Skin: Structure and Function

Introduction

• The human skin is the outer covering of the body.
• In humans, it is the largest organ of the human body.
• The integumentary system helps to maintain a constant body temperature, protects the body, and provides sensory information about the surrounding environment.

Dermatology:

• It is the branch of medical science that deals with the diagnosis and treatment of skin disorders.

Structure of Skin

• The skin or cutaneous membrane covers the external surface of the body and is the largest organ of the body in both surface area and weight.
• In adults, it covers an area of about 2 square meters and weighs about 4.5-5 kg, about 16% of total body weight.
• It ranges in thickness from 0.5 mm on the eyelids to 4.0 mm on the heels.
• It consists of two main parts.
• Epidermis: The superficial, thinner portion, composed of epithelial tissue, is the epidermis.
• Dermis: The deeper, thicker connective tissue portion is the dermis.
• Under the dermis, but not part of the skin is the subcutaneous layer called as hypodermis.
• This layer consists of areolar and adipose tissues.

Epidermis:

• The epidermis is the outermost layer of skin.
• It is composed of keratinized stratified squamous epithelium.
• It contains four principal types of cells:
  • Keratinocytes
  • Melanocytes
  • Langerhans cells
  • Merkel cells

Keratinocytes:

• About 90% of epidermal cells are keratinocytes arranged in four or five layers and produce the protein keratin.
• Keratin is a tough, fibrous protein that helps to protect the skin and underlying tissues from heat, microbes and chemicals.

Melanocytes:

• About 8% of the epidermal cells are melanocytes and produce the pigment melanin.
• Melanin is a yellow-red or brown-black pigment that contributes to skin colour and absorbs damaging ultraviolet (UV) light.

Langerhan’s cells:

• They arise from red bone marrow and migrate to the epidermis and constitute a small fraction of the epidermal cells.
• They participate in immune responses mounted against microbes that invade the skin and are easily damaged by UV light.

Merkel Cells:

• These are the least numerous epidermal cells.
• They are located in the deepest layer of the epidermis.
• The epidermis is composed of five layers:
  • Stratum corneum (Outermost layer)
  • Stratum lucidum
  • Stratum granulosum
  • Stratum spinosum
  • Stratum basale (innermost layer)

Stratum Basale:

• Deepest layer of the epidermis is the stratum basale.
• It is composed of single layer of cuboidal or columnar keratinocytes.

Stratum Spinosum:

• Superficial to stratum basale is the stratum spinosum.
• It consists of 8-10 layers of keratinocytes fits closely together.

Stratum Granulosum:

• In the middle of the epidermis is the stratum granulosum.
• It consists of 3-5 layers of flattened keratinocytes.

Stratum Lucidum:

• It lies below the stratum corneum and consists of 3-5 layers of clear, flat, dead cells, lacking granules and nuclei.
• The cells of stratum lucidum are also filled with keratin.

Stratum Corneum:

• It is the outermost layer of epidermis.
• It is made up of 25-30 layers of flat, dead cells, completely filled with keratin.
• It is effective barrier against light, heat, bacteria, water and many chemicals.

Dermis:

• The second, deeper layer of the skin is dermis.
• It is composed of mainly connective tissue.
• Blood vessels, nerves, glands and hair follicles are embedded in the dermal tissue.
• The dermis can be divided into;

1. Papillary region
2. Reticular region

• The papillary region makes up to the 1/5^th thickness of total layer.
• It consists of areolar connective tissue containing fine elastic fibres.
• Its surface area is greatly increased by small, finger-like structure called as dermal papillae.
• These nipple shaped structure project into the epidermis and contains capillary loops (blood capillaries).
• The reticular region is attached to the subcutaneous layer, consists of dense irregular tissue containing fibroblast, bundles of collagen and some elastic fibres.
• A few adipose cells, hair follicles, nerves, sebaceous (oil) glands and sudoriferous (sweat) glands are present in the reticular region.
• The combination of collagen and elastic fibres in the reticular region provides the skin with strength, extensibility and elasticity.

Accessory Structure of the Skin

It includes;

• Skin glands: It regulates body temperature.
• Hairs: It protects the body.
• Nails: It protects the body.

Hairs

• Hairs or pili are present on most skin surfaces except palms, fingers and feet surfaces.
• Each hair is composed of columns of dead and keratinized cells bonded together by extracellular proteins.
• The shaft is the superficial portion of the hair which projects above the surface of the skin.
• The root is the deeper portion of the hair that penetrates into the dermis and sometimes into the subcutaneous layer into the dermis.
• The shaft and root of the hair both consists of three concentric layers of cells.
  • Medulla: Inner/made-up of soft keratin
  • Cortex: Middle/made-up of hard keratin

Cuticle of the Hair

• Surrounding the root hair is the hair follicle which is made up of external root sheath and internal root sheath.
• Together these are referred as epithelial root sheath.
• The dense dermis surrounding the hair follicle is called as dermal root sheath.
• The base of each hairs follicle is an anion shaped structure called as bulb.
• The bulb contains nipple shaped structure called as papilla of the hair contains many blood vessels that nourishes the growing hair follicle.
• The function of hair is to keep out dust particles from entering the eyes, ear canals and nasal chambers.
• Hair also acts as insulating coat of the human body as it keeps the body warm.

Growth Cycle:

• It consists of two stages;
• Growth stage: Matrix of cells differentiates, keratinize and die. New cells are added at the base of hair root and hair grows longer.
  • Resting stage: In time growth of hair stops and the resting stage begins.
  • After the resting stage, a new growth cycle begins.
• Normal hair loss in adult scalp is about 70-100 hairs/day.
• Both the rate of growth and replacement cycle may be altered by illness, radiation therapy, chemotherapy, age, genetics, gender and emotional stress.

Nails

• These are plates of tightly packed, hard, dead, keratinized epidermal cells over the dorsal surface of digits.
• Each nail consists of;
  • Nail body: It is the visible portion of nail.
  • Free edge: It is the free part of nail.

Nail root:

• It is the portion that is burried in the fold of skin.
• Below the nail body is the region of epithelium and deeper layer of dermis.
• Nail body appears pink in colour because of blood flowing through the capillaries in the dermis.
• The free edge is white in colour because of no blood capillaries.
• The whitish, crescent shaped area of the proximal end of nail body is called as lunula.
• Below, the free edge a thickened region of stratum corneum called as hyponychium, which secures the nail to the finger tip.
• The eponychium or cuticle is a narrow band of epidermis that covers the nail body.
• The portion of epithelium present below the nail root called as nail matrix.
• Nails help us to grasp and manipulate small objects in various ways, provide protection against trauma to the ends of the digits and allow us to scratch various parts of the body.

Skin Glands

Two types of skin glands are present.

• Sebaceous (oil) glands
• Sudoriferous (sweat) glands

Sebaceous Glands:

• The sebaceous glands or oil glands are simple, branched acinar glands.
• These are associated with hair follicles.
• The secreting portion of sebaceous glands lies in the dermis and usually opens into the neck of hair follicle.
• These glands are absent in palms and soles, sebaceous glands are small in most area of trunk and limbs, but large in the skin of the breast, face and neck.
• Asebaceous gland secretes an oily substance called as sebum.
• Sebum is a complex mixture of naturally produced fats, oils, waxes, cholesterol and other molecules.

Functions of Sebum:

• It prevents excessive evaporation of water from the skin.
• It keeps the skin soft and pliable.
• It inhibits the growth of certain bacteria.
• It coats the surface of hairs and keeps them from drying and becoming brittle.

Sweat Glands:

• These are also called as sudoriferous glands.
• These glands release sweat or perspiration into hair follicles or into the skin surface through pores.
• A sweat gland helps in regulating temperature of the body and also helps in removing the wastes outside the body.
• Sweat glands are of two types.
• Eccrine sweat glands
• Apocrine sweat glands

Eccrine Sweat Glands:

• These are also called merocrine sweat glands.
• These are simple, coiled, tubular glands.
• The sweat produced by eccrine sweat glands (about 600 ml per day) consists of water, ions (sodium and chloride), urea, uric acid, ammonia, amino acid, glucose and lactic acid.
• The main function is the regulation of body temperature through evaporation.
• It also removes wastes such as urea, uric acid and ammonia from the body.

Apocrine Sweat Body:

• These are simple, coiled and tubular glands.
• The secretory portion of these sweat glands is mostly located in the subcutaneous layer and excretory duct opens into hair follicles.
• The secretion of apocrine sweat gland is more viscous than the eccrine sweat gland.
• These glands are inactive during childhood.
• They develop and begin function during puberty as they get stimulated by sex hormones.
• The components of sweat glands are same as present in eccrine sweat glands plus lipids
  and proteins.

Ceruminous Glands:

• The modified sweat glands in the external ear called as ceruminous glands, produces a waxy secretions.
• The combined secretion of ceruminous and sebaceous glands called as ceruminous earwax.
• Cerumen together with hairs in the external auditory canal, provides a sticky barrier that prevents the entry of foreign bodies.

Functions of Skin

Thermoregulation:

• The skin plays an important role in thermoregulation by two ways:
  • By liberating sweat at its surface.
  • By adjusting the blood flow in the dermis.
• In response to high environment temperature or heat produced during exercise, heat production increases producing evaporation of sweat from the skin helps to lowers the body temperature.
• Blood vessels in the dermis of skin shows dilation producing more blood flows through the dermis producing increased amount of heat loss from the body.
• In response to low environment temperature sweat production decreases that helps in conserving heat.

Blood Reservoir:

• Dermis houses as extensive network of blood vessels that carry 8-10% of total blood flow in a resting adult.

Protection:

• The skin provides protection to the body in various ways.
• Keratin protects underlying tissue from microbes, abrasion, heat and chemicals.
• The oily sebum from sebaceous glands keeps skin and hairs from drying out and contains bactericidal chemicals that kill surface bacteria.
• The acidic pH of sweat retards the growth of microbes.
• The pigments melanin helps against the damaging effect of UV light.

Cutaneous Sensations:

• These are the sensations that arises in the skin including touch, pressure, vibrations and tickling.
• It also includes thermal sensations such as warmth and coolness.
• Another cutaneous sensation is pain usually indicates tissue damage.

Excretion and Absorption:

• The skin normally has a small role in excretion (the elimination of substances from the body) and absorption the passage of materials from the external environment into the body.
• A person produces 400 ml of sweat daily.
• Sweat act as a vehicle for excretion of small amount of salts, carbon dioxide and two organic molecules that results from breakdown of proteins i.e. ammonia and urea.
• The absorption of water-soluble substances is possible, but certain lipid-soluble materials do not penetrates the skin.
• These include fat-soluble vitamins (A, D, E and K) certain drugs, gases oxygen and carbon dioxide.

Synthesis of Skin:

• Synthesis of vitamin-D requires activation of a precursor molecule in the skin by UV rays of sunlight.
• Enzymes in the liver and kidneys then modify the activated molecule finally producing calcitriol, the most active form of vitamin-D.
• Calcitriol is a hormone that helps in absorption of calcium in foods from GIT into the blood.