Skin is the largest organ of body covering the body. In addition to protection, skin has many functions.

Thickness of Skin

Skin is not uniformly thick. At some places it is thick and at some places it is thin.

Average thickness of skin is about 1 to 2 mm.

Skin of foot, palm of hand and interscapular region is considerably thick, measuring about 5 mm.

In other areas of the body, skin is thin. It is thinnest over eyelids and penis, measuring only about 0.5 mm.

Skin is made up of two layers:

1. Outer epidermis.
2. Inner dermis.

Epidermis

Epidermis is the superficial layer of skin protecting the body from external environment. It is formed by stratified epithelium.

Five Layers of Epidermis

1. Stratum corneum.
2. Stratum lucidum.
3. Stratum granulosum.
4. Stratum spinosum.
5. Stratum germinativum.

Dermis

Dermis is the inner layer of skin situated between epidermis and subcutaneous tissue.

Dermis is made up of two layers:

1. Superficial papillary layer.
2. Deeper reticular layer.

Hair follicles with hairs, nails, sweat glands, sebaceous glands and mammary glands are considered appendages of the skin.

Color of the skin depends upon two factors:

1. Pigmentation of skin.
2. Hemoglobin in blood.

Pigmentation of Skin

Cells of the skin contain a brown pigment called melanin.

Melanin is synthesized by melanocytes, which are present mainly in the stratum germinativum and stratum spinosum of epidermis.

After synthesis, this pigment spreads to cells of other layers.

Melanin

Melanin is the skin pigment and it forms the major color determinant of human skin.

Skin becomes dark when melanin content increases. It is protein in nature and it is synthesized from the amino acid tyrosine via dihydroxy phenylalanine (DOPA).

Deficiency of melanin leads to albinism (hypopigmentary congenital disorder).

Hemoglobin in Blood

Coloration of the skin is also determined by the amount and nature of hemoglobin in cutaneous blood vessels.

1. Skin becomes pale when hemoglobin content decreases.
2. It becomes pink when blood rushes to skin due to cutaneous vasodilation (blushing).
3. Skin becomes bluish during cyanosis which is caused by excess amount of reduced hemoglobin.

Skin contains two types of glands:

1. Sebaceous glands.
2. Sweat glands.

Sebaceous Glands

Sebaceous glands are simple or branched alveolar glands situated at the side of hair follicle.

Sebaceous glands develop from hair follicles. So, sebaceous glands are absent over the thick skin, which is devoid of hair follicles.

Each gland is covered by a connective tissue capsule and alveoli of gland are lined by stratified epithelial cells.

Sebaceous glands open into neck of the hair follicle through a duct.

In some areas like face, lips, nipple, glands of penis and labia minora, sebaceous glands open directly into exterior.

Secretion of Sebaceous Gland: Sebum

Sebaceous glands secrete an oily substance called sebum.

Sebum is formed by liquefaction of alveolar cells and poured out through the ducts either via hair follicle or directly into exterior.

Composition of Sebum

Sebum contains:

1. Free fatty acids.
2. Triglycerides.
3. Squalene.
4. Sterols.
5. Waxes.
6. Paraffin.

Functions of Sebum

1. Free fatty acid content of the sebum has antibacterial and antifungal actions. Thus, it prevents the infection of skin by bacteria or fungi.
2. Lipid nature of sebum keeps the skin smooth and oily. It protects the skin from unnecessary desquamation and injury caused by dryness.
3. Lipids in sebum prevent heat loss from the body. It is particularly useful in cold climate.

Activation of Sebaceous Glands at Puberty

Sebaceous glands are inactive till puberty. At the time of puberty these glands are activated by sex hormones in both males and females.

At the time of puberty particularly in males, due to increased secretion of sex hormones especially dehydroepiandrosterone, the sebaceous glands are stimulated suddenly. It leads to development of acne on the face.

Acne

Acne is the localized inflammatory condition of skin characterized by pimples on face, chest and back. It occurs because of over activity of sebaceous glands.

Acne vulgaris is the common type of acne that develops during adolescence. Acne disappears within few years when the sebaceous glands become adapted to the sex hormones.

Sweat Glands

Sweat glands are of two types:

1. Eccrine glands.
2. Apocrine glands.

Eccrine Glands

Eccrine glands are tubular glands distributed throughout the body. These glands open out through sweat pore.

Secretory Activity of Eccrine Glands

Eccrine glands which function throughout life secrete a clear watery sweat. The secretion increases during increase in temperature and emotional conditions.

Eccrine glands play important role in regulating body temperature by secreting sweat.

Sweat contains water, sodium chloride, urea and lactic acid.

Control of Eccrine Glands

Eccrine glands are under nervous control and are supplied by sympathetic postganglionic cholinergic nerve fibers, which secrete acetylcholine. Stimulation of these nerves causes secretion of sweat.

Apocrine Glands

Apocrine glands are situated only in certain areas of the body like axilla, pubis, areola and umbilicus. These glands are also tubular in nature but open into the hair follicles.

Secretory Activity of Apocrine Glands

Apocrine sweat glands are nonfunctional till puberty and start functioning only at the time of puberty. In old age, function of these glands gradually declines.

Secretion of apocrine glands is thick and milky. At the time of secretion, it is odorless. When microorganisms grow in this secretion, a characteristic odor develops in the regions where apocrine glands are present.

Secretion increases in emotional conditions.

Apocrine glands do not play any role in temperature regulation.

Control of Apocrine Glands

Apocrine glands are innervated by sympathetic adrenergic nerve fibers. But the secretory activity is not under nervous control. However, adrenaline from adrenal medulla causes secretion by apocrine glands.

Differences Between Eccrine and Apocrine Sweat Glands

Pheromones

Pheromones or vemerohormones are a group of chemical substances secreted by apocrine glands.

When secreted into the environment by an organism, pheromones produce some behavioral or physiological changes in other members of the same species.

Pheromones are present in urine, vaginal fluid and other secretions of mammals and influence the behavior and reproductive cycle of these animals.

It is found that pheromones excreted in axilla of a woman affects the menstrual cycle of her room-mate or another woman living with her.

These substances stimulate receptors of vomeronasal organ which detect the odor of pheromones. Impulses from these receptors are transmitted to hypothalamus, which influences menstrual cycle via pituitary hormones.

This effect of pheromones on the menstrual cycle of other individuals is called domor effect.

Primary function of skin is the protection of organs. However, it has many other important functions also.

1. Protective Function

Skin forms covering of all organs of body and protects the organs from the following factors:

1. Bacteria and toxic substances.
2. Mechanical blow.
3. Ultraviolet rays.

Protection from Bacteria and Toxic Substances

Skin covers and protects the organs from having direct contact with external environment. Thus, it prevents the bacterial infection.

Lysozyme secreted in skin destroys the bacteria.

Langerhans cells in epidermis are responsible for protective immune reactions following microbial infection of skin.

This layer also offers resistance against toxic chemicals like acids and alkalis.

Protection from Mechanical Blow

Skin is not tightly placed over the underlying organs or tissues. It is somewhat loose and moves over the underlying subcutaneous tissues.

So, the mechanical impact of any blow to skin is not transmitted to underlying tissues.

Protection from Ultraviolet Rays

Skin protects the body from ultraviolet rays of sunlight. Exposure to sunlight or to any other source of ultraviolet rays increases the production of melanin in skin.

Melanin absorbs ultraviolet rays. At the same time, thickness of stratum corneum increases. This layer of epidermis also absorbs the ultraviolet rays.

2. Sensory Function

Skin is considered as largest sense organ in the body. It has many nerve endings which form the specialized cutaneous receptors.

Cutaneous receptors are stimulated by impulses of touch, pain, pressure or temperature sensation and convey the sensations to brain via afferent nerves.

At the brain level, perception of sensations occurs.

3. Storage Function

Skin stores fat, water, chloride and sugar.

It can also store blood by dilatation of the cutaneous blood vessels.

4. Synthesis Function

Vitamin D₃ is synthesized in skin by the action of ultraviolet rays from sunlight on cholesterol.

5. Regulation of Body Temperature

Skin plays an important role in the regulation of body temperature.

Excess heat is lost from the body through skin by radiation, conduction, convection and evaporation.

Sweat glands of skin play active part in heat loss by secreting sweat.

The lipid part of sebum prevents loss of heat from body in cold environment.

6. Regulation of Water and Electrolyte Balance

Skin regulates water balance and electrolyte balance by excreting water and salts through sweat.

7. Excretory Function

Skin excretes small quantities of waste materials like urea, salts and fatty substance.

8. Absorptive Function

Skin absorbs the fat-soluble substances and some ointments.

9. Secretory Function

Skin secretes sweat through sweat glands and sebum through sebaceous glands.

By secreting sweat, skin regulates body temperature and water balance.

Sebum keeps the skin smooth and moist.