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It isn't good to take for granted something as important as skin. Take a moment
and imagine the following scene. You're standing in the moist, shadowy
heat of an orchard in the late afternoon of a summer's day. You are
able to stand outside in comfort without overheating, thanks to your skin's
ability to regulate your body temperature and shield you from ultraviolet
radiation. Only a few beads of sweat on your brow and upper lip betray the
fact that your skin is working to keep you cool. As you flick away the fly that
tried to settle on your face, you don't give a thought to the way your skin is
protecting you from the microorganisms on the insect's feet and snout.
You have your eye on a peach dangling from a branch above your head,
and you want to pick it and eat it. As you reach up toward that lovely peach,
you're distracted again by the fly, and the back of your hand scrapes against
the snag of an old branch. Thanks to your skin's fairly tough surface, the
scrape isn't a problem. A welt starts to rise in a few minutes, but your skin
is unbroken because its outermost layer is quite scuff-resistant. You reach
up again, and the elastic properties of the skin on your arm and trunk allow
you tostretch effortlessly until, on tiptoe, you touch the peach. As you
grasp the fruit, you squeeze it ever so slightly and register its subtle softness
through the exquisitely sensitive pressure sensors in the skin of your
fingertips. It is ripe. As you pull the peach off the tree, the temperature sensors
in the skin of your hand let you appreciate its slight warmth. As you
lower your arm, the stretched skin of your arm and trunk returns instantly
to its resting shape.
You bring the peach to your nose and smell it, and then brush it gently
against your cheek, enjoying the feeling of the soft fuzz against your face.
Your sensitive facial skin, with its high density of delicate touch sensors,
is transmitting information about the texture of the peach to your brain.
Just as you prepare to bite into the fruit, an annoying tickle at your ankle
disturbs your reverie, and you realize that a mosquito has just bitten you
while you were so pleasantly distracted with the smell and feel of the peach.
Your skin and its wide-ranging capabilities made the various parts of this
scenario possible. To understand how this is so, an introductory tour of human
skin, exploring its structure and its essential functions, is in order.
One of the most striking features of human skin is that it is basically
naked; in this way it differs from the skin of most of our warm-blooded relatives.
The ancestors of birds and mammals evolved fine, threadlike appendages
on their skin-feathers and hairs, respectively, which regulate
heat interchange and also help to prevent water loss and mechanical
trauma. Lacking such protection, human skin had to undergo numerous
structural changes to give it strength, resilience, and sensitivity. Our skin
is not perfect, but it does a remarkably good job. Our fabric doesn't wear
out, our seams don't burst, we don't spontaneously sprout leaks, and we
don't expand like water balloons when we sit in the bathtub.
Some of the most important properties of skin are related to sunlight.
In humans, the skin and the pigments it contains selectively filter the ultraviolet
radiation emanating from the sun. Our skin has the amazing ability
not only to serve as a protective shield against the damaging effects of
sunlight but also to utilize some of that same sunlight to the body's advantage,
by beginning the process of producing vitamin D right there in
the skin. Thus our skin, like so many other parts of the body, is a compromise
hammered out at the negotiating table of evolution. Its complex properties
reflect a balance, brought about through natural selection, between
conflicting needs-in this case, protection against harmful solar radiation
and production of an essential vitamin.
Skin is made up of layers with different physical and chemical properties.
This laminar, or layered, construction gives the skin its resistance to
abrasions and punctures and allows it to avoid absorbing most substances.
The skin's two major layers, the epidermis and the dermis, differ remarkably
in their composition and function (figure 1). The skin also includes
special types of cells that insinuate themselves into the skin during early
embryonic development. These aptly named immigrant cells play varied
and important roles in protecting the skin, as we'll see later in the chapter.
The skin's outermost layer, the epidermis, shields us from environmental
oxidants and heat, while it also resists water, abrasion, stains, microbes, and
many chemicals-a list of qualities that makes the epidermis sound more
like a revolutionary new type of carpeting than a natural material. It is all
the more astonishing, then, that these useful attributes are found in a self-renewing
layer only about one millimeter thick, which continuously performs
all its functions despite being in a constant state of turnover, with
its outermost cells being shed as they are replaced from below. The epidermis
is composed mostly of a specialized type of epithelium consisting
of multiple layers, or strata, of flattened cells. (An epithelium is a covering
of any external or internal surface of the body.) Because these cells contain
high concentrations of the protective protein keratin, this epithelium is
known scientifically as stratified keratinizing epithelium.
The very surface of the epidermis is its most remarkable layer, the stratum
corneum (figure 2). The stratum corneum is sometimes called the epidermal
horny layer because it consists of a relatively thin sheet of dead,
flattened cells with a smooth, fairly tough, and water-resistant surface. The
only things that interrupt its surface are hair follicles, the pores of sweat
glands, and parts of some of the so-called immigrant cells that help to form
the complex mosaic of the skin. The skin's effectiveness as a barrier against
environmental insult of all kinds, especially oxidative stress such as ultraviolet
radiation (UVR), ozone, air pollution, pathological microorganisms,
chemical oxidants, and topically applied drugs, depends primarily on the
integrity of the stratum corneum.
One of the ways the skin defends itself against some environmental stressors
is to become thicker. When the skin is repeatedly exposed to UVR, for
instance, cell division increases in the deepest layer of the epidermis, the
stratum basale, which is the source of epidermal cells; and, as a result, the
stratum corneum thickens. If the stress, whether external or internal, is
extreme-too much UVR, too much heat, a corrosive chemical such as acid,
some diseases or genetic problems-the stratum corneum can cease to be
an effective barrier. This can have disastrous results if a large area of the
skin is affected.
Keratinocytes, the main types of cells found in the epidermis, are made
up of proteins called keratins. They are responsible for the strength, resistance,
and stretchability of the skin's surface. Within keratinocytes, filaments
of keratin are embedded in a gelatin-like matrix, and layer after layer of these
cells build up from below to make up the epidermis. Between the cells, a
substance rich in proteins and lipids fills the narrow spaces. The elasticity
and imperviousness of the epidermis, especially the stratum corneum, result
from its "brick and mortar" construction, that is, the tight and strong
physical interconnections between adjacent cells and the protein and lipid
material between them. In people with dark skin, the keratinocytes also
contain flecks of the pigment melanin ("melanin dust"), which provide another
layer of protection against UVR.
Scientists have long thought that human epidermis is unique because it
does such a good job of protecting us even though we are effectively hairless.
But the genetic basis for that uniqueness had not been appreciated until
the past few years. One of the ways in which the genetic makeup of humans
varies from that of our closest relatives, chimpanzees, is in the genes
determining the structure of the epidermis. The recent sequencing of the
chimpanzee genome has revealed that one of the few areas of the genome
where humans and chimps differ significantly is in a cluster of functionally
related genes that regulate the differentiation of the epidermis and contribute
to coding the proteins that make up the keratin-rich layer of the skin.
At least as far as primate skin goes, our epidermis is tough stuff.
The immigrant cells in the epidermis are a diverse lot that work with
the other cells in the skin. They migrate into the skin from other parts of
the body during early development to provide special physical and chemical
protection against potent environmental agents such as UVR, disease-causing
microorganisms, and dangerously high physical pressures. Although
they are developmental interlopers, the immigrant cells don't in any
way weaken the physical fabric of the skin. There are three main types of
immigrant cells in the epidermis. Melanocytes (shown in figures 1 and 2)
produce the skin's primary pigment and natural sunscreen, melanin. These
cells migrate to the skin from a position flanking the spine during early
embryonic development. Once they arrive, they set up shop near the interface
of the dermis and the epidermis in order to manufacture melanin.
Some people produce a lot of melanin in their melanocytes, whereas others
produce only a little, depending on the amount of UVR present in the environment
of their ancestors. Skin color, which is determined by the activity
of melanocytes and their manufacture of melanin, has evolved under
the close watch of natural selection.
Two other types of immigrant cells are also important. Langerhans cells
are specialized cells of the immune system that respond to foreign substances
coming in contact with the skin. They constitute the body's first
line of defense against bacteria and viruses that land on the skin. Merkel
cells are associated with the ends of sensory nerves in the skin, where they
appear to assist in the transfer of mechanical signals from the skin to sensory
nerves and then on to the brain. Merkel cells, which are common on
the smooth skin of our fingertips and lips, contribute to our finely discriminating
sense of touch. They are also of great importance to our furred
and feathered relatives: in mammals and birds, Merkel cells occur in the
collars of cells that support hair and feather follicles, including those surrounding
the sensitive whiskers of dogs, cats, and rats.
Probing beneath the epidermis, we reach the second of the skin's two
primary layers, a thick layer of dense connective tissue called the dermis.
This is the layer that really imparts toughness to skin. It is pliable, elastic,
and has considerable tensile strength. Most of the thickness of our own
skin-and most of the thickness of the hide of any animal-comes from
the dermis. Its thickness, in addition to its chemical and physical properties,
helps to insulate the body and makes the skin resistant to mechanical
injury. Leather is composed mainly of tough animal dermis that has been
tanned so that it will be more pliable.
The dermis is a composite tissue that gets its strength and toughness
from a combination of collagen fibers and elastin fibers. These fibers are
maintained in a gel composed of salts, water, and large protein molecules
called glycosaminoglycans. The primary cells of the dermis are collagen-rich
cells known as fibroblasts. Collagen, which constitutes 77 percent of
the dry weight of skin, accounts for most of the tensile strength of the skin
and for some of its ability to scatter visible light (figure 3). Collagen acts
just the way it looks, like tough little ropes of protein holding the dermis
together. Interwoven with the collagen is a network of abundant elastin
fibers that restore the skin to its normal configuration after stretching.
The production of collagen and elastin fibers slows down as we get older,
and it is adversely affected by UVR from excessive sun exposure. Many products
on the beauty market today claim to stimulate production of these materials
to keep skin looking young. But there is only so much that creams,
treatments, and "cosmeceuticals" can do to change the appearance and composition
of skin, especially when people have caused irreparable damage
through their incautious behavior in the sun. Many of the processes in the
skin that control the production of collagen and elastin are governed by internal
mechanisms of cellular aging that are not affected or are only weakly
affected by what we apply to the skin's surface.
Amid the complex tangle of connective tissue fibers in the dermis, we
find a branching network of blood vessels, an extensive network of nerves,
numerous sweat glands, and an assortment of hair follicles, hair-raising
arrector pili muscles, and oil-producing glands (refer back to figure 1). The
blood vessels are critical because they supply the appetites of the sweat
glands, the hair follicles, and the rapidly multiplying cells in the lowest layer
of the epidermis. The density of blood vessels varies over the body's surface.
They are especially concentrated on the head, for instance, where temperature
regulation is particularly important to protect the brain and where
the hair follicles of the scalp require good nutrition from a rich blood supply
so that hair can grow. Blood vessels are also quite dense in areas where
the skin must be kept moist by sweat and sebaceous (oil-producing)
glands-for example, on the palms of the hands, the soles of the feet, and
the nipples. In addition, blood vessel density is related to different postures.
In both humans and primates, some of the densest concentrations of blood
vessels in the body are found on the bottom of the buttocks, supplying the
skin in this area with blood so that it does not deteriorate when we sit for
long periods. In some of our close primate relatives, the skin around the
female genitals is richly supplied with blood vessels, which permit the skin
to become engorged with fluid when the animals are sexually receptive, creating
puffy pink sexual swellings that are highly attractive to males.
The blood vessels of the dermis carry red blood cells, which derive their
color from hemoglobin. Hemoglobin is a pigment that is bright red when
it is carrying oxygen to cells and a dull reddish-blue after it has discharged
its ferried oxygen and is heading back to the heart and lungs. Hemoglobin
is one of the skin's main pigments, but it is most visible in people who have
relatively little of the dark brown melanin pigment in their skin. Rosy cheeks
and blue veins are more evident in people with light skin than in those with
dark skin. The painfully bright red appearance of sunburned skin actually
results from an increase in the number and diameter of the tiny blood vessels
in the skin as well as an increase in the blood flow through each of
these vessels. Sunburned skin feels hot to the touch because it is infused
with blood and because it is mounting a hot and vigorous inflammatory
response in order to repair the damage caused by UVR.
The nerves of the dermis are highly complex because the skin is one of
the body's main sensory portals. Skin contains several specialized types
of receptor cells, which send signals to the central nervous system about
the external environment and the state of the skin. These include two types
of temperature receptors, diverse mechanical receptors associated with
both hairy and smooth skin, and an important group of pain sensors that
specialize in detecting potentially dangerous physical stimuli or the presence
of injury or inflammation. Although this formidable battery of receptor
cells is extremely important, their evolutionary history is not yet
A tour of the skin would not be complete without a side trip to examine
hair. Hair in humans is significant largely because we have so little of it. If
we cast our gaze back in time to consider the evolution of skin in our earliest
warm-blooded ancestors and cousins, the story of hair becomes very
interesting. As the forebears of mammals and birds evolved toward endothermy,
or warm-bloodedness, one of the key innovations that allowed
this development was good external insulation on the body. In other words,
if you want a warm house, but not a high heating bill, you must have good
insulation in your walls. Warm bodies permitted higher activity levels
throughout the day, but at the cost of greatly increased energy expenditure.
In the ancient physiological economy of proto-birds and proto-mammals,
keeping the lid on energy costs was a high priority so that the animals would
not have to spend excessive amounts of time finding and eating food. The
solution was found in the development of complex, built-in insulation such
as hair and feathers.
Excerpted from Skin
by Nina G. Jablonski
Copyright © 2006 by Nina G. Jablonski.
Excerpted by permission.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
1 Skin Laid Bare
4 Skin and Sun
5 Skin’s Dark Secret
8 Emotions, Sex, and Skin
9 Wear and Tear
11 Future Skin
Posted October 4, 2011
" Skin. The organ you probably take the most for granted. It seems extremely simple, but as Nina G. Jablonski shows us in this book, 'Skin: A Natural History', it is extremely complex. I chose this book for my alternate reading assignment in my Biological Anthropology class this past semester, and it's fascinating. She goes over, of course, the "basics" that most (if not all) of us know from our introductory biology courses in high school or college, but she takes it a little more in depth as well. ..."
For full review, please visit me (Les Livres) on Blogger!