Fundamentals of Anatomy and Physiology - 8e - M05 MART5891 08 SE C05, Angielskie [EN](1)
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The Integumentary System
Did you know...?
Hair follicles are found nearly everywhere on
the body surface. Usually most abundant on the
scalp, they are absent on the palms and soles.
Learning Outcomes
After completing this chapter, you should be able to do the following:
5-1
Describe the main structural features of the epidermis, and
explain the functional significance of each.
5-2
Explain what accounts for individual differences in skin color,
and discuss the response of melanocytes to sunlight exposure.
5-3
Describe the interaction between sunlight and vitamin D
3
production.
5-4
Describe the roles of epidermal growth factor.
5-5
Describe the structure and functions of the dermis.
5-6
Describe the structure and functions of the hypodermis.
5-7
Describe the mechanisms that produce hair, and explain the
structural basis for hair texture and color.
5-8
Discuss the various kinds of glands in the skin, and list the
secretions of those glands.
5-9
Describe the anatomical structure of nails, and explain how
they are formed.
5-10
Explain how the skin responds to injury and repairs itself.
5-11
Summarize the effects of aging on the skin.
5-12
Give examples of interactions between the integumentary
system and each of the other organ systems.
Clinical Notes
Skin Cancers, Melanomas, and Sunblocks p. 164
Ulcers p. 167
Liposuction p. 168
Skin Abnormalities p. 178
Burns and Grafts p. 180
158
Unit 2
Support and Movement
An Introduction to the
Integumentary System
2.
The accessory structures include hair, nails, and multicel-
lular exocrine glands. These structures are located prima-
rily in the dermis and protrude through the epidermis to
the skin surface.
This chapter considers the many and varied functions of the
skin, the organ system with which you are probably most fa-
miliar. No other organ system is as accessible, large, and un-
derappreciated as the integumentary system. Often referred to
simply as the
integument
(in-TEG-u-ment), this system ac-
counts for about 16 percent of your total body weight. Its sur-
face, 1.5–2 m
2
in area, is continually abraded, attacked by
microorganisms, irradiated by sunlight, and exposed to envi-
ronmental chemicals. The integumentary system is your
body’s first line of defense against an often hostile environ-
ment—the place where you and the outside world meet.
The integumentary system has two major components:
the
cutaneous membrane
or skin, and the
accessory struc-
tures
(
Figure 5–1
).
The integument does not function in isolation. An exten-
sive network of blood vessels branches through the dermis,
and sensory receptors that monitor touch, pressure, temper-
ature, and pain provide valuable information to the central
nervous system about the state of the body. Deep to the der-
mis, the loose connective tissue of the
hypodermis
, also
known as the superficial fascia or
subcutaneous layer
, sepa-
rates the integument from the deep fascia around other or-
gans, such as muscles and bones.
l
p. 137
Although the
hypodermis is often considered separate from the integu-
ment, we will consider it in this chapter because its connec-
tive tissue fibers are interwoven with those of the dermis.
The general functions of the skin and hypodermis in-
clude the following:
•
Protection
of underlying tissues and organs against
impact, abrasion, fluid loss, and chemical attack.
1.
The cutaneous membrane has two components: the
epidermis
(
epi
-, above) or superficial epithelium, and
the
dermis
, an underlying area of connective tissues.
Hair shaft
Epidermis
Pore of sweat gland duct
Papillary
layer
Tactile corpuscle
Dermis
Sebaceous gland
Reticular
layer
Arrector pili muscle
Sweat gland duct
Hair follicle
Lamellated corpuscle
Subcutaneous
layer
(hypodermis)
Nerve fibers
Sweat gland
Artery
Cutaneous plexus
Vein
Fat
Figure 5–1
The Components of the Integumentary System.
This diagrammatic section of skin illustrates the relationships among the two
components of the cutaneous membrane (epidermis and dermis) and the accessory structures of the integumentary system (with the exception
of nails, shown in
Figure 5–13
).
Chapter 5
The Integumentary System
159
•
Excretion
of salts, water, and organic wastes by
integumentary glands.
•
Maintenance
of normal body temperature through either
insulation or evaporative cooling, as needed.
•
Production
of melanin, which protects underlying tissue
from ultraviolet radiation.
•
Production
of keratin, which protects against abrasion and
serves as a water repellent.
•
Synthesis of vitamin D
3
,
a steroid that is subsequently
converted to calcitriol, a hormone important to normal
calcium metabolism.
•
Storage
of lipids in adipocytes in the dermis and in
adipose tissue in the subcutaneous layer.
•
Detection
of touch, pressure, pain, and temperature
stimuli, and the relaying of that information to the
nervous system. (These
general senses
, which provide
information about the external environment, will be
considered further in Chapter 15.)
the epidermis is avascular. Because there are no local blood
vessels, epidermal cells rely on the diffusion of nutrients and
oxygen from capillaries within the dermis. As a result, the epi-
dermal cells with the highest metabolic demands are found
close to the basal lamina, where the diffusion distance is
short. The superficial cells, far removed from the source of
nutrients, are either inert or dead.
The epidermis is dominated by
keratinocytes
(ke-RAT-i-
no-sits), the body’s most abundant epithelial cells. These cells,
which form several layers, contain large amounts of the protein
keratin
(discussed shortly).
Thin skin
(
Figure 5–2a,b
), which
covers most of the body surface, contains four layers of kerat-
inocytes, and is about as thick as the wall of a plastic sandwich
bag (roughly 0.08 mm).
Thick skin
(
Figure 5–2c
), which occurs
on the palms of the hands and the soles of the feet, contains a
fifth layer, the
stratum lucidum
, and because it has a much
thicker superficial layer (the
stratum corneum
) it is about as
thick as a standard paper towel (roughly 0.5 mm). Note that the
terms
thick
and
thin
refer to the relative thickness of the epider-
mis, not to the integument as a whole.
Figure 5–3
shows the layers of keratinocytes in a section of
the epidermis in an area of thick skin. The boundaries between
the layers are often difficult to see in a standard light micro-
graph. You will notice that the various layers have Latin names.
The word
stratum
(plural,
strata
) means “layer”; the rest of the
name refers to the function or appearance of the layer. The
strata, in order from the basal lamina toward the free surface, are
the
stratum germinativum
, the
stratum spinosum
, the
stratum
granulosum
, the
stratum lucidum
, and the
stratum corneum
.
Tips
&
Tricks
Strat
um
means layer, as in
strat
ified.
In the next several sections we will consider the various
components of the integument.
5-1
The epidermis is composed
of strata (layers) with various
functions
The epidermis consists of a stratified squamous epithelium.
Recall from Chapter 4 that such an epithelium provides con-
siderable mechanical protection and also helps keep microor-
ganisms outside the body.
l
p. 118
Like all other epithelia,
Stratum
corneum
Epidermis
Basal
lamina
Epidermal
ridge
Stratum
lucidum
Dermal
papilla
Dermis
Dermal
papilla
Epidermal
ridge
Dermis
(a) Relationship between the epidermis
and dermis
(b) Thin skin
LM
154
(c) Thick skin
LM
154
Figure 5–2
The Basic Organization of the Epidermis.
(a)
A drawing showing the structural relationship and interface between the
epidermis and underlying dermis. The proportions of the various layers differ with the location sampled.
(b)
A micrograph of thin skin, which
covers most of the exposed body surface.
(c)
A micrograph of thick skin, which covers the surfaces of the palms and soles.
160
Unit 2
Support and Movement
Surface
Stratum
corneum
Stratum
lucidum
Stratum
granulosum
Stratum
spinosum
Stratum
germinativum
Pores of
sweat gland
ducts
Epidermal
ridge
Basal lamina
Dermis
LM
210
Figure 5–4
The Epidermal Ridges of Thick Skin.
Fingerprints
reveal the pattern of epidermal ridges. This scanning electron
micrograph shows the ridges on a fingertip. The pits are the openings
into the ducts of merocrine sweat glands. [©R. G. Kessel and R. H.
Kardon,
Tissues and Organs: A Text-Atlas of Scanning Electron
Microscopy
, W. H. Freeman & Co., 1979. All Rights Reserved.]
Figure 5–3
The Structure of the Epidermis.
A portion of the
epidermis in thick skin, showing the major layers of stratified
epidermal cells. Note that Langerhans cells cannot be distinguished
in standard histological preparations.
Stratum Germinativum
The innermost epidermal layer is the
stratum germina-
tivum
(STRA-tum jer-mi-na-TE-vum), or
stratum basale
(
Figure 5–3
). Hemidesmosomes attach the cells of this layer
to the basal lamina that separates the epidermis from the are-
olar tissue of the adjacent dermis.
l
p. 114
The stratum
germinativum and the underlying dermis interlock, increas-
ing the strength of the bond between the epidermis and der-
mis. The stratum germinativum forms
epidermal ridges
,
which extend into the dermis and are adjacent to dermal pro-
jections called
dermal papillae
(singular,
papilla
; a nipple-
shaped mound) that project into the epidermis (
Figure 5–2a
).
These ridges and papillae are significant because the strength
of the attachment is proportional to the surface area of the
basal lamina: The more and deeper the folds, the larger the
surface area becomes.
The contours of the skin surface follow the ridge pat-
terns, which vary from small conical pegs (in thin skin) to the
complex whorls seen on the thick skin of the palms and soles.
Ridges on the palms and soles increase the surface area of the
skin and increase friction, ensuring a secure grip. Ridge
shapes are genetically determined. The pattern of your epi-
dermal ridges is unique and does not change during your life-
time. The ridge patterns on the tips of the fingers are the basis
of fingerprints (
Figure 5–4
), which have been used to identify
individuals in criminal investigations for more than a century.
Basal cells
, or
germinative cells
, dominate the stratum
germinativum. Basal cells are stem cells whose divisions re-
place the more superficial keratinocytes that are lost or shed
at the epithelial surface. Skin surfaces that lack hair also
contain specialized epithelial cells known as
Merkel cells
scattered among the cells of the stratum germinativum.
Merkel cells are sensitive to touch; when compressed, they
release chemicals that stimulate sensory nerve endings.
(The skin contains many other kinds of sensory receptors,
as we will see in later sections.) The brown tones of skin re-
sult from the synthetic activities of pigment cells called
melanocytes
,
l
p. 126
which are distributed throughout
the stratum germinativum, with cell processes extending
into more superficial layers.
Tips
&
Tricks
Associate the word
germinativum
with
germinate
, which
means to sprout or grow. Just as blades of grass sprout
upward and extend beyond the soil, the daughter cells of
stem cells dividing in the stratum germinativum are pushed
toward the skin surface; on the way, they elongate, acquire
organelles, and mature.
Chapter 5
The Integumentary System
161
Stratum Spinosum
Each time a stem cell divides, one of the daughter cells is pushed
superficial to the stratum germinativum into the
stratum spi-
nosum
(
Figure 5–3
), which consists of 8 to 10 layers of kerat-
inocytes bound together by desmosomes.
l
p. 114
The name
stratum spinosum
, which means “spiny layer,” refers to the fact
that the cells look like miniature pincushions in standard histo-
logical sections. They look that way because the keratinocytes
were processed with chemicals that shrank the cytoplasm but
left the cytoskeletal elements and desmosomes intact. Some of
the cells entering this layer from the stratum germinativum con-
tinue to divide, further increasing the thickness of the epithe-
lium. The stratum spinosum also contains
dendritic
(Langerhans) cells
, which participate in the immune response by
stimulating a defense against (1) microorganisms that manage
to penetrate the superficial layers of the epidermis and (2) su-
perficial skin cancers. Dendritic cells and other cells of the im-
mune response will be considered in Chapter 22.
normally contains 15 to 30 layers of keratinized cells.
Kera-
tinization
, or
cornification
, is the formation of protective, super-
ficial layers of cells filled with keratin. This process occurs on all
exposed skin surfaces except the anterior surfaces of the eyes.
The dead cells in each layer of the stratum corneum remain
tightly interconnected by desmosomes. The connections are so
secure that keratinized cells are generally shed in large groups
or sheets rather than individually.
It takes 15 to 30 days for a cell to move from the stratum
germinativum to the stratum corneum. The dead cells gener-
ally remain in the exposed stratum corneum for an additional
two weeks before they are shed or washed away. This
arrangement places the deeper portions of the epithelium
and underlying tissues beneath a protective barrier of dead,
durable, and expendable cells. Normally, the surface of the
stratum corneum is relatively dry, so it is unsuitable for the
growth of many microorganisms. Maintenance of this barrier
involves coating the surface with lipid secretions from seba-
ceous glands.
The stratum corneum is water resistant, but not water-
proof. Water from interstitial fluids slowly penetrates the sur-
face, to be evaporated into the surrounding air. You lose
roughly 500 mL (about 1 pt) of water in this way each day.
The process is called
insensible perspiration
, because you
are unable to see or feel the water loss. In contrast, you are
usually very aware of the
sensible perspiration
produced by
active sweat glands. Damage to the epidermis can increase the
rate of insensible perspiration. If the damage breaks connec-
tions between superficial and deeper layers of the epidermis,
fluid will accumulate in pockets, or
blisters
, within the epi-
dermis. (Blisters also form between the epidermis and dermis
if the basal lamina is damaged.) If damage to the stratum
corneum reduces its effectiveness as a water barrier, the rate
of insensible perspiration skyrockets, and a potentially dan-
gerous fluid loss occurs. This is a serious consequence of se-
vere burns and a complication in the condition known as
xerosis
(excessively dry skin).
When the skin is immersed in water, osmotic forces
may move water into or out of the epithelium.
l
p. 91
Sit-
ting in a freshwater bath causes water to move into the epi-
dermis, because fresh water is hypotonic (has fewer
dissolved materials) compared with body fluids. The ep-
ithelial cells may swell to four times their normal volumes,
a phenomenon particularly noticeable in the thickly kera-
tinized areas of the palms and soles. Swimming in the ocean
reverses the direction of osmotic flow; because the ocean is
a hypertonic solution, water leaves the body, crossing the
epidermis from the underlying tissues. The process is slow,
but long-term exposure to seawater endangers survivors of
a shipwreck by accelerating dehydration.
Stratum Granulosum
The region superficial to the stratum spinosum is the
stratum
granulosum
, or “grainy layer” (
Figure 5–3
). The stratum
granulosum consists of three to five layers of keratinocytes
derived from the stratum spinosum. By the time cells reach
this layer, most have stopped dividing and have started mak-
ing large amounts of the proteins
keratin
(KER-a-tin;
keros
,
horn) and
keratohyalin
(ker-a-to-HI-a-lin). Keratin, a tough,
fibrous protein, is the basic structural component of hair and
nails in humans.
l
p. 118
As keratin fibers develop, the
cells grow thinner and flatter, and their membranes thicken
and become less permeable. Keratohyalin forms dense cyto-
plasmic granules that promote dehydration of the cell as well
as aggregation and cross-linking of the keratin fibers. The nu-
clei and other organelles then disintegrate, and the cells die.
Further dehydration creates a tightly interlocked layer of cells
that consist of keratin fibers surrounded by keratohyalin.
Stratum Lucidum
In the thick skin of the palms and soles, a glassy
stratum
lucidum
(“clear layer”) covers the stratum granulosum
(
Figure 5–3
). The cells in the stratum lucidum are flattened,
densely packed, largely devoid of organelles, and filled with
keratin.
Stratum Corneum
At the exposed surface of both thick skin and thin skin is the
stratum corneum
(KOR-ne-um;
cornu
, horn) (
Figure 5–3
). It
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