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CHAPTER 2:
THE INTEGUMENTARY SYSTEM
Learning Outcome:
1. Differentiate the layers of the epidermis and the cells that compose them.
2. Contrast the structure, distribution, and functions of hair, skin glands, and nails.
3. Examine how the skin contributes in maintaining homeostasis and wound healing.
Introduction to Integumentary System
The integumentary system is composed of the skin, hair, oil, and sweat
glands, nails and sensory receptors. It helps maintain a constant body
temperature, protects the body and provides sensory information about
surrounding environment. It is the only organ of the body that is on show all the
time which shows what we feel emotionally (blushing, frowning) and physically,
bluish skin indicates hypoxia. The most recognized function of the
integumentary system is the protection that the skin gives to underlying tissues.
The skin not only keeps most harmful substances out, but also prevents the loss
of fluids. Its functions include protection of internal structures, prevention of
entry of disease-causing microorganisms, temperature regulation, excretion
through perspiration, pigmentary protection against ultraviolet sunrays, and
production of vitamin D. The body stores about half its fat in the underlying
hypodermis.
STRUCTURE OF THE SKIN
The skin is composed of two distinct regions: the dermis and the
epidermis. The subcutaneous facia (sometimes referred to as the hypodermis)
lies under the dermis (Colbert et al., 2012); these masses of loose connective and
adipose tissue attach to the skin and organs beneath; they are not part of the
skin (see fig 2.1).
Fig 2.1 Skin Structure. Pearson 2018
The Epidermis
The superficial and thinnest aspect of the skin, the epidermis is the area
of skin that can most commonly be seen. while the skin covers the whole of the
body, there are several regional distinctions, and these are associated with
flexibility, distribution and type of hair, density and types of gland, pigmentation,
vascularity, innervations and thickness (Jenkins et al., 2013). The thinnest part
of the skin can be found on the eyelids; here, it is just 0.5 mm in thickness,
whereas at the heel it is 4.0 mm thick.
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The epidermis is made up of epithelium, called keratinised stratified
squamous epithelium, and contains four key cell types (Fig. 2.2):
• Keratinocytes- produce keratin, a protein that gives skin its strength and
flexibility and waterproofs the skin surface. Keratin is a tough, fibrous protein
that aids in the protection of the skin and tissues below from the heat,
microorganisms and chemicals.
• Melanocytes- produce melanin, the dark pigment that gives skin its color.
• Langerhans cells- help the immune system by processing antigens (foreign
bodies).
• Merkel cells- involved with touch reception
Fig. 2.2 Epidermis cell types
Pearson 2018
Epidermis has four strata or layer- stratum basale, stratum spinosum,
startum granulosum, stratum lucidum and a thick skin called stratum corneum
as shown in fig. 2.3.
Fig. 2.3 The Layers of Epidermis. Pearson 2018
Stratum Basale is a single layer
of cells resting on a basement
membrane (layer between the
dermis and epidermis). The
stratum basale cells divide
continuously. As new cells form,
older ones are pushed toward
the skin surface.
The next layer, the stratum
spinosum, consists of spiny
prickle cells that interlock to
support the skin. The stratum
granulosum, the thin middle
layer, initiates keratinization
(production of keratin). This
process starts the death of
epithelial cells (the cell type that
makes up skin).
During
desquamation,
keratinocytes are pushed toward the surface. These cells begin to produce the
keratin that eventually will dominate their contents. When these cells reach the
epidermis outer layer, they are little more than keratin-filled sacs. Millions of
these dead cells are worn off daily, creating a new epidermis every 35 to 45 days.
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The stratum lucidum protects against sun ultraviolet-ray damage. This
thick layer appears only in frequently used areas such as palms of the hands
and soles of the feet. Thick skin epidermis has all five strata. Thin skin covers
thinner epidermal areas such as eyelids. Thin skin has three or four of the five
strata; it never has stratum lucidum.
The stratum corneum, the fifth, outermost layer is thick with rows of dead
cells. These cells contain soft keratin, which keeps the skin elastic and protects
underlying cells from drying out.
Thin skin- covers most of the body surface, contains four strata. The skin is
about as thick as the wall of plastic sandwich bag (see Fig. 2.4).
Thick skin- found on the palms of the hand and sole of the feet and is about as
thick as a standard paper towel. The thickness and thinness is referred to the
relative thickness of the epidermis (see Fig 2.4).
Fig. 2.4 Thin and thick skin
Pearson 2018
Dermis
The deepest part of the skin is called the dermis and lies directly below the
epidermis; it is predominantly composed of dense connective tissue that contains
collagen and elastic fibres. Its major parts are collagen (a protein that adds
strength), reticular fibers (thin protein fibers that add support), and elastic fibers
(a protein that adds flexibility).
The dermis has two layers:
1. Papillary region- consists of thin collagen and fine elastic fibers. Some
papillae contain capillaries that nourish the epidermis; others contain
Meissner's corpuscles, sensory touch receptors. Different free nerve
endings initiate signals that give rise to sensations of warmth, coolness,
pain, tickling and itching. Fingerprints and footprints keep skin from
tearing and aid in gripping objects.
2. Reticular Region- it is attached to the subcutaneous layer are irregular,
dense, connective tissues containing fibroblasts and collagen bundles, and
coarse elastic fibre. The combination of collagen and elastic fibers provides
the skin with strength, extensibility, the ability to stretch and elasticity
(the ability to return to its original shape after stretching). The reticular
layer of the dermis contains criss-crossing collagen fibers that form a
strong elastic network. This network forms a pattern called cleavage
(Langer's) lines. Surgical incisions that are made parallel to cleavage lines
heal faster and with less scarring than those made perpendicular. Parallel
incisions disrupt collagen fibers less and require less scar tissue (cells that
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aid in healing) to close up a wound. The reticular layer also contains
Pacinian corpuscles, sensory receptors for deep pressure. This layer
contains sweat glands, lymph vessels, smooth muscle, and hair follicles,
described in the discussion on hair follicles.
Hypodermis
This is attached it to underlying bone and muscle and supplies it with
blood vessels and nerves. The hypodermis consists of loose connective tissue
with collagen and elastin fibers. The main types of cells within the hypodermis
are fibroblasts, adipose cells, and macrophages. The hypodermis, which is not
part of the skin, is sometimes called subcutaneous (su˘ b-koo-ta¯ne¯-u˘s) tissue.
It is responsible in the storing fat where it functions as a source of energy,
insulation, and padding.
Skin Color
The primary pigments involved in skin coloration are carotene and melanin.
The micrograph at right shows a section through the thin skin of a light-skinned
person. The ratio of melanin-producing melanocytes to basal cells ranges
between 1:4 and 1:20, depending on the body region. The skin covering most
areas of the body has about 1000 melanocytes per square millimeter. Skin color
differences are due to the amount of melanin produced, not the number of
melanocytes. Light-skinned people produce less melanin than dark-skinned
people. Absence of melanin production leads to a disorder called albinism.
ACCESSORY STRUCTURES OF THE SKIN
The accessory structures are also known as the appendages.
A. Hair
Fig. 2.5 Hair Structure (Timby, 2012).
The hair can play a part in a person’s
distinctive appearance. The colour of the hair
influenced by the melanocytes that are found
within the hair bulb. A progressive decline in
melanin results in hair that is grey in colour.
Hair growth is determined by genetic and
hormonal factors.
The primary role of hair is to inhibit heat
loss. The whole of the skin surface has hair
follicles; every pore is an opening to a follicle,
and these are located deep in the dermis on
of the subcutaneous layer. The hair protects
specific parts of the body. Example: hair on
head protects the scalp from the sun, the
eyelashes and eyebrows guards the eyes
from foreign particles, cilia in the nostrils
help protect the inhalation of foreign particles.
is
top
the
Types of hair
Humans have three different types of hair:
•
Lanugo, the fine, unpigmented hair that covers nearly the entire body of
a fetus, although most has been replaced with vellus by the time of the
baby's birth
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•
•
Vellus hair, the short, downy, "peach fuzz" body hair (also unpigmented)
that grows in most places on the human body. While it occurs in both
sexes, and makes up much of the hair in children, men have a much
smaller percentage (around 10%) vellus whereas 2/3 of a female's hair is
vellus.
Terminal hair, the fully developed hair, which is generally longer, coarser,
thicker, and darker than vellus hair, and often is found in regions such as
the axillary, male beard, and pubic.
B. Nails
Fig. 2.6 Nail structure Pearson 2018
The nails provide a protective covering
for the ends of the fingers and the toes (see
fig. 3). Nails are tightly packed, dead, hard,
keratinised epidermal cells that form a
clean, solid covering over the digits. The
horn‐like structure of the nails is a result of
the concentrated amount of keratin present;
there are no nerve endings in nails. Lawton
(2006) notes that the nails act as a
counterforce to the fingertips, the fingertips
have numerous nerve endings, permitting a
person to receive information about objects
that are touched.
The majority of the nail body is pink, a
result of the blood capillaries lying
underneath. The white crescent present at
the proximal ends of the nail is known as the lunula and is formed by air mixed
with keratin matrix. The size of the lunula varies with individuals. The cuticle
(also called the eponychium) is stratum corneum extending over the proximal
end of the nail body.
Fingernails grow faster than toenails; as a person ages, the growth of nails slows.
In some cases, evidence of systemic diseases can be identified by the condition
of the nails, for example, chronic cardiopulmonary disease or fungal infection.
C. Sweat Glands
There are a number of glands located within the skin; these can be thought
of as mini‐organs of the skin, which have a number of functions to fulfil. The
sweat glands are coiled tubes composed of epithelial tissue and open out to pores
that are located on the skin surface. All of the glands have separate nerve and
blood supplies; each secretes a slightly acidic fluid made up of water and salts.
There are two kinds of sweat gland: eccrine and apocrine.
•
Eccrine glands- located all over the body; numerous in some areas such
as forehead, axillae, soles and palms. Reaction to fear and heat the
production of secretions in response to the activity of the sympathetic
system. It primary function is on thermoregulation. This is accomplished
through the cooling effect of the evaporation of sweat on the surface of the
skin. During hot weather, stress, exercise and pyrexia these glands
produce more sweat.
•
Apocrine glands- The apocrine glands are also coiled; there are not as
many of these in comparison with the eccrine glands, and they are found
in more localised sites, for example, the pubic and axillary areas, the
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nipples and perineum. not fully active until the person reaches puberty;
they are larger, deeper and produce thicker secretions than the eccrine
glands. During periods of stress and when in a heightened emotional state
these glands produce more sweat.
Modified apocrine glands
o Ceruminous – secrete earwax, for lubrication
o Mammary – secrete milk
Fig. 2.7 Sweat gland
Sebaceous Gland
They secrete an oily substance called sebum (Latin, meaning fat or tallow)
that is made of fat (lipids) and the debris of dead fat-producing cells. Sebum acts
to protect and waterproof hair and skin, and keep them from becoming dry,
brittle, and cracked. It can also inhibit the growth of microorganisms on skin.
Sebum is odorless, but its bacterial breakdown can produce odors. Sebum is
the cause of some people experiencing "oily" hair if it is not washed for several
days. Earwax is partly sebum, as is mucopurulent discharge, the dry substance
accumulating in the corners of the eye after sleeping.
Fig. 2.8 Sebaceous gland and its relationship to hair follicle
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Functions of the Skin
A fundamental understanding of the structure of the skin allows the reader
to begin to understand the numerous functions of the skin.
Sensation- There are several receptor sites on the skin that have the ability to
sense change in the external environment in respect to temperature and
pressure; these receptors throughout the skin are made up of a wide and varied
range of nerve endings. The messages picked up in the skin are then usually
transferred to the brain.
Sensations that arise in the skin are known as cutaneous sensations;
other sensations are those associated with vibration and tickling and irritations.
There are some areas of the body that have more sensory receptors than others;
for example, the lips, genitalia and tips of the fingers. The sensation of pain can
signify actual or potential tissue injury.
Thermoregulation- The skin has a role to play in homeostasis through
thermoregulation, helping to keep the temperature of the body within narrow
ranges, adapting and adjusting as a person engages in a number of different
activities. Changes in the size of blood vessels in the skin can help to regulate
temperature. When the temperature increase, the blood vessels dilate
(vasodilation). To prevent further rise in temperature, the hot blood goes to the
surface of the skin to cool down. The skin is cooler as heat radiates away from
the body. As this is happening, the sweat glands secrete water onto the surface
of the skin. Evaporation occurs and, as a result of this, so too does cooling. The
opposite will occur when the person is in a cold environment. The blood vessels
constrict – vasoconstriction – and blood stays closer to the core of the body,
preserving heat.
Protection- the skin protects the body in many ways; example is the production
of melanin against the harmful effects of ultraviolet light. Through its ability to
intensify normal cell replacement when needed and the ability to shed dead skin
and cause the migration of cells, the skin maintains the integrity of the body.
Wound healing is an example of the skin’s protective mechanism. Eliminating
waste product through the pores of the skin can prevent buildup of poisonous
substances. Preventing the escape of fluid, preventing dehydration, and
regulating the amount of fluid through the content and volume of sweat
produced. It serves also as a water-proof barrier against harmful fluids in the
environment.
Excretion and Absorption- has the ability to excrete substances; sweat is
composed of water, sodium, carbon dioxide, ammonia and urea. It has also the
ability to absorb from the external environment; therapeutic and nontherapeutic medications, fat- soluble vitamins- A, D, E and K- oxygen and carbon
dioxide are also absorbed.
Synthesis of vitamin D- For Vitamin D to synthesise effectively, activation of a
precursor molecule in the skin by ultraviolet rays in the sunlight (ultraviolet
radiation) is required. Enzymes present in the kidneys and liver alter the
molecules, producing calcitriol. Calcitriol (a hormone) assists in the absorption
of calcium present in food in the intestines into the blood.
Wound Healing
The integumentary system often functions independently, responding directly
and automatically to local influences without involving the nervous or endocrine
systems. A dramatic display of local regulation can be seen after an injury to the
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skin. There are four phases in skin regeneration after injury: inflammation,
migration, proliferation, and scarring (see fig. 2.9).
There are four classical signs of inflammation
•
•
•
•
Calor- heat,
Dolor- pain,
Rubor- redness,
Tumor- swelling
Fig 3 Skin regeneration by Pearson 2018
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Types of Lesions
Burn
Burn, injury to the skin and deeper tissues caused by hot liquids,
flames, radiant heat, direct contact with hot solids, caustic chemicals,
electricity, or electromagnetic (nuclear) radiation. Skin exposed to temperatures
as low as 120°F is burned after about 5 minutes. It has three classifications as
seen in Fig. 3.1
Fig. 3.1 Classification of burn. Pearson 2018
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Summary Review: The Integumentary System
Here is what we have learned from the chapter:
•
•
•
•
In humans, the skin appendages, or structures embedded in the skin,
include hair, nails, and several types of glands.
Glands are groups of cells that produce and secrete substances needed
by other parts of the body.
In other vertebrates, the skin appendages include scales (in fish and
reptiles) and feathers (in birds).
Together, the skin and the skin appendages are known as the
integumentary system of the body.