Download PHYSIOLOGY OF SKIN FUNCTION

PHYSIOLOGY OF SKIN FUNCTION
SKIN
→ The skin, the body’s largest organ, is the outer body covering of an animal.
→ It forms a barrier that helps prevent harmful microorganisms and chemicals
from entering the body, and it also prevents the loss of life-sustaining body fluids.
→ It protects the vital structures inside the body from injury and from the
potentially damaging ultraviolet rays of the sun.
→ The skin also helps regulate body temperature, excretes some waste products,
and is an important sensory organ.
→ Both delicate and resilient, the skin constantly renews itself and has a
remarkable ability to repair itself after injury.
Structure of the Skin
The skin consists of an outer, protective layer (epidermis) and an inner, living layer
(dermis). The top layer of the epidermis is composed of dead cells containing
keratin, the horny protein that also makes up hair and nails.
MAJOR LAYERS of the SKIN
●EPIDERMIS
→The outer layer of skin which is only a few cells thick; it contains pigments and
pores, and its surface is made of dead cells that it sheds from the body.
●DERMIS
→ the thick sensitive layer of skin or connective tissue beneath the epidermis
EPIDERMIS
● Keratinocytes
● Melanocytes
● Langerhans Cells
● Merkel Cells
KERATINOCYTES
About 90% of the cells in the epidermis are keratinocytes,
produces a tough, fibrous protein called keratin. This protein is the
main structural protein of the epidermis, and it provides many of
the skin’s protective properties.
The old keratinocytes at the upper surface of the skin constantly slough off.
Meanwhile, cells in the lower layers of the epidermis divide
continually, producing new keratinocytes to replace those that
have sloughed off. By the time the cells reach the uppermost
layer of the epidermis, they are dead and completely filled with
the tough protein.
MELANOCYTES
Scattered among the keratinocytes in the epidermis are melanocytes, cells that
produce a dark pigment called melanin. This pigment gives color to the skin and
protects it from the sun’s ultraviolet rays.
Differences in skin color result from differences in the
amount of melanin produced and how melanosomes,
packets of melanin, are arranged in the keratinocytes.
Particularly in people with light skin, melanin sometimes
accumulates in patches, forming freckles, age spots, or liver spots.
A type of melanin called pheomelanin makes redheaded people more
sensitive to the sun.
A total lack of melanin, a genetic condition called albinism, makes people
extremely sensitive to the sun. Peoplewith albinism have very light skin,
hair, and eyes.
LANGERHANS CELLS
The epidermis also contains a type of immune cell known as a Langerhans
cell. Produced in the bone marrow, Langerhans cells take up sentrylike
positions in the epidermis, where they help cells of the immune system
recognize potentially dangerous microorganisms and chemicals.
MERKEL CELLS
Another cell in the epidermis is the Merkel cell, found in sensitive, hairless areas
such as the fingertips and lips. Located in the deepest layer of the epidermis,
Merkel cells contact nerve endings in the dermis below and function as a type of
touch receptor.
DERMIS
● Collagen
●Elastin
●Papillary Layer
● Reticular Layer
COLLAGEN
The main structural component of the dermis is a protein called collagen. Bundles
of collagen molecules pack together throughout the dermis, accounting for
three-fourths of the dry weight of skin. Collagen is also responsible for the skin’s
strength.
ELASTIN
Another protein in the dermis, elastin, is the main component of elastic fibers.
These protein bundles give skin its elasticity. Collagen and elastin are produced by
cells called fibroblasts, which are found scattered throughout the dermis.
PAPILLARY LAYER
The upper part of the dermis is known as the papillary layer. It is characterized by
dermal papillae, tiny, fingerlike projections of tissue that indent into the epidermis
above.
In the thick skin on the palms and soles, the epidermis conforms to the shape of
the underlying dermal papillae, forming ridges and valleys that we know as
fingerprints. These ridges provide traction that helps people grasp objects and
surfaces.
Some dermal papillae contain touch receptors called Meissner’s corpuscles,
and many contain loops of tiny blood vessels. The extensive network of
blood vessels in the dermal papillae plays an important role in the
regulation of body temperature.
The blood vessels dilate in hot environments to help dissipate heat, and
they constrict to conserve heat in cold environments.
The lower layer of the dermis is called the reticular layer. It is made
primarily of coarse collagen and elastic fibers.
Skin appendages such as glands and hair follicles are often anchored in the
reticular layer of the dermis.
The reticular layer also contains several different types of sensory receptors,
nerve cells specialized to detect various stimuli.
SKIN APPENDAGES
● HAIR
● NAILS
● GLANDS
HAIR
Hair is composed primarily of keratin. The dead keratinocytes fuse together to
form the hair. At the base of the follicle is the bulb, which contains cells that give
rise to the keratinocytes that make up the hair, as well as blood vessels that
nourish the growing hair.
Lengthening fibers of keratin-filled dead cells, grouped around the semi hollow
medulla, make up the cortex. A living structure called the bulb (visible as a white
lump at the end of a plucked hair) surrounds and feeds the root, which lies in a
pocket of the epidermis called the follicle.
Each hair follicle also contains the arrector pili, a muscle that
contracts in response to cold, fright, and other emotions. When the
muscle contracts, it pulls the hair in the follicle into a vertical position.
This response may help some mammals keep warm or look bigger to frighten or
intimidate their enemies.
But in humans, because of our sparse coat of body hair, it merely produces “goose
bumps.”
The color of hair is due to melanin. Dark hair contains true melanin like that found
in the skin.
Blond and red hair result from types of melanin that contain sulfur and iron.
Hair goes gray when melanocytes age and lose the enzyme necessary to produce
melanin.
White hair occurs when air bubbles become incorporated into the growing hair.
NAILS
Nails on the fingers and toes are made of hard, keratin-filled epidermal cells. They
protect the ends of the digits from injury, help us grasp small objects, and enable
us to scratch.
The part of the nail that is visible is called the nail body or nail plate, and the
portion of the nail body that extends past the end of the digit is called the free
edge. The nail groove is the fold on the side and the eponychium is the skin that
holds the nail root.
Most of the nail body appears pink because of blood flowing in the tissue
underneath. The pale, semicircular area called the lunula appears white due to an
underlying thick layer of epidermis that does not contain blood vessels.
The part of the nail that is buried under the skin is called the root. Nails grow as
epidermal cells below the nail root and transform into hard nail cells that
accumulate at the base of the nail, pushing the rest of the nail forward.
GLANDS
●SWEAT GLANDS
An adult human has between 1.6 to 4 million sudoriferous glands, or sweat
glands. Most are of a type known as eccrine sweat glands, which are found
almost all over the surface of the body and are most numerous on the palms and
soles. Eccrine sweat glands begin deep in the dermis and connect to
the surface of the skin by a coiled duct.
In addition, nerve fibers that encircle the sweat glands stimulate the
glands in response to fear, excitement, or anxiety.
Other sweat glands, known as apocrine sweat glands, are much less numerous
than eccrine sweat glands. They are also anchored deep in the dermis, but open
into hair follicles rather than onto the surface of the skin.
Apocrine sweat glands are located mainly in the armpit, genital area, and around
the nipples of the breasts.
● OIL GLANDS
Oil, or sebaceous, glands are found all over the body except on the
palms, the soles, and the top of the feet. They are most numerous
on the face and scalp. Most sebaceous glands open into hair
follicles, but the glands also occur in some hairless areas, such as the lips and
inside the mouth.
Glands of this type produce an oily substance called sebum, which keeps
the skin and hair from drying out and inhibits the growth of certain
harmful bacteria.
● WAX GLANDS
Wax, or ceruminous, glands are located in the ear canal. They secrete a
waxy substance that helps prevent foreign particles from entering the
ear. Ceruminous glands are modified sweat glands.