The outermost covering over the whole body is called skin. It is the largest organ of our body specialized to act as a physical barrier to protect us against harmful organisms and other hazards.

Structure of skin: Skin Anatomy

It consists of three layers:

• Epidermis
• Dermis
• Hypodermis

Epidermis:

The outermost layer of skin is called the epidermis. It consists of the following different types of cells:

• Keratinocytes (keratin protein-producing cells)
• Melanocytes (melanin pigment-producing cells, which are responsible for our skin tone)
• Langerhans cells (they enable our immune system to develop immunity against any foreign substance or microorganism)
• Merkel cells (produce light touch sensations)

The epidermis has the following five structural layers from deep to superficial:

• Stratum basale
• Stratum spinosum
• Stratum granulosum
• Stratum lucidum
• Stratum corneum

1. Stratum basale:

This is the deepest epidermal layer, lying over the dermis. It is also called Stratum germinativum, as new keratinocytes are produced in this layer. This layer consists of keratinocytes, melanocytes, and Merkel cells.

• Stratum spinosum:

Above the stratum basale, there lies the stratum spinosum. It consists of several layers of keratinocytes. In deeper layers, keratinocytes are polyhedral, while in upper layers the keratinocytes are flattened.

The cells in the stratum spinosum are strongly bound together, thus enabling the skin to resist abrasion. In addition to keratinocytes, this layer also contains Langerhans cells and occasional lymphocytes.

This layer is responsible for skin strength and flexibility.

• Stratum granulosum:

This layer gets its name by the presence of lamellar granules.

Lamellar granules:

The keratinocytes in this layer contain small, oval membrane-bound which exhibit lamellar (multilayered) structure, therefore called lamellar granules

These granules release their contents which include special lipids that diffuse into intercellular spaces. The presence of these lipids makes our skin impermeable to water.

• Stratum lucidum:

It is composed of three to five layers of extremely flattened keratinocytes. It appears as a thin, translucent, and refractile layer, therefore, named stratum lucidum.

This epidermal layer is only present in thick skin. It is not observed in thin skin.

• Stratum corneum:

The outermost epidermal layer which consists of many layers of highly keratinized flattened keratinocytes is called Stratum corneum.

• Corneocytes:

These highly keratinized, flattened keratinocytes are now devoid of nuclei and organelles and are called corneocytes.

• Desquamation:

The outermost layer of corneocytes eventually becomes dead, breaking down its connection with neighbouring cells and constantly exfoliating from the skin surface, this process is called desquamation.

Dermis:

Beneath the epidermis,  is a second layer which is called the dermis. It consists of connective tissue, blood vessels,  follicles, sweat glands, sebaceous glands, and nerves.

It is composed of two layers:

• The thinner upper layer is called the papillary layer. It is rich in blood vessels and contains Meissner's corpuscles(receptors that respond to fine touch and pressure).

• The thicker lower layer is called the reticular layer. It consists of many thick elastic fibres, Pacinian corpuscles( receptors that detect coarse touch and deep pressure changes on the skin), and Ruffini corpuscles ( receptors that detect stretch, high-frequency vibrations).

Glands of Skin: Skin Anatomy

What are sebaceous glands?

These are spherical or ovoid structures present in the dermis while their ducts open near the hairs follicles.

• Location of sebaceous glands:

These glands are present all over the skin except in the thick skin of palms and soles, where they are completely absent. They are abundantly present in the skin of the face and scalp.

• The function of sebaceous glands:

They secrete a fatty material called sebum, which passes through the ducts of the gland into the hair follicle and then onto the skin surface. It performs two main functions.

1. It maintains the proper skin texture and hair flexibility.

• It contributes to the antibacterial and antifungal functions of the skin.

What are sweat glands? Skin Anatomy

These are coiled tubular structures present in the dermis. They are exocrine glands (which are the glands that secrete and release their substances onto the epithelial surface through a duct).

Classification of sweat glands:

Generally,  these glands are divided into two main types:

• Eccrine sweat glands
• Apocrine sweat glands

Eccrine sweat glands:

These are the sweat glands with the ducts opening directly on the skin surface with the help of a sweat pore. They are distributed throughout the skin but are present abundantly on the skin of palms and soles.

Apocrine sweat glands:

These are the sweat glands having ducts opening in the canal of the hair follicle rather than opening directly on the skin surface. They are located on specific areas of the body which include the scalp, axilla, groin, areola of the breast, and labia majora.

The function of sweat glands:

They produce a clear and odorless fluid having acidic pH called sweat. It consists of 99%  water, sodium, chloride, potassium, ammonia, urea, and uric acid. It performs the following functions:

• It helps in thermoregulation by evaporative cooling.
• It helps in excreting excess water and salts.
• It has antimicrobial properties, thus protecting the skin.

Hypodermis:

The innermost, thickest, and third layer of skin is called the hypodermis. It is also known as the subcutaneous layer of subcutaneous tissue.

It has the following structures:

• Collagen and elastic fibres (that attach dermis to underlying muscles and bones).
• Fat cells
• Nerve endings
• Hair follicle roots
• Blood vessels

Classification of skin: Skin Anatomy

Skin is classified into two following types based on the thickness of its epidermis:

• Thick skin
• Thin skin

Thick skin:

The skin covering the palms and soles of our body is called thick skin due to its relative thickness. It has the following characteristics:

• Thick epidermis (All five layers are present)
• Thin dermis
• Lacks hair follicles
• Lacks arrector pili muscles(these muscles upon contraction cause the hairs to straighten up).
• Lacks sebaceous glands
• Numerous sweat glands
• Dense sensory receptors
• Has ridges and furrows on the surface
• More resistant to mechanical abrasion

Thin skin:

The skin covering the whole body except palms and soles is called thin skin. It has s following characteristics:

• Thin epidermis (Only four layers are present – stratum lucidum is absent).
• Thick dermis
• Contains arrector pili muscles
• Contains sebaceous glands
• Fewer sweat glands
• Sparse sensory receptors
• Lacks ridges and furrows on the surface
• Perform all functions of the skin

Functions of skin:

Skin primarily has the following main functions:

1. Protection
2. Thermoregulation
3. Act as a Sensory organ
4. Excretory organ
5. Vitamin D synthesis
6. Energy reservoir

Protection of body:

• It protects the body from any physical, thermal, and mechanical injury.

• It protects our body from harmful UV rays mainly coming from the sun with the help of melanin pigment. This is the reason when our body is exposed to more UV rays, it produces more melanin, thus becoming tanned.

• It also protects our body from bacteria and other harmful microorganisms, this is usually done with the help of sebum which has antifungal and antibacterial properties.

• Due to its acidic nature and antimicrobial contents, sweat also protects our bodies against the colonization of microorganisms.

• It also has antioxidative properties and protects itself during inflammation.

Thermoregulation:

Thermoregulation is mainly done by the dermis by sweating, vasodilation, and vasoconstriction.

Loss of excess heat occurs through the following mechanisms:

• In hot surroundings, sweat glands secrete sweat which produces a film of moisture, thus helping in lowering the temperature by evaporative cooling.

• Blood vessels dilate ( vasodilation), there is more blood supply to the skin, more heat loss occurs through the surface of the skin.

Conservation of heat occurs through the following mechanisms:

• In a cold environment, sweat production is minimized.

• Blood vessels constrict (vasoconstriction), the blood supply to the skin is reduced, less heat loss occurs through the skin surface.

Sensory Functions:

The skin has many sensory receptors which produce touch, pressure, heat, cold, pain, and itching sensations. Sensory parts of the skincare are:

• Meissner’s corpuscles
• Pacinian corpuscles
• Merkel's discs ( receptors made up of Merkel cells)
• Ruffini corpuscles

Function as an excretory organ:

Skin plays its role in excretion through the production of sweat by sweat glands. Sweating eliminates excess water and salts as well as a small amount of urea, a byproduct of protein catabolism.

Vitamin D synthesis:

The keratinocytes in the epidermis are responsible for producing Vitamin D. When skin is exposed to sunlight, UV rays penetrate the epidermis and photolyzes ( breakdown with the help of light) provitamin D3 to vitamin D3, which can isomerize to vitamin D3.

Energy reservoir:

The fat cells present in the hypodermis serve as an energy reservoir.

Diseases related to skin:

• Acne vulgaris:

It is commonly known as acne. It is the most common disease of the skin. As an individual reaches puberty, sebaceous glands become more active, there is excessive production of sebum, as a result, pores of the skin are blocked by sebum, this sebum is usually invaded by bacterial cells. Treatment may involve antibiotics.

• Atopic dermatitis:

It is the most common inflammatory disease worldwide. Symptoms usually include dry, red, itchy, and bumpy skin. It is common in children but can occur at any age. It occurs in genetically predisposed persons. It spreads from its place of contact.

• Contact dermatitis:

It is the delayed allergic reaction that develops hours to days after contact with the allergen. It occurs in all individuals contacting that specific allergen. Symptoms are usually restricted to the area of contact.

• Sunburn:

When the skin has too much exposure to UV rays, it turns red and becomes painful. If It becomes severe, it can develop swelling and skin blisters. Home remedies are usually applied.

• Urticaria:

It is an immediate allergic reaction on the skin which results in redness or pinkness of the skin. It develops red, itchy raised areas of skin usually accompanied by angioedema  (painful swelling of deeper layers of skin). Treatment usually involves antihistamines.

• cancer:

It is characterized by the abnormal growth of skin cells. Symptoms include an open sore that may bleed, a reddish irritated area, or a pinkish, red, pearly white translucent bump. It is mostly caused by exposure to UV rays.

It can be of the following types:

• Basal cell carcinoma(a most common type of skin cancer)
• Merkel cells cancer
• Squamous cell  carcinoma
• Melanoma (cancer of melanocytes)