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Structure and Function of Skin… 1/18
4/22/2021 Structure and Function of Skin | Biology for Majors II


Describe the layers of the skin and the functions of
each layer

The skin is composed of two major layers: a super cial epidermis and a
deeper dermis. The epidermis consists of several layers The topmost
layer consists of dead cells that shed periodically and is progressively
replaced by cells formed from the basal layer. The dermis connects the
epidermis to the hypodermis, and provides strength and elasticity due to
the presence of collagen and elastin bers. The hypodermis, deep to the
dermis of skin, is the connective tissue that connects the dermis to un-
derlying structures; it also harbors adipose tissue for fat storage and



Describe the di erent layers of the skin

Describe the epidermis and identify its di erent

Describe the dermis and identify its di erent layers

Identify and describe the hypodermis and deep fascia

Describe the role of melanocytes in skin pigmentation


Layers of the Skin

Although you may not typically think of the skin as an organ, it is in fact
made of tissues that work together as a single structure to perform
unique and critical functions. The skin and its accessory structures make
up the integumentary system, which provides the body with overall pro-
tection. The skin is made of multiple layers of cells and tissues, which
are held to underlying structures by connective tissue (Figure 1). The
deeper layer of skin is well vascularized (has numerous blood vessels). It… 2/18
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also has numerous sensory, and autonomic and sympathetic nerve bers
ensuring communication to and from the brain.


Figure 1. The skin is composed of two main layers: the epidermis, made of
closely packed epithelial cells, and the dermis, made of dense, irregular
connective tissue that houses blood vessels, hair follicles, sweat glands, and
other structures. Beneath the dermis lies the hypodermis, which is
composed mainly of loose connective and fatty tissues.


The skin consists of two main layers and a closely associated
layer. View this animation to learn more about layers of the skin.
What are the basic functions of each of these layers?



The epidermis is composed of keratinized, strati ed squamous epithe-
lium. It is made of four or ve layers of epithelial cells, depending on its
location in the body. It does not have any blood vessels within it (i.e., it is
avascular). Skin that has four layers of cells is referred to as “thin skin.”
From deep to super cial, these layers are the stratum basale, stratum
spinosum, stratum granulosum, and stratum corneum. Most of the skin
can be classi ed as thin skin. “Thick skin” is found only on the palms of… 3/18
4/22/2021 Structure and Function of Skin | Biology for Majors II


the hands and the soles of the feet. It has a fth layer, called the stratum
lucidum, located between the stratum corneum and the stratum granulo-
sum (Figure 2).


Figure 2. These slides show cross-sections of the epidermis and dermis of
(a) thin and (b) thick skin. Note the signi cant di erence in the thickness of
the epithelial layer of the thick skin. From top, LM × 40, LM × 40.
(Micrographs provided by the Regents of University of Michigan Medical
School © 2012)


The cells in all of the layers except the stratum basale are called ker-
atinocytes. A keratinocyte is a cell that manufactures and stores the pro-
tein keratin. Keratin is an intracellular brous protein that gives hair, nails,
and skin their hardness and water-resistant properties. The ker-
atinocytes in the stratum corneum are dead and regularly slough away,
being replaced by cells from the deeper layers (Figure 3).


Figure 3. The epidermis is epithelium composed of multiple
layers of cells. The basal layer consists of cuboidal cells,
whereas the outer layers are squamous, keratinized cells, so… 4/18
4/22/2021 Structure and Function of Skin | Biology for Majors II


Show Answer


Stratum Basale

The stratum basale (also called the stratum germinativum) is the deep-
est epidermal layer and attaches the epidermis to the basal lamina, be-
low which lie the layers of the dermis. The cells in the stratum basale
bond to the dermis via intertwining collagen bers, referred to as the
basement membrane. A nger-like projection, or fold, known as the der-
mal papilla (plural = dermal papillae) is found in the super cial portion of
the dermis. Dermal papillae increase the strength of the connection be-
tween the epidermis and dermis; the greater the folding, the stronger
the connections made (Figure 4).… 5/18
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Figure 4. The epidermis of thick skin has ve layers: stratum basale, stratum
spinosum, stratum granulosum, stratum lucidum, and stratum corneum.


The stratum basale is a single layer of cells primarily made of basal cells.
A basal cell is a cuboidal-shaped stem cell that is a precursor of the ker-
atinocytes of the epidermis. All of the keratinocytes are produced from
this single layer of cells, which are constantly going through mitosis to
produce new cells. As new cells are formed, the existing cells are
pushed super cially away from the stratum basale. Two other cell types
are found dispersed among the basal cells in the stratum basale. The
rst is a Merkel cell, which functions as a receptor and is responsible for
stimulating sensory nerves that the brain perceives as touch. These cells
are especially abundant on the surfaces of the hands and feet. The sec-
ond is a melanocyte, a cell that produces the pigment melanin. Melanin
gives hair and skin its color, and also helps protect the living cells of the
epidermis from ultraviolet (UV) radiation damage.

In a growing fetus, ngerprints form where the cells of the stratum
basale meet the papillae of the underlying dermal layer (papillary layer),
resulting in the formation of the ridges on your ngers that you recog-
nize as ngerprints. Fingerprints are unique to each individual and are… 6/18
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used for forensic analyses because the patterns do not change with the
growth and aging processes.

Stratum Spinosum

As the name suggests, the stratum spinosum is spiny in appearance
due to the protruding cell processes that join the cells via a structure
called a desmosome. The desmosomes interlock with each other and
strengthen the bond between the cells. It is interesting to note that the
“spiny” nature of this layer is an artifact of the staining process. Un-
stained epidermis samples do not exhibit this characteristic appearance.
The stratum spinosum is composed of eight to 10 layers of keratinocytes,
formed as a result of cell division in the stratum basale (Figure 5). Inter-
spersed among the keratinocytes of this layer is a type of dendritic cell
called the Langerhans cell, which functions as a macrophage by engulf-
ing bacteria, foreign particles, and damaged cells that occur in this layer.


Show Answer… 7/18
4/22/2021 Structure and Function of Skin | Biology for Majors II


Figure 5. The cells in the di erent layers of the epidermis originate from
basal cells located in the stratum basale, yet the cells of each layer are
distinctively di erent. EM × 2700. (Micrograph provided by the Regents of
University of Michigan Medical School © 2012)


View the University of Michigan WebScope to explore the tissue
sample in greater detail. If you zoom on the cells at the outermost
layer of this section of skin, what do you notice about the cells?



If you zoom on the cells of the stratum spinosum (Figure 5),
what is distinctive about them?


Show Answer


The keratinocytes in the stratum spinosum begin the synthesis of keratin… 8/18
4/22/2021 Structure and Function of Skin | Biology for Majors II


and release a water-repelling glycolipid that helps prevent water loss
from the body, making the skin relatively waterproof. As new ker-
atinocytes are produced atop the stratum basale, the keratinocytes of
the stratum spinosum are pushed into the stratum granulosum.

Stratum Granulosum

The stratum granulosum has a grainy appearance due to further
changes to the keratinocytes as they are pushed from the stratum
spinosum. The cells (three to ve layers deep) become atter, their cell
membranes thicken, and they generate large amounts of the proteins
keratin, which is brous, and keratohyalin, which accumulates as lamel-
lar granules within the cells (see Figure 4). These two proteins make up
the bulk of the keratinocyte mass in the stratum granulosum and give
the layer its grainy appearance. The nuclei and other cell organelles dis-
integrate as the cells die, leaving behind the keratin, keratohyalin, and
cell membranes that will form the stratum lucidum, the stratum corneum,
and the accessory structures of hair and nails.

Stratum Lucidum

The stratum lucidum is a smooth, seemingly translucent layer of the epi-
dermis located just above the stratum granulosum and below the stra-
tum corneum. This thin layer of cells is found only in the thick skin of the
palms, soles, and digits. The keratinocytes that compose the stratum lu-
cidum are dead and attened (see Figure 4). These cells are densely
packed with eleiden, a clear protein rich in lipids, derived from kerato-
hyalin, which gives these cells their transparent (i.e., lucid) appearance
and provides a barrier to water.

Stratum Corneum

The stratum corneum is the most super cial layer of the epidermis and
is the layer exposed to the outside environment (see Figure 4). The in-
creased keratinization (also called corni cation) of the cells in this layer
gives it its name. There are usually 15 to 30 layers of cells in the stratum
corneum. This dry, dead layer helps prevent the penetration of microbes
and the dehydration of underlying tissues, and provides a mechanical
protection against abrasion for the more delicate, underlying layers.… 9/18
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Cells in this layer are shed periodically and are replaced by cells pushed
up from the stratum granulosum (or stratum lucidum in the case of the
palms and soles of feet). The entire layer is replaced during a period of
about 4 weeks. Cosmetic procedures, such as microdermabrasion, help
remove some of the dry, upper layer and aim to keep the skin looking
“fresh” and healthy.


The dermis might be considered
the “core” of the integumentary
system (derma- = “skin”), as dis-
tinct from the epidermis (epi– =
“upon” or “over”) and hypoder-
mis (hypo– = “below”). It contains
blood and lymph vessels, nerves,
and other structures, such as hair
follicles and sweat glands. The
dermis is made of two layers of
connective tissue that compose
an interconnected mesh of
elastin and collagenous bers,
produced by broblasts
(Figure 6). Figure 6. This stained slide shows
the two components of the dermis—
the papillary layer and the reticular
Papillary Layer layer. Both are made of connective
tissue with bers of collagen
extending from one to the other,
The papillary layer is made of making the border between the two
loose, areolar connective tissue, somewhat indistinct. The dermal
papillae extending into the
which means the collagen and epidermis belong to the papillary
elastin bers of this layer form a layer, whereas the dense collagen
ber bundles below belong to the
loose mesh. This super cial layer reticular layer. LM × 10. (credit:
of the dermis projects into the modi cation of work by
“kilbad”/Wikimedia Commons)
stratum basale of the epidermis
to form nger-like dermal papillae
(see Figure 6). Within the papillary layer are broblasts, a small number
of fat cells (adipocytes), and an abundance of small blood vessels. In ad-
dition, the papillary layer contains phagocytes, defensive cells that help
ght bacteria or other infections that have breached the skin. This layer… 10/18
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also contains lymphatic capillaries, nerve bers, and touch receptors
called the Meissner corpuscles.

Reticular Layer

Underlying the papillary layer is the much thicker reticular layer, com-
posed of dense, irregular connective tissue. This layer is well vascular-
ized and has a rich sensory and sympathetic nerve supply. The reticular
layer appears reticulated (net-like) due to a tight meshwork of bers.
Elastin bers provide some elasticity to the skin, enabling movement.
Collagen bers provide structure and tensile strength, with strands of
collagen extending into both the papillary layer and the hypodermis. In
addition, collagen binds water to keep the skin hydrated. Collagen injec-
tions and Retin-A creams help restore skin turgor by either introducing
collagen externally or stimulating blood ow and repair of the dermis,


The hypodermis (also called the subcutaneous layer or super cial fas-
cia) is a layer directly below the dermis and serves to connect the skin to
the underlying fascia ( brous tissue) of the bones and muscles. It is not
strictly a part of the skin, although the border between the hypodermis
and dermis can be di cult to distinguish. The hypodermis consists of
well-vascularized, loose, areolar connective tissue and adipose tissue,
which functions as a mode of fat storage and provides insulation and
cushioning for the integument.

Lipid Storage

The hypodermis is home to most of the fat that concerns people when
they are trying to keep their weight under control. Adipose tissue
present in the hypodermis consists of fat-storing cells called adipocytes.
This stored fat can serve as an energy reserve, insulate the body to pre-
vent heat loss, and act as a cushion to protect underlying structures from

Where the fat is deposited and accumulates within the hypodermis de-… 11/18
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pends on hormones (testosterone, estrogen, insulin, glucagon, leptin,
and others), as well as genetic factors. Fat distribution changes as our
bodies mature and age. Men tend to accumulate fat in di erent areas
(neck, arms, lower back, and abdomen) than do women (breasts, hips,
thighs, and buttocks). The body mass index (BMI) is often used as a mea-
sure of fat, although this measure is, in fact, derived from a mathematical
formula that compares body weight (mass) to height. Therefore, its accu-
racy as a health indicator can be called into question in individuals who
are extremely physically t.

In many animals, there is a pattern of storing excess calories as fat to be
used in times when food is not readily available. In much of the devel-
oped world, insu cient exercise coupled with the ready availability and
consumption of high-calorie foods have resulted in unwanted accumula-
tions of adipose tissue in many people. Although periodic accumulation
of excess fat may have provided an evolutionary advantage to our an-
cestors, who experienced unpredictable bouts of famine, it is now be-
coming chronic and considered a major health threat. Recent studies in-
dicate that a distressing percentage of our population is overweight
and/or clinically obese. Not only is this a problem for the individuals af-
fected, but it also has a severe impact on our healthcare system.
Changes in lifestyle, speci cally in diet and exercise, are the best ways
to control body fat accumulation, especially when it reaches levels that
increase the risk of heart disease and diabetes.


The color of skin is in uenced by a number of pigments, including
melanin, carotene, and hemoglobin. Recall that melanin is produced by
cells called melanocytes, which are found scattered throughout the stra-
tum basale of the epidermis. The melanin is transferred into the ker-
atinocytes via a cellular vesicle called a melanosome (Figure 7).… 12/18
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Figure 7. The relative coloration of the skin depends of the amount of
melanin produced by melanocytes in the stratum basale and taken up by

Melanin occurs in two primary forms. Eumelanin, the most common form
of melanin, exists as black and brown, whereas pheomelanin provides a
red color. Dark-skinned individuals produce more melanin than those
with pale skin. Exposure to the UV rays of the sun or a tanning salon
causes melanin to be manufactured and built up in keratinocytes, as sun
exposure stimulates keratinocytes to secrete chemicals that stimulate
melanocytes. The accumulation of melanin in keratinocytes results in the
darkening of the skin, or a tan. This increased melanin accumulation pro-
tects the DNA of epidermal cells from UV ray damage and the break-
down of folic acid, a nutrient necessary for our health and well-being. In
contrast, too much melanin can interfere with the production of vitamin
D, an important nutrient involved in calcium absorption. Thus, the
amount of melanin present in our skin is dependent on a balance be-
tween available sunlight and folic acid destruction, and protection from
UV radiation and vitamin D production.

It requires about 10 days after initial sun exposure for melanin synthesis
to peak, which is why pale-skinned individuals tend to su er sunburns of… 13/18
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the epidermis initially. Dark-skinned individuals can also get sunburns,
but are more protected than are pale-skinned individuals. Melanosomes
are temporary structures that are eventually destroyed by fusion with
lysosomes; this fact, along with melanin- lled keratinocytes in the stra-
tum corneum sloughing o , makes tanning impermanent.

Too much sun exposure can eventually lead to wrinkling due to the de-
struction of the cellular structure of the skin, and in severe cases, can
cause su cient DNA damage to result in skin cancer. When there is an
irregular accumulation of melanocytes in the skin, freckles appear. Moles
are larger masses of melanocytes, and although most are benign, they
should be monitored for changes that might indicate the presence of
cancer (Figure 8).


Figure 8. Moles range from benign accumulations of melanocytes to
melanomas. These structures populate the landscape of our skin. (credit:
the National Cancer Institute)



What determines the color of skin, and what is the process that
darkens skin when it is exposed to UV light?


Show Answer


4/22/2021 Structure and Function of Skin | Biology for Majors II


The rst thing a clinician sees is the skin, and so the examina-
tion of the skin should be part of any thorough physical exami-
nation. Most skin disorders are relatively benign, but a few, in-
cluding melanomas, can be fatal if untreated. A couple of the
more noticeable disorders, albinism and vitiligo, a ect the ap-
pearance of the skin and its accessory organs. Although nei-
ther is fatal, it would be hard to claim that they are benign, at
least to the individuals so a icted.

Albinism is a genetic disor-
der that a ects (completely
or partially) the coloring of
skin, hair, and eyes. The de-
fect is primarily due to the
inability of melanocytes to
produce melanin. Individuals
with albinism tend to appear
white or very pale due to the
lack of melanin in their skin
and hair. Recall that melanin
helps protect the skin from
the harmful e ects of UV ra-
diation. Individuals with al- Figure 9. Individuals with
vitiligo experience
binism tend to need more depigmentation that results in
protection from UV radiation, lighter colored patches of skin.
The condition is especially
as they are more prone to noticeable on darker skin.
sunburns and skin cancer. (credit: Klaus D. Peter)

They also tend to be more
sensitive to light and have vision problems due to the lack of
pigmentation on the retinal wall. Treatment of this disorder
usually involves addressing the symptoms, such as limiting UV
light exposure to the skin and eyes. In vitiligo, the melanocytes
in certain areas lose their ability to produce melanin, possibly
due to an autoimmune reaction. This leads to a loss of color in
patches (Figure 9). Neither albinism nor vitiligo directly a ects
the lifespan of an individual.

Other changes in the appearance of skin coloration can be in-
dicative of diseases associated with other body systems. Liver
disease or liver cancer can cause the accumulation of bile and
the yellow pigment bilirubin, leading to the skin appearing yel-… 15/18