THE SKIN
Skin is the largest organ in the body. It has a surface area of 1.5- 2 square meters in
adults. It varies in thickness in different parts of the body. It is thinnest on the eyelids and the
thickest on the palms and soles of the feet.
LAYERS OF THE SKIN
Epidermis or cuticle: is the superficial layer
Dermis or corium: is the layer below the epidermis
the subcutaneous layer:is the layer between the dermis and underlying structures and it is
composed of areolar tissue and adipose tissue.
The Epidermis: is the superficial layer. It is composed of stratified keratinized squamous
epithelium. The epidermis has no blood vessels or nerve endings. It is entirely dependent on the
underlying dermis for nutrients and blood supply. The main types of cells in the epidermis are
Keratinocytes Melanocytes ,and Langerhans cells.
Lyers of epidermis: There are several layers of cells in the epidermis which extends from
the deepest basal or germinative layer to the most superficial stratum corneum (horney layer) .
The epidermis contains 5 layers, from bottom to top the layers are named
- STRATUM BASAL OR GERMINATIVE LAYER
- STRATUM SPINOSUM
- STRATUM GRANULOSUM
- STRATUM LUCIDUM
- STRATUM CORNEUM OR HORNEY LAYER
THE BASAL OR GERMINATIVE LAYER
It is the deepest layer and consists of single layer of columnar
cells and is attached by a membrane to the under surface of dermis. These are the cells from
which new epidermal cells are constantly being produced. The new cells push the older cells
upwards away from the basal layer towards the surface. Upward projections from the dermal
layer the dermal papillae anchor the dermis securely to the epidermis and allow passage and
exchange of nutrients and wastes to the lower part of the epidermis. As they approach the
surface of the epidermis these cells become flattened and become the stratum granulosum.
(granular layer)
STRATUM GRANULOSUM
The nucleus of cells in the granular layer contains granules of
keratiohyaline. Enzymes in the nuclei convert keratiohyaline in to keratin. In the thickest
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areas of the skin over the palms of the hands and soles of the feet granular layer become in
to stratum lucidum and the other areas it become in to Stratum corneum (horney layer).
STRATUM CORNEUM
it consists of thin, flat, non-nucleated cells composed of
keratin. The cells of the honey layer are constantly being shed and replaced by cells from
the deeper layer. The life span of epidermal cell is between 28- 30 days. Between the cells
of the basal layer there is other type of cells known as melanocytes. Melanocytes produce
melanin pigment which is responsible for the color of the skin and they also protect the
deeper structures from ultra violet rays and other injurious light. Melanin is produced from
tyrosine under the influence of enzyme tyrosinase.
THE DERMIS
Dermis or corium lies deep to the epidermis. It is composed of dense
connective tissue, and matrix contains collagen fibers interlaced with elastic fibers.
Collagen fibers bind water and give the skin its tensile strength. Fibroblasts, macrophages,
and mast cells are the main cells found in the dermis.
Structure found in the dermis are
a. small blood and lymph vessel
b. sensory nerve endings
c. sweat glands and their ducts
d. hairs,arector pili muscles and sabeceous gland
BLOOD AND LYMPH VESSELS
arterioles from a fine network with capillary branches supplying , sweat glands, sebaceous glands, hair follicles, and the dermis. Lymph vessels
also form a network throughout the dermis.
SENSORY NERVE ENDINGS
The sensory nerve endings are widely distributed in the
dermis and it contains the nerve endings of touch, pain, temperature, and presser. Skin is an
important sensory organ through which individuals receive information about their
environment. Nerve impulses, generated in the sensory receptors in the dermis are
transmitted to the spinal cord by sensory nerves. From there impulses are conducted to the
sensory area of the cerebrum, where the sensations are perceived.
SWEAT GLAND
these are widely distributed throughout the skin and are most numerous in the palms of the hands, soles of the feet, axillae, and groins. The bodies of the glands lie coiled in the subcutaneous tissue. There are two types of sweat glands. Eccrine and the apocrine.
THE ACCRINE GLANDS
are the more common type and open on the surface through tiny
pores. Sweat glands have a coil which is buried deep in the dermis and a duct which opens
on to the surface of the skin at minute depression called pores. Sweat is a clear watery Fluid
important in regulating body temperature.
THE APPOCRINE GLANDS
these are large sweat glands, which open in to the hair follicles above the
sabaous gland. They are found in the rectal and anal area, axilla, nipples and areola and
begin to function after puberty. The most important function of the sweat is the regulation
of body temperature.
HAIR
Hair grows from a hair follicle. The part extending above the skin surface is called
shaft and the part which is embedded in the skin is the root. The lower end of the follicle
expands to form the hair-bulb. At the base of the follicle is a cluster of cells called hair
papilla or hair bulb. Papilla contains blood vessels, nerve endings and melanocytes. Hair is
formed by multiplication of cells of the bulb and as the hairs are pushed upwards away from
their source of nutrition, the cells die and become keratinized. The color of the hair is due
to pigment melanin.
The arrector pili; the walls of the hair follicle ,there is minute bundle of smooth muscle
fibres called arrector pili. Contraction of the muscle fibers makes the hair to stand erect.
This causes the skin around the hair to become elevated giving the appearance of
gooseflesh.
Sebaceous gland ; : Sebaceous glands are found all over the bodies except the palms of the
hand and soles of the feet. They are most numerous on the scalp, fore-head, cheeks and
chin. Secretion of the Sebaceous gland is called sebum which is an oily antimicrobial
substance. Sebum is discharged in to the hair follicle by their ducts. Sebaceous glands
which are in the lips, eyelids, nipple, labia minora, and glans penis discharge their Secretion
directly on to the surface. Sebum keeps the hair soft and pliable, and gives it’s a shiny
appearance. It waterproofs the skin and acts as a bactericidal and fungicidal agent
preventing infection. It also prevents drying and cracking of skin.
Nails; are the appendages of the skin. It is a modified horny cells and forms protective
covering over the fingers and toes. It has got a body or nail plate which is attached to the
nail bed. Each nail has a root which is embedded in the skin and covered by the cuticle,
which forms the hemispherical pale area called lunula.
The Subcutaneous layer: Subcutaneous layer, containing areolar tissues and varying
amounts of adipose tissue, lies under the dermis.
Function of the skin
- PROTECTION
- Invasion by micro-organism
- chemicals
- Physical agent’s e.g. mild trauma, ultraviolet light: The sensory nerve endings the dermis conveys information about the external environment to the brain, and serves as an important protective mechanism for the body. The pigment Melanin protects against harmful ultraviolet rays in sunlight.
- Dehydration. It helps to maintain a stable internal environment by preventing excessive loss of water, and electrolytes.
The constant level of body temp. is maintained
by a balance between the heat produced and the heat lost by the body.
Heat production; Heat is constantly produced in the body by varies chemical
reactions in the body. The principle organs involved in heat production are .
- Skeleton contraction : skeletal Muscle contraction which produces large amount of heat. The more strenuous the muscular exercise the greater the heat produced.
- The liver : the liver performs metabolic activities and heat is produced.
- The digestive organs : heat is produced by the contraction of alimentary tract and by digestion of food.
Heat is lost from the skin by; radiation, evaporation ,conduction
and convection. Heat is lost from the body in several ways. 97% by the skin. 25 in
expired air 1% in urine and faces.
Heat is lost from the skin by; radiation, conduction and convection.
Rdiation: in radiation the exposed part of the body radiat heat away from the body.
conduction: the heat is transferred to any object in direct contact to the body. In
conduction the cloths in contact with the skin conduct heat away from the body.
convection: heat is transferred away from the body surface by movement of air. In
convection the air passing over the exposed parts of the body is heated and rises cool air
replaces it.
3. Formation of vitamin D
Vitamin D is formed by the action of sunlight on 7- dehydrocholesterol, a fatty substance
widely distributed in the skin.
4. Cutaneous sensation: Sensory receptors in the dermis can be sensitive to touch,
pressure, temperature and pain. Stimulation generates impulses in sensory nerves
which are transmitted to the cerebral cortex.
5. Absorption: Substances can be absorbed through the intact skin into the blood
stream.e.g. Administration of certain drugs (sublingually) because it is quickly
absorbed through the mucous membrane of the mouth to bring rapid relief from the
chest pain. Also some toxic chemicals e.g. mercury.
6. Excretion: The skin is a minor excretory organ for some substances including:
Sodium chloride in sweating, urea, when kidney function is impaired, and
aromatic substances e.g. garlic and other spices.
7. Storage :The skin and subcutaneous tissues act as a store for water and fat. Approximately 15% of the total water content of the body is contained in the skin, while the subcutaneous tissue serves as one of the main fat depots of the body.
8. Secretion:The secretion of sebum by the sebaceous glands helps to maintain the
integrity of the skin. Sweat produced by the eccrine and apocrine gland plays an
important part in the regulation of body temperature and aids in the excretion of
metabolic waste products.
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