Structure and Functions of the Skin

Introduction

The skin is the body’s largest organ and serves as the primary protective barrier against physical, chemical, and microbial injury. It also plays essential roles in vitamin D synthesis, temperature regulation, sensation, and maintenance of internal homeostasis.

Structure of Skin

• The skin is the outermost covering of the body and forms the first line of defense against the external environment.
• It separates the internal environment from physical, chemical, and microbial hazards.
• Together with its appendages, the skin is known as the integument.
• Skin is the largest organ of the body and weighs approximately 3.5 kilograms in an average adult.
• Because it is continuously exposed to the surroundings, the skin rapidly responds to environmental changes.
• The skin contributes to innate immunity by acting as a physical barrier and by producing antimicrobial substances.

Layers of the Skin

Structurally, the skin consists of two principal layers: the epidermis and the dermis.

Epidermis

• The epidermis is composed of keratinized stratified squamous epithelium.
• It is arranged into five layers from superficial to deep: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.
• The stratum lucidum is prominent only in thick skin, such as the palms and soles, and is absent in thin skin.
• The epidermis is avascular and receives nutrients by diffusion from blood vessels in the underlying dermis.
• Cells of the epidermis are continuously renewed through mitotic activity in the stratum basale.

Stratum Corneum

• The stratum corneum is the outermost layer of the epidermis.
• It consists of flattened, dead, keratin-filled cells that form a strong protective barrier.
• Most cells in this layer lack nuclei and intracellular organelles.
• Superficial cells are continuously shed and replaced by newly formed cells from deeper epidermal layers.
• The stratum corneum is especially thick over the palms and soles, providing increased resistance to friction and mechanical stress.

Stratum Lucidum

• The stratum lucidum is a thin, translucent layer present only in thick skin, such as the palms and soles.
• It consists of flattened, non-living cells containing eleidin, an intermediate protein formed during keratinization.
• Eleidin is gradually converted into keratin as the cells migrate toward the surface.
• This layer provides additional protection against friction and mechanical stress.

Stratum Granulosum

• The stratum granulosum contains several layers of flattened cells rich in keratohyalin granules.
• Keratohyalin proteins promote the aggregation of keratin filaments during keratinization.
• Cells in this layer begin to lose their nuclei and organelles before entering the superficial layers.
• Lipid-rich granules released from these cells help establish the epidermal water barrier.

H5: Stratum Spinosum

• The stratum spinosum is composed of several layers of polyhedral keratinocytes connected by numerous desmosomes.
• These intercellular junctions produce a characteristic spiny appearance under the microscope.
• Cells gradually migrate toward the skin surface while undergoing keratinization.
• Langerhans cells, which function as antigen-presenting immune cells, are abundant in this layer and contribute to cutaneous immune defense.

Stratum Germinativum

• The stratum basale is the deepest layer of the epidermis and rests on the basement membrane.
• It consists of a single layer of actively dividing cells responsible for continuous epidermal renewal.
• Newly formed cells migrate upward to replace cells lost from the skin surface.
• Melanocytes located in this layer synthesize melanin from tyrosine through the enzyme tyrosinase.
• Melanin is packaged into melanosomes and transferred to neighboring keratinocytes, where it protects nuclear DNA from ultraviolet radiation.

Dermis

• The dermis is the connective tissue layer located beneath the epidermis.
• It contains hair follicles, glands, blood vessels, lymphatic vessels, nerves, fibroblasts, immune cells, and small bundles of smooth muscle.
• The dermis is divided into a papillary layer and a reticular layer.

Papillary Layer

• The papillary layer forms dermal papillae that project into the epidermis.
• These papillae contain capillaries, lymphatic vessels, and sensory nerve endings, providing nourishment and sensory support to the avascular epidermis.

Reticular Layer

• The reticular layer contains dense irregular connective tissue with collagen and elastic fibers.
• It blends with the subcutaneous tissue, providing strength, elasticity, and firm attachment of the skin to underlying structures.

Glands in the Skin

• The principal skin glands are the sweat glands and sebaceous glands.
• Sweat glands are distributed throughout most of the body but are absent from the tympanic membrane, lips, and glans penis.
• Sweat glands are classified into merocrine (eccrine) and apocrine types.

Merocrine Glands

• Merocrine glands are simple coiled tubular glands widely distributed over the body surface.
• They are especially numerous on the palms, soles, and forehead.
• These glands produce the majority of sweat, which contributes to thermoregulation through evaporative cooling.
• Their secretions also aid in maintaining skin hydration and surface acidity.
• Following skin injury, cells from merocrine glands contribute to epidermal regeneration and wound healing.

Apocrine Glands

• Apocrine glands are located in the axilla, areola, mons pubis, and perianal region.
• Ceruminous and mammary glands are modified apocrine glands.
• Their secretion begins after puberty and contributes to characteristic body odor following bacterial decomposition.

Sebaceous Glands

• Sebaceous glands are branched alveolar glands that usually open into hair follicles.
• They secrete sebum, which lubricates the skin and hair.
• Blockage of sebaceous ducts may lead to acne vulgaris, particularly during puberty due to increased androgen activity.

Skin Blood Supply

• The cutaneous circulation supplies oxygen and nutrients to the skin and helps remove metabolic waste products.
• Skin blood flow is highly variable and changes according to physiological needs.
• Cutaneous blood flow ranges from approximately 1 to 150 milliliters per 100 grams of tissue per minute.
• The average skin blood flow is about 13 milliliters per 100 grams of tissue per minute, corresponding to nearly 460 milliliters per minute for the entire skin.
• Blood flow increases during heat exposure to facilitate heat loss and decreases in cold conditions to conserve body heat.
• Cutaneous vessels also contribute to wound healing, immune responses, and maintenance of normal skin function.

Skin Appendages

• The principal skin appendages are hair and nails.
• Both structures are derived from the epidermis and are composed primarily of keratin.

Hairs

Types of Hair

• Lanugo hair is the fine, soft hair covering the fetus and is usually shed before or shortly after birth.
• Vellus hair is thin, lightly pigmented hair that covers most of the body during childhood and adulthood.
• Terminal hair is coarse, thick, and pigmented.
• Terminal hair develops in specific regions during puberty under the influence of androgens.
• In adult females, vellus hair remains predominant over most facial areas.

Parts of the Hair

• Each hair consists of a shaft, root, and hair bulb.
• The shaft projects above the skin surface.
• The root lies within the skin and is enclosed by the hair follicle.
• The hair bulb surrounds the dermal papilla, which contains blood vessels supplying nutrients required for hair growth.

Structure of the Hair

• Hair is composed of three layers: medulla, cortex, and cuticle.
• The medulla forms the central core and may be absent in fine hairs.
• The cortex contains densely packed keratin fibers and pigment granules that determine hair strength and color.
• The cuticle is the thin outer protective layer composed of overlapping keratinized cells.
• The hair follicle is continuous with the epidermis and is surrounded by connective tissue.
• Sebaceous glands usually open into the hair follicle and lubricate the hair with sebum.
• The arrector pili muscle attaches to the follicle and contracts during sympathetic stimulation, producing hair erection or goosebumps.

Nails

• Nails are modified keratinized structures located at the tips of the fingers and toes.
• Each nail consists of a root, body, and free edge.
• The nail root is continuous with the stratum basale of the epidermis.
• Continuous cell division in the nail matrix produces new keratinized cells, resulting in lifelong nail growth.
• Healthy nails protect the distal digits and improve precision during grasping and fine motor activities.

Functions of the Skin

• The skin is a multifunctional organ that protects the body and helps maintain homeostasis.
• It performs protective, sensory, metabolic, immune, and thermoregulatory functions essential for normal health.

Protection

• The skin forms the body’s primary physical barrier against mechanical injury, microorganisms, and harmful environmental agents.
• The keratinized epidermis reduces the entry of pathogens and toxic substances.
• Intact skin prevents excessive loss of body fluids and electrolytes.
• Extensive skin loss, such as in severe burns, markedly increases the risk of dehydration, infection, and sepsis.

Temperature Regulation

• The skin plays a major role in maintaining normal body temperature.
• Sweat secretion promotes evaporative cooling during heat exposure.
• Continuous insensible water loss also contributes to heat dissipation.
• Cutaneous blood vessels dilate to increase heat loss and constrict to conserve body heat.
• Sebum helps maintain skin hydration by reducing excessive evaporation from the skin surface.

Sensory Function

• The skin contains numerous sensory receptors that detect external stimuli.
• Specialized receptors respond to touch, pressure, vibration, pain, temperature, and itch.
• Hair follicles contain receptors that detect even slight hair movement.
• Sensory information from the skin helps protect the body from injury and assists interaction with the environment.

Vitamin D Synthesis

• The skin synthesizes vitamin D3 when exposed to ultraviolet B radiation from sunlight.
• Ultraviolet B converts 7-dehydrocholesterol in the epidermis into cholecalciferol.
• Vitamin D is essential for normal calcium absorption and bone mineralization.

Blood Reservoir

• The cutaneous circulation serves as a temporary blood reservoir.
• During sympathetic activation, cutaneous vasoconstriction redistributes blood to vital organs such as the heart and brain.
• This response is important during hemorrhage, exercise, and circulatory shock.

Water and Electrolyte Balance

• Sweat contains water, sodium, chloride, potassium, and small quantities of other electrolytes.
• Excessive sweating may cause dehydration and electrolyte imbalance if losses are not replaced.
• Aldosterone reduces sodium loss by increasing sodium reabsorption in sweat glands.
• Sweat also eliminates small amounts of metabolic waste products such as urea and lactate.

Protection Against Ultraviolet Radiation

• Melanin absorbs and scatters ultraviolet radiation, reducing cellular damage.
• Increased melanin production after sun exposure provides additional protection against ultraviolet-induced DNA injury.

Immune Defense

• The skin provides an important component of innate immunity.
• The acidic surface of the skin inhibits the growth of many microorganisms.
• Sebum and sweat contain antimicrobial substances that limit bacterial colonization.
• Langerhans cells capture and present foreign antigens, initiating immune responses.
• Resident immune cells and normal skin microbiota further strengthen cutaneous defense mechanisms.

Route for Drug Administration

• The skin permits administration of selected drugs through the transdermal route.
• Lipid-soluble medications can be delivered continuously using transdermal patches.
• Examples include nicotine, nitroglycerin, estrogen, and certain analgesic preparations.
• Topical medications primarily produce local therapeutic effects with minimal systemic absorption.

Clinical Diagnosis

• Examination of the skin provides valuable clues to systemic disease.
• Pallor may indicate anemia, cyanosis suggests reduced oxygenation, and jaundice reflects elevated bilirubin levels.
• Rashes, pigmentation changes, edema, and nail abnormalities may indicate endocrine, autoimmune, hepatic, renal, or nutritional disorders.
• Skin examination therefore forms an important part of routine clinical assessment.

Appearance

• Healthy skin contributes to normal physical appearance and psychological well-being.
• Adequate hydration, nutrition, circulation, and skin integrity maintain normal texture, elasticity, and appearance.
• Healthy skin reflects overall physiological health rather than skin color alone.