The Integumentary System

Overview of the Integument

The integumentary system, commonly known as the skin, is the body's largest organ system and serves as a protective barrier. It consists of three major regions, each with distinct structures and functions.

• Epidermis: The outermost, superficial region.

• Dermis: The middle region, providing strength and flexibility.

• Hypodermis (Superficial fascia): The deepest region, primarily composed of adipose tissue.

Example: The hypodermis anchors the skin to underlying tissues and acts as an insulator and shock absorber.

Structure of the Skin

Cross-Sectional Anatomy

The skin is organized into layers, each containing specialized cells and structures. The following diagram (not shown here) typically illustrates the arrangement of hair follicles, sweat glands, blood vessels, and nerve endings within these layers.

• Hair shaft and follicle: Extend from the epidermis into the dermis.

• Sweat glands (eccrine and apocrine): Involved in thermoregulation and excretion.

• Blood vessels: Supply nutrients and aid in temperature regulation.

• Nerve endings: Detect touch, pressure, pain, and temperature.

Epidermis

Composition and Cell Types

The epidermis is composed of keratinized stratified squamous epithelium. It consists of four or five distinct layers, depending on the region of the body, and contains four main cell types:

• Keratinocytes: Produce the fibrous protein keratin, providing strength and waterproofing.

• Melanocytes: Synthesize the pigment melanin, which protects against ultraviolet (UV) radiation.

• Langerhans' cells: Specialized macrophages involved in immune defense.

• Merkel cells: Function as touch receptors in association with sensory nerve endings.

The outer portion of the skin is exposed to the external environment and functions primarily in protection.

Layers of the Epidermis

The epidermis is organized into several layers, each with unique characteristics and functions:

• Stratum Basale (Basal Layer): The deepest layer, firmly attached to the dermis. It consists of a single row of the youngest keratinocytes, which undergo rapid division (also called the stratum germinativum).

• Stratum Spinosum (Prickly Layer): Contains several layers of keratinocytes connected by desmosomes. Melanin granules and Langerhans' cells are abundant here.

• Stratum Granulosum (Granular Layer): Consists of three to five cell layers where keratinocytes undergo significant changes. Keratohyaline and lamellated granules accumulate in the cells.

• Stratum Lucidum (Clear Layer): A thin, transparent band of flat, dead keratinocytes, present only in thick skin (e.g., palms, soles).

• Stratum Corneum (Horny Layer): The outermost layer, composed of dead, keratinized cells. It accounts for most of the epidermal thickness and provides waterproofing and protection from abrasion and penetration.

Additional info: The process of keratinocyte maturation and migration from the stratum basale to the stratum corneum is called keratinization.

Dermis

Structure and Cell Types

The dermis is the second major skin region, composed of strong, flexible connective tissue. It contains fibroblasts, macrophages, and occasionally mast cells and white blood cells. The dermis is divided into two layers:

• Papillary Layer: Made of areolar connective tissue with collagen and elastic fibers. Its surface forms dermal papillae, which contain capillary loops, Meissner's corpuscles (touch receptors), and free nerve endings.

• Reticular Layer: Accounts for about 80% of the dermal thickness. Dense irregular connective tissue provides strength and resiliency, while elastin fibers allow stretch and recoil.

Hypodermis

Composition and Function

The hypodermis (superficial fascia) lies deep to the skin and is composed mainly of adipose and areolar connective tissue. It anchors the skin to underlying structures, insulates the body, and serves as an energy reserve.

Skin Color

Pigments and Determinants

Three main pigments contribute to skin color:

• Melanin: Ranges from yellow to reddish-brown to black. Responsible for dark skin tones. Local accumulations cause freckles and moles.

• Carotene: Yellow to orange pigment, most visible in the palms and soles.

• Hemoglobin: The red pigment in blood, giving skin a pinkish hue, especially in fair-skinned individuals.

Glands of the Skin

Types and Functions

• Eccrine (Merocrine) Sweat Glands: Widely distributed, especially on palms, soles, and forehead. Secrete watery sweat for thermoregulation.

• Apocrine Sweat Glands: Found in axillary and anogenital areas. Secrete a thicker, milky sweat that may produce body odor.

• Ceruminous Glands: Modified apocrine glands in the external ear canal that secrete cerumen (earwax).

• Mammary Glands: Specialized sweat glands that secrete milk.

• Sebaceous (Oil) Glands: Simple alveolar glands found throughout the body (except palms and soles). Secrete sebum, an oily substance that lubricates skin and hair.

Hair

Structure and Function

Hair consists of dead, keratinized cells produced by hair follicles. It contains hard keratin, which is more durable than the soft keratin of the skin. The hair shaft projects from the skin, while the root is embedded within the skin. The hair structure includes:

• Medulla: Central core.

• Cortex: Surrounds the medulla.

• Cuticle: Outermost layer.

Hair color is determined by melanocytes at the base of the hair follicle.

Functions of Hair:

• Helps maintain warmth.

• Alerts the body to the presence of insects on the skin.

• Guards the scalp against physical trauma, heat loss, and sunlight.

Distribution: Hair is found on most skin surfaces except the palms, soles, lips, nipples, and parts of the external genitalia.

Hair Follicle

The hair follicle is a sheath extending from the epidermal surface into the dermis. The expanded deep end forms a hair bulb, which is surrounded by a knot of sensory nerve endings (root hair plexus) that function as touch receptors.

Types of Hair

• Vellus Hair: Pale, fine body hair found in children and adult females.

• Terminal Hair: Coarse, long hair of the scalp, eyebrows, axillary, and pubic regions.

Hair Thinning and Baldness

• Alopecia: Hair thinning in both sexes.

• True (Frank) Baldness: Genetically determined and sex-influenced. Male pattern baldness is caused by a follicular response to dihydrotestosterone (DHT).

Nails

Structure

Nails are scale-like modifications of the epidermis found on the distal, dorsal surface of fingers and toes. They protect the tips of the digits and assist in picking up small objects.

Functions of the Integumentary System

Protection

• Chemical, physical, and mechanical barriers protect against pathogens, chemicals, and physical trauma.

Body Temperature Regulation

• Accomplished by dilation (cooling) and constriction (warming) of dermal blood vessels and by increasing sweat gland secretions.

Cutaneous Sensation

• Exteroceptors sense touch, pressure, pain, and temperature.

Metabolic Functions

• Synthesis of vitamin D in dermal blood vessels.

Blood Reservoir

• Skin blood vessels store up to 5% of the body's blood volume.

Excretion

• Limited amounts of nitrogenous wastes are eliminated in sweat.

Skin Cancer

Types and Characteristics

• Basal Cell Carcinoma: Most common and least malignant. Arises from stratum basale cells. Slow-growing and rarely metastasizes. High cure rate with surgical excision.

• Squamous Cell Carcinoma: Arises from keratinocytes of the stratum spinosum. Common on scalp, ears, and lower lip. Grows rapidly and may metastasize if not removed. Good prognosis if treated early.

• Melanoma: Cancer of melanocytes. Most dangerous due to high metastatic potential and resistance to chemotherapy.

ABCDE Rule for Melanoma

• A: Asymmetry – two sides of the pigmented area do not match.

• B: Border irregularity – edges are notched or indented.

• C: Color – contains multiple colors (black, brown, tan, red, or blue).

• D: Diameter – larger than 6 mm (about the size of a pencil eraser).

Melanoma is treated by wide surgical excision and immunotherapy. Prognosis is poor if the lesion is over 4 mm thick.

Burns

Classification

• First-degree burns: Only the epidermis is damaged. Symptoms include redness, swelling, and pain.

• Second-degree burns: Damage to the epidermis and upper dermis. Symptoms include blisters in addition to first-degree symptoms.

• Third-degree burns: Entire thickness of the skin is damaged. Burned area appears gray-white, cherry red, or black. No initial pain due to destruction of nerve endings.

Rule of Nines

The rule of nines is used to estimate the severity of burns. Burns are considered critical if:

• Over 25% of the body has second-degree burns.

• Over 10% of the body has third-degree burns.

• There are third-degree burns on the face, hands, or feet.

Developmental Aspects of the Integument

Fetal Development

• The epidermis develops from ectoderm.

• The dermis and hypodermis develop from mesoderm.

• Lanugo: Downy coat of delicate hairs covering the fetus.

• Vernix caseosa: Oily substance produced by sebaceous glands that protects fetal skin.

Adolescence to Adulthood

• Skin and hair become oilier; acne may appear.

• Skin shows effects of cumulative environmental exposure by age 30.

• Scaling and dermatitis become more common with age.

Aging

• Replacement of skin cells slows; skin becomes thinner and drier.

• Subcutaneous fat layer diminishes, increasing cold intolerance.

• Decreased elasticity and loss of subcutaneous tissue lead to wrinkles.

• Decreased numbers of melanocytes and Langerhans' cells increase the risk of skin cancer.