THE INTEGUMENTARY SYSTEM
Prepared by Hugh Potter
Biology Department
Union County College
Source of Images: ADAM, Inc
INTRODUCTION TO THE SKIN
The integumentary system consists of the skin and the
derivatives of the epidermis such as hair, glands and nails.
The skin is also classified as a cutaneous epithelial
membrane.
A. Stratum corneum is a dead
upper portion of the epidermis
A
B. Living epidermis consists
of several layers of cells
which manufacture keratin.
C. Dermis is the layer of skin
containing blood vessels,
glands and nerve endings
B
C
EPIDERMIS - THIN SKIN
The epidermis of skin is a
stratified squamous epithelium
consisting of several layers of
cells. These cells are called
keratinocytes because of their
ability to produce the tough,
fibrous protein keratin. The
surface of the epidermis is
covered with a layer of dead tissue
called the stratum corneum. On
some of the surfaces of the body,
the epidermis is thin and delicate,
e.g., the under surfaces of the
upper arms.
KERATIN FORMATION
Keratin fiber is formed from
keratohyaline. Keratohyaline is
formed in the stratum
granulosum (black asterisk). It
is in this region that the
keratinocytes die. Immediately
above this layer is a clear
translucent layer, the stratum
lucidum (red asterisk). The
stratum corneum (sc) is the
uppermost layer
*
*
sc
Keratinocytes – Most abundant cell type. Cells are held together by
desmosomes and are organized into layers.
1. Stratum germinativum – Cells found just above the basement membrane
which undergo rapid cell divisions.
2. Stratum spinosum is a middle portion of the epidermis several cells thick..
The cells have a "spiny appearance".
3. Stratum granulosum is a layer just above the spinosum in which large
amounts of keratohyalin is being synthesized.
4. Stratum lucidum - The keratinocytes in this layer are filled with
keratohyalin and a fibrous protein, keratin. Keratin is tough, durable and
water resistant. Keratin is also the chief component in hair and nails.
5. Stratum corneum is the outermost layer of the epidermis composed of
flattened, dead, keratin-filled cells. This dead layer provides protection from
abrasion, strong chemicals, microbial invasion and dehydration.
Other cell types in the epidermis include:
1. Langerhans cells are T lymphocytes (cells that mount an immune
response) located within the stratum spinosum. These cells will initiate an
immune response against microbes and epidermal skin cancers.
2. Merkel cells are located among the cells of the germinativium. They are
sensitive to touch.
3. Melanocytes are also located in the germinativium often under the basal
cells just above the basement membrane. They manufacture a brown
pigment called melanin. Melanin is packaged within vesicles called
melanosomes in the cytoplasm of the melanocytes. These vesicles are
transferred into the overlying keratinocytes, darkening the skin. Melanin is a
helpful material in the skin. It tends to surround the nucleus of the
keratinocyte protecting the DNA of that cell from damage due to ultraviolet
light from the sun. It also seems to neutralize damaging chemicals called
free radicals that accumulate in traumatized tissues. Cancer of the
melanocytes, melanoma, is an extremely dangerous and malignant
neoplasm.
Changes in Skin Color
1. Tanning Effect –Due to the activity of melanocytes.
2. Reddening of skin occurs due to an increased blood flow in the
underlying dermis. For example, an increase in body temperature which
leads to a vasodilation of blood vessels in the dermis.
3. Strong emotional states will lead to an increase in blood flow in the
dermis.
4. Blanching of Skin ("turning pale") occurs due to decreased blood flow to
the skin. This decreased blood flow may be caused by a sudden drop in
blood pressure, hypothermia and emotional states.
5. Cyanosis – bluish color in skin due to sustained reduction in blood supply
to the skin
6. Jaundice – When liver function is interrupted due to cirrhosis, liver cancer
or blockage of bile flow, yellow bile pigments and bilirubin accumulate in the
skin and whites of the eyes.
The Dermis
1. Located below the epidermis it contains all of the accessory
organs of epidermal origin, such as, hair follicles and glands.
2. It also has extensive networks of blood vessels, lymphatic
vessels nerve endings and nerve fibers.
3. The dermis consists of two major layers:
a. The Papillary layer contains loose (areolar) connective
tissue with a rich supply of blood capillaries. It also contains
the nerve endings for touch and pain.
b. The Reticular layer contains dense irregular bundles of
collagen, elastic and reticular fibers. These fibrous bundles
blend into the papillary layer above and into the underlying
subcutaneous layer.
Dermal Blood Supply
1. The Cutaneous Plexus
When arteries supplying the skin reach the subcutaneous layer, they form
a network of branches called the cutaneous plexus. Branches from this
plexus supply the subcutaneous fat and various structures in the dermis
such as hair follicles and sweat glands.
2. The Papillary plexus – capillary beds that follow the contours of the
epidermal-dermal boundary. Interruptions in this circulatory flow can result
in epidermal and dermal deterioration and necrosis, e.g., decubitis ulcers
and diabetic foot.
B. Innervation of the skin – The skin is richly innervated. The functions of
these nerves include:
1. Control of blood flow through the skin.
2. Adjusting the rate of glandular secretions.
3. Sensory reception – The sensory receptors of the skin respond to two
basic types of stimulation, mechanical change and pain.
Meissner’s Corpuscle
Pacinian Corpuscle
Mechanoreceptors of the skin
1. Meissner’s corpuscles – located within the dermal papillae, they
respond to light touch. Meissner’s corpuscles are located immediately
under the epidermis within the dermal papillae. These nerve endings
respond to light touch.
2. Pacinian corpuscles – located deep within the reticular layer, Pacinian
corpuscles are sensitive to deep pressure and vibrations. They are
located in the deep dermis or subcutaneous layer. They are also
common in the glans penis of the male and in the pancreas. The role
they may play in the latter organ has never been determined.
3. Merkel’s disks – are specialized nerve endings found at the ends of
nerve fibers. These disks are closely associated with the epidermal
Merkel’s cells and respond to fine touch.
Nociceptors – Pain receptors are especially abundant in
the upper skin, joint capsules, the periosteum of bone and
the walls of blood vessels. Very few pain receptors are
located in visceral organs or deep tissues. There are three
types of pain receptors:
1. Those sensitive to temperature extremes.
2. Those sensitive to mechanical damage.
3. Those sensitive to chemicals, e.g., metabolites from
traumatized tissues such as aracidonic acid and
prostaglandins.
Very strong pain stimuli will excite all three types of
receptors. For example, severe trauma of any kind (burns,
cuts, corrosive chemicals) will often be described as
"burning" or acute pain.
Hair Follicle –Hair follicles project deep into the dermis and subcutaneous fat
from the surface of the skin. Each hair follicle consists of the following:
1. Hair Papilla – a small clump of connective tissue, capillaries and nerves
which supports the growth of the hair.
2. Hair Bulb – consists of the epithelium that surrounds the papilla. This
structure is an invagination of the epidermis. Cells from the basal layer of the
bulb divide and are pushed up into the root of the hair.
3. Hair shaft – begins at a point about midway between the papilla and the
skin surface.
4. The arrector pili are small bands of smooth muscle which extend from the
connective tissue sheath of the follicle and anchor in the papillary layer of the
dermis. This muscle is stimulated to contract by strong emotional states or
cold temperatures. These stimuli operate through the nervous system to
cause the hairs to become erect.
SUBCUTANEOUS LAYER
The subcutaneous layer of
skin contains abundant
adipose tissue. In addition,
many blood vessels, nerve
endings and hair follicles
can usually be seen.
Adipose
tissue
Hair Follicle
HISTOLOGY OF THE HAIR FOLLICLE
The hair follicle is an epidermal sheath that surrounds the hair.
Sebaceous glands are usually attached to the side of a follicle. Their oily
secretion enters the follicle and follows it to the surface. The hair grows
from the bulb, the swollen lower end of the follicle. The bulb is invested
with blood vessels and nerves which are essential for the continued
growth of the hair.
Glands in the Skin – The skin contains a number of exocrine glands
Sebaceous glands – Holocrine glands which discharge an oily secretion
called sebum.
The gland cells originate in the periphery of the gland. As they mature,
the cells manufacture sebum, a mixture of triglycerides, cholesterol,
proteins and electrolytes.
As the cells reach the opening or lumen of the gland, they rupture
releasing their product (holocrine secretion).
There are two types of sebaceous glands:
1. Simple branched alveolar glands – empty their secretion into the
follicle of a hair.
2. Sebaceous follicles – large sebaceous glands that are connected
directly to the epidermis and are not associated with a hair. They are
found in the skin of the face, back, chest and nipples.
Sebum functions by lubricating the skin and retarding the growth of
bacteria.
ACNE
Acne is a condition of the skin
which is shared by individuals
of all ages. Each pimple is in
fact an inflammed sebaceous
gland. The causes of the
inflammation include inadequate
cleansing of the skin to remove
superficial oils, endocrine
changes during puberty and
pregnancy, dietary effects and
emotional stress. It is easy to
understand why teenagers so
often exhibit this problem.
SEBACEOUS AND SWEAT GLANDS
Sebaceous glands (Sb) are usually
associated with a hair and release their
holocrine secretion (sebum) into the hair
shaft which carries it to the surface. Sweat
glands come in two varieties. The most
numerous are the eccrine glands (Sw). The
watery merocrine product of these glands
travels through a duct to the surface of the
skin. Here it evaporates and cools the
body. Apocrine sweat glands are found
around the areola of the nipple, in the labia
majora and axilla. Their secretion is
thicker and more odiferous than the eccrine
secretion.
Sb
Sw
Sb
Sw
Merocrine (eccrine) sweat glands
1. Very numerous. In the adult, the skin may contain 2 to 5
million merocrine glands per square inch. Palms and the soles
of the feet have the highest concentration.
2. Merocrine glands are smaller than the apocrine glands.
They produce a watery sweat containing electrolytes,
lysozymes, antibodies and other ingredients.
3. The functions of these glands include:
a. Removing heat from body’s surface to lower body
temperature. b. Excretion of water, electrolytes and
nitrogenous wastes.
c. Protection from chemical and microbial
Apocrine glands
Located in the armpits, groin and around the nipples.
They produce a sticky, cloudy, odorous secretion into a hair
follicle. These secretions become intensified at the time of
puberty under the influence of the nervous and endocrine
SWEAT GLANDS
Sweat glands are located in the deep dermis. They are usually
associated with small blood vessels and nerves. In response to
hyperthermia, the eccrine sweat glands release a serous liquid to the
surface of the skin via a duct. The evaporation of this liquid from the
surface of the skin cools the body. In the arm pits and groin, apocrine
sweat glands secrete an oily material. Sweat glands (sg) are unbranched,
coiled, tubular glands distributed throughout the skin. They are not found in the
nail beds, margins of the lips, glans penis or ear drum. They are most
numerous in the palms of the hands and soles of the feet.
Functions of the Skin
A. Regulation of body temperature
1. During hyperthermia blood vessels (arteries) in the skin
dilate due to nerve stimulation from the brain. Blood flow to
the skin increases.
2. Warm water moves from the blood to the merocrine
sweat glands by filtration. This warm sweat now moves
to the skin’s surface by the duct from the gland.
3. The warm sweat evaporates from the skin’s surface
taking excess body heat with it.
4. Body temperature comes down and sweating is
reduced (negative feedback).
During hypothermia
1. Blood vessels in the skin constrict due to nerve
stimulation from the brain. This reduces blood flow to the
skin.
2. Radiant heat loss is reduced from the skin.
Sweat production decreases.
3. Body heat is retained in the trunk and head or "core" of
the body.
4. Shivering, an involuntary contraction of skeletal muscles
due to nerve stimulation, helps to generate body heat. This
is the result of the breakdown or "burning" of glucose in the
skeletal muscles.
B. Protection from ultraviolet light
1. Ultraviolet light penetrates the epidermis and stimulates the melanocytes
to increase their production of melanin.
2. Melanocytes inject packages of melanin called melanosomes into cells of
the lower epidermis. As these darkened cells divide, they are pushed to the
top of the epidermis.
3. Eventually, the entire epidermis becomes a "sunscreen" blocking out
much of the harmful ultraviolet light.
The arrow indicates the layer
of melanocytes in the lowest
level of the epidermis.
MELANOMA
Cancer of the Melanocytes
Melanoma is a neoplasm of the
melanin producing cells. The
cancer usually begins when a mole
(actually a benign tumor) becomes
malignant. Overexposure to
sunlight is thought to be a cause of
melanoma. However, some forms
may have a hereditary basis.
Melanoma is one of the most
metastatic and invasive cancers.
The prognosis for an individual
with advanced melanoma is not
good. Any change in a mole or
nevus should be followed up with
a visit to the dermatologist.
C. Protection from infection by microbes
1. The tough impervious epidermis forms a barrier to
microbes, as long as it is intact.
2. The formation of sweat and sebum flushes away microbes
on the surface of the skin, as well as those in sweat gland
pores and hair follicles. The pH of sweat and sebum tends to
retard the growth of bacteria. Sweat also contains lysozymes
and antibodies which attack microbes.
3. The blood-rich dermis reacts quickly to microbial invasion
through the activities of various cells. Tissue basophils
release chemicals which increase the blood supply to an
infected region of skin. Phagocytic cells such as the
macrophages and neutrophils physically attack microbes. The
combined reaction of these cells is referred to as
inflammation.
D. Protection from excessive water loss or dehydration. The
tough, keratinized epidermis forms a watertight cover on the body
keeping body water in.
E. Production of Vitamin D - Some of the ultraviolet light striking
the skin passes through the melanin sunscreen and causes a
chemical change in the blood leading to the formation of vitamin
D. This vitamin is required for the body to absorb calcium from the
intestine. Calcium is essential for the development of teeth and
bones.