Anatomy & Physiology of the Skin and Its Derivatives: Structure, Function, and Clinical Aspects

Introduction to the Integumentary System

The integumentary system, primarily composed of the skin and its derivatives (hair, nails, glands), serves several protective and regulatory functions essential for human health. This system is the largest organ of the body and plays a vital role in protection, sensation, temperature regulation, and more.

Main Functions of the Skin

• Protection: Acts as a physical, chemical, and biological barrier against environmental hazards.

• Body Temperature Regulation: Sweat glands and blood vessels help maintain homeostasis.

• Sensation: Contains receptors for touch, pain, and temperature.

• Excretion: Removes salts and metabolic wastes via sweat.

• Immunity: Secretes substances that inhibit bacterial growth.

• Synthesis of Vitamin D: Initiated by exposure to sunlight.

• Blood Reservoir: Stores blood in dermal vessels.

• Nonverbal Communication: Facial expressions and skin appearance contribute to social interaction.

Structure of the Skin (Cutaneous Membrane)

The skin consists of two main layers: the epidermis and the dermis, with a subcutaneous layer beneath.

Epidermis

• Type: Keratinized stratified squamous epithelium.

• Layers of the Epidermis: (from deep to superficial)

  1. Stratum basale: Deepest layer, site of new cell generation. Contains keratinocytes, melanocytes (produce melanin), and Merkel cells (touch receptors).

  2. Stratum spinosum: Contains dendritic (Langerhans) cells for immune defense.

  3. Stratum granulosum: Cells begin to die and keratinize.

  4. Stratum lucidum: Present only in thick skin (palms, soles).

  5. Stratum corneum: Outermost layer, composed of dead, keratinized cells; provides barrier function.

• Growth: Keratinocytes migrate from the basal layer to the surface, flaking off after 30-40 days. Growth is regulated by epidermal growth factors and hormones.

• Clinical Note: Psoriasis is a chronic autoimmune disorder causing rapid keratinocyte turnover and scaling.

Dermis

• Composition: Connective tissue with fibroblasts, collagen, and elastic fibers.

• Regions:

  • Papillary layer: Contains dermal papillae, pain receptors, and touch receptors.

  • Reticular layer: Contains cleavage lines, sweat glands, sebaceous glands, and pressure receptors (Pacinian corpuscles).

Subcutaneous Layer (Hypodermis)

• Composition: Adipose tissue, not technically part of the skin.

• Functions: Anchors skin to underlying tissues, provides insulation and energy storage.

• Clinical Note: Subcutaneous fat is thicker in women and varies with age.

Structure and Basis of Skin Color

Skin color is determined by three main pigments: melanin, hemoglobin, and carotene.

• Melanin: Synthesized by melanocytes from tyrosine via the enzyme tyrosinase. Protects against UV radiation.

• Hemoglobin: Contributes to pinkish hue in fair skin.

• Carotene: Yellow-orange pigment from diet.

• Clinical Signs:

  • Vitiligo: Loss of melanocytes, causing white patches.

  • Erythema: Redness due to increased blood flow.

  • Pallor: Pale skin from reduced blood flow.

  • Albinism: Genetic lack of melanin production.

  • Jaundice: Yellowing from bilirubin accumulation (liver dysfunction).

Skin Disorders

Skin Cancer

• Basal Cell Carcinoma: Most common, least dangerous.

• Squamous Cell Carcinoma: Arises from keratinocytes.

• Malignant Melanoma: Most dangerous, arises from melanocytes.

Other Disorders

• Pressure/Bed Sores (Decubitus Ulcers): Caused by prolonged pressure.

• Athlete's Foot: Fungal infection.

• Boils: Bacterial infection.

• Contact Dermatitis: Allergic reaction.

• Shingles: Reactivation of herpes zoster virus.

Burns

• First-degree: Only epidermis affected; redness and pain.

• Second-degree: Epidermis and part of dermis; blisters.

• Third-degree (Full-thickness): Destroys entire skin; risk of shock and infection.

• Rule of Nines: Used to estimate burn area. $\text{Percent Burned Area} = \text{Sum of affected body regions as per Rule of Nines}$

Accessory Structures of the Skin

Hair

• Types:

  • Lanugo: Fine, unpigmented hair on fetus.

  • Vellus: Fine, pale hair on children and women.

  • Terminal: Coarse, pigmented hair on scalp, eyebrows, etc.

• Anatomy: Shaft (above skin), root (below skin), follicle, cuticle, and associated structures (sebaceous glands, arrector pili muscles).

• Growth Cycle:

  • Growth stage: 2-6 years.

  • Resting stage: 3 months.

Nails

• Structure: Hard, dead, keratinized cells; consists of body, root, lunule, and cuticle (eponychium).

• Function: Protection, manipulation, and grasping.

• Clinical Note: Swollen or clubbed nails may indicate oxygen deficiency.

Skin Glands

• Sebaceous (Oil) Glands: Secrete sebum for lubrication.

• Eccrine (Merocrine) Sweat Glands: Widely distributed, regulate temperature.

• Apocrine Sweat Glands: Found in axillary/genital areas; active after puberty.

• Ceruminous Glands: Produce earwax.

Age-Related Changes to the Skin

• Fewer melanocytes (less pigment).

• Thinner epidermis and dermis.

• Reduced blood supply and slower healing.

• Diminished immune response.

• Fewer active follicles (less hair).

• Reduced fat distribution.

Comparison Table: Sweat and Oil Glands

Feature	Sweat Glands (Eccrine & Apocrine)	Oil Glands (Sebaceous)
Location	Eccrine: widespread; Apocrine: axilla/genital	Associated with hair follicles
Secretion	Sweat (water, salts, waste)	Sebum (lipids)
Function	Temperature regulation, excretion	Lubrication, waterproofing
Activation	Eccrine: lifelong; Apocrine: puberty	Puberty

Summary of Skin Cancer Types

Type	Origin	Characteristics
Basal Cell Carcinoma	Basal cells of epidermis	Most common, slow-growing, rarely metastasizes
Squamous Cell Carcinoma	Keratinocytes of epidermis	May metastasize, often from sun-exposed areas
Malignant Melanoma	Melanocytes	Highly metastatic, most dangerous

Example: Determining Burn Severity

• First-degree burn: Sunburn affecting only the epidermis.

• Second-degree burn: Blistering from scalding water.

• Third-degree burn: Full-thickness burn from fire exposure.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables have been recreated and summarized for comparison and classification purposes.