, that black skin is black
and white skin is white. The glutathione system is also involved in many vital
biological processes. The more we learn about the regulation of mela-
nogenesis, the more we can check certain misconceptions and prejudices
about ethnic skin colors.
There is increasing evidence that black skin is endowed with specific
biological properties that are lacking in white skin. For example, black skin is
smoother and firmer to the touch than white skin. Black people also appear
to age more gracefully than white people, developing fewer and less conspic
uous wrinkles in their middle-aged years. Black skin is also considerably
more resistant to dermal elastotic changes when chronically exposed to
ultraviolet radiation. Furthermore, epidemiological studies indicate that
black skin is largely spared from skin cancers, including malignant melano
mas (Willis, 1988).
We believe that the greater amounts of melanin and the packaging of
melanosomes in black skin are not the only agents responsible for the
biologic uniqueness of black skin. A biochemical search for the agent or
agents that protect black skin from actinic damages has not yet been pursued
seriously; finding such agent or agents might teach us a great deal about what
causes photoaging and the higher incidence of skin cancer in white people.
This book is aimed at gathering facts and making possible correlations in
our scattered and, as yet, incomplete knowledge of the biology, physiology,
and chemistry of black skin. We include some details of the dermatology and
cosmetology of black skin, from a practical view of skin care and current
treatments of pigmentary disorders. We also discuss the structure of black
skin and current and past thinking about skin pigmentation, and we mention
the multifaceted sociological implications of skin color differences in ancient
and modern times.
We hope that the information and references assembled in this book will
provide a guide to the functional properties of black skin, as well as a
stimulus to further research on the intriguing relationship between skin color
and skin physiology. We believe that this book should be of interest to a wide
range of readers from lay persons to specialists in different scientific and
medical disciplines, such as anthropology, skin biology and biochemistry,
clinical dermatology, and the ever increasing group of research-oriented
practitioners in the field of functional cosmetology.
Preface xiii
Throughout the book we refer to people with black skin as Blacks
(African or American) and to Caucasians as Whites. American Blacks also
refer to themselves as African Americans.
Chapter 2, The Structure of Black Skin, was written in collaboration
with cell biologist Kay Carlisle, who did all of the transmission electron
microscopy research. She also assisted in all aspects of the preparation of this
book. Ms. Carlisle is a silent author of this book.
We are indebted to many people in the preparation of this book. The
entire library staff of the Oregon Regional Primate Research Center has
helped in many ways in ferreting out needed details in the literature. We are
also deeply indebted to M. d’Ischia and G. Misuraca for supplying crucial
references and for their willingness to discuss important points with us. They
also helped in many other ways. We are grateful to the staff of the Depart
ment of Organic and Biological Chemistry at the University of Naples for
their valuable assistance. We could not have completed this book without the
help of all these people.
We thank Ms. Bernice Marcks who was always cheerful and patient even
when we asked her to type all-but-illegible, scribbled material. We thank the
medical illustrator, Mr. Joel Ito, who accepted with his usual good humor the
task of drawing the diagrams. Although we were seldom specific, he always
understood what we needed.
Our special thanks to Vaughn B. Critchlow, Director of the Primate
Center and Chairman of the Division of Neurosciences; the late Wilbur
McNulty, Chairman of the Primate Center’s Division of Pathology; and
Michael K. Axthelm, current Chairman of the Center’s Division of Primate
Medicine, for their generosity and for allowing us to use the Primate Center
facilities.
1
Skin Color and
the Evolution
of Mankind
I. INTRODUCTION
Melanin is a pigmented substance characteristically found in the
skin of vertebrates including that of all human races. It is pro
duced by specialized cells called melanocytes. The functional
state of these cells is primarily under genetic and hormonal
controls, and establishes the level of constitutive skin pigmenta
tion (the basic color of protected, unexposed skin). Facultative
skin color (inducible), or tan, is the intensified skin color pro
duced by exposure to sunlight. The ability of facultative skin
color to change is related to constitutive skin color. This means
that the capability of light-skinned persons to tan when exposed
1
2 1. Skin Color and the Evolution of Mankind
to ultraviolet light (UVL) is largely dependent on the gene action expressed in
constitutive skin color (Pathak et al., 1982). Color would seem to be only a
superficial difference, but differences in skin color are also related to the
biology and biochemistry of the skin as well as to the whole individual and
can provide important clues to a variety of internal problems. Physicians, for
example, have long used skin color as an indicator of the health of indi
viduals.
People are familiar with the tanning that develops after exposure to
sunlight. This increased pigmentation is one of the many responses of skin to
actinic (sun-induced; solar) exposure, beginning with cell injury and leading
to erythema (the reddening that signals inflammation) and an increased
proliferation of melanocytes and keratinocytes. Since our knowledge of
photobiology of the skin is incomplete we cannot explain why some individ
uals are more susceptible than others to the harmful effects of sunlight. What
seems clear is that heavily pigmented skin, be it constitutive or UV-induced
(facultative), is far less susceptible to actinic damage than lightly pigmented
skin.
The degree of pigmentation also appears to be a factor in determining
the risk of skin cancers, including melanomas. Fair-skinned, blue-eyed, and
red-haired people, who tan poorly and burn easily, are at much greater risk
than good tanners or dark-skinned people (Willis, 1988). All these observa
tions are consistent with the notion that melanin plays an important role in
protecting skin (photoprotection) and could serve as a critical “handle” for
natural selection. However, things are not so simple for reasons that will
become apparent as we proceed further in the analysis of the origin, varia
tion, and significance of skin color in human beings.
The other principal source of human skin color is carotene. There is also
oxyhemoglobin (red) and deoxyhemoglobin (bluish), respectively, from the
arterioles and venules in the superficial dermis, that impart color, in particu
lar to the glabrous vermillion border of the lips of white people (Quevedo et
al., 1985).
II. FACTORS THAT REGULATE HUMAN SKIN COLOR
The Hungarian-born American dermatologist and physiologist, Stephen
Rothman (1954), like earlier investigators, concluded that, regardless of
race, there are no qualitative differences in the pigmentation of human skin.
Robins (1991) states in his book on human pigmentation that the evolution
of skin color will continue to puzzle investigators for a long time. Even
though many genes may influence human skin color, its pigmentation can
involve as few as three to six pairs that “interact during melanogenesis”
(Quevedo et al., 1985).
Figge and Allen (1944) and then Rothman et al. (1946) first showed that
II. Factors That Regulate Human Skin Color 3
the glutathione in the epidermis inhibits melanogenesis. Halprin and Ohka-
wara (1966) later showed that the correct amount of the tripeptide-reduced
glutathione (y-glutamyl cisteinyl-glycine) in the epidermis exists in greater
amounts in the skin of Whites than in that of Blacks, and that this could be
the chemical mechanism that controls human skin color. When the sulfhy-
dryl groups are poisoned (inactivated), their inhibitory activity on mela
nogenesis is removed. Halprin and Ohkawara (1966) have also shown that
the hyperpigmentation (tan) that follows the exposures of white skin to UVL
is always preceded by a drop in glutathione reductase and reduced gluta
thione in the epidermis. It would seem that there is little more to say about
the genes that control human skin pigmentation. This conclusion, however,
has been challenged, and the only thing we know for certain is that skin
pigmentation is controlled by genes (see Chapter 5, sections VIII and IX).
Since there is no conclusive information about the origins of human skin
color, Leakey and Lewin (1977) made an “intuitive guess” that the loss of
dark skin pigmentation and thick, kinky hair probably occurred early in the
evolution of Homo erectus, “when our ancestors were still in Africa.” These
authors believe that as H. erectus migrated into the cooler northern hemi
sphere, their skin became lighter and lighter colored through “genetic selec
tion,” another guess. It is not necessary to invoke genetic selection since
completely isolated populations can differentiate through chance, called drift
(genetic drift refers to random change in the gene complex of a population)
(Cavalli-Sforza, 1991).
Skin color is regulated primarily by genetic factors, hormones, and
sunlight. Social interaction and sexual attraction are influenced by differ
ences in skin pigmentation. Skin color can also vary according to the time of
the year and to the sex and age of individuals.
A. Gloger’s Rule and Skin Color
In 1833, Gloger wrote about his observations on the regularities that occur
between the pigmentation of feathers and furs and the relationships of these
colors to the geographic areas in which the animals lived. From this article,
scholars have formulated what is now known as Gloger’s rule, which states
that the darkest forms of a species or subspecies and race occur in the hot,
humid areas of their range. Rensch (1935) interpreted Gloger’s rule to mean
that within a polytypic species of warm-blooded vertebrates, those members
that live in cool, moist regions have less dark pigmentation than those in hot,
moist regions. In hot, arid regions, there is a lessening of the dark-brown
color (eumelanin) and an increase of the reddish-brown color (pheomelanin),
a phenomenon known as desert coloration. In cold regions the reddish-
brown melanin diminishes, and in the Arctic dark-brown melanin is dras
tically reduced (polar coloration). Cowles (1959) believes that animals are
dark-skinned in hot, humid places as an adaptation to their environments
4 1. Skin Color and the Evolution of Mankind
(hot, humid areas are dark) and Robins (1991) agrees with Cowles’ conclu
sions that the “determining criterion” of an animal’s pigmentation is the
albedo (coefficient of reflection) of its terrain. At first, one may be struck with
the perspicacity of Gloger’s rule. However, when one evaluates it critically,
he/she is likely to find faults in it. Kelso (1974), LaBarre (1955), and others
believe that dark-skinned people who live near the equator may be one of
Nature’s major blunders since dark skin absorbs solar radiation whereas
light-colored skin reflects it.
B. Exceptions to Gloger’s Rule
LaBarre (1955), like many others, is skeptical about the adaptive properties
of black skin to warm areas. He reminds us that “it is not immediately
apparent to the heating engineer” why either dark skin or kinky hair is
adaptive. If insulation from a tropical sun is needed, longer and looser hair
which traps air spaces would be more thermodynamically efficient.
However, Caucasoids have such adaptations to insulate them from the cold
and it is not very clear why Blacks need skin adapted to keeping them warm
in the tropics. LaBarre concludes that many such alleged racial adaptations
“may actually be maladaptions.” Black skin, he says, is well-adapted to hot,
sunny places only as long as “the owner” stays in the shade.
It seems to us important that since man’s fur is at best scanty, one ought
to consider the color of skin rather than that of hair. Investigators of skin
colors do not mention (or are not aware of) the many apparent vagaries that
occur in the pigmentation of mammalian skin (Post et al., 1975; Montagna,
1985). Arctic mammals with heavy white fur, for example, polar bears and
arctic foxes, have black skin, a glabrous (hairless) rhynarium (nose), and
periocular rings of glabrous black skin. Black-, brown-, or silver-gray-furred
African bushbabies (Galago crassicaudatus), are forest and savannah dwell
ers, respectively, with white skin regardless of fur color. Most seals have
dark-brown or black skin. The Hainan langur (Pigathrix nemaeus nemaeus)
from Hainan Island, which has mostly light-gray (whitish) fur, has black skin
(Fig. 1-1). A variant of the same species (a subspecies), the Douc langur
(.Pigathrix nemaeus negripes) from Laos and Vietnam (Fig. 1-2), has white
fur on its face and neck and dark fur over the rest of the body (white spots
may also occur anywhere on the body of this leaf-eating monkey), but the
skin is white even where the fur is dark. And yet, Douc langurs are born with
dark (blackish-gray) skin (Fig. 1-3) that gradually becomes white after a
period of weeks or months. By contrast, the crested Celebes black macaques
{Macaca maura or M. nigra), in which the adult animals have black fur and
black skin all over the body (Fig. 1-4), have young born with “white” skin
that gradually becomes black (Fig. 1-5) as the juveniles grow to maturity. All
chimpanzees have black fur (Fig. 1-6), but their normally black skin is often
splattered with white blotches, particularly on the face. These white spots of
