Skin and Membranes: Structure, Function, and Clinical Conditions

Body Membranes

Classification of Body Membranes

Body membranes are essential structures that line or cover body surfaces, cavities, and organs. They are classified based on their tissue composition and function.

• Epithelial membranes: Composed of epithelial tissue and an underlying layer of connective tissue. Types include cutaneous, serous, and mucous membranes.

• Connective tissue membranes: Composed exclusively of various types of connective tissue, such as synovial membranes.

Epithelial Membranes

• Cutaneous membrane: The skin, primary organ of the integumentary system.

• Serous membranes: Simple squamous epithelium on a connective tissue basement membrane.

  • Parietal layer: Lines walls of body cavities.

  • Visceral layer: Covers organs found in body cavities.

• Mucous membranes: Line body surfaces that open directly to the exterior and produce mucus to keep membranes soft and moist.

Connective Tissue Membranes

• Synovial membranes: Produce synovial fluid to reduce friction between bones in movable joints and line bursae.

Skin Structure

Layers of the Skin

The skin, or cutaneous membrane, is a sheetlike organ that acts as a barrier between the internal and external environment. It consists of two primary layers:

• Epidermis: Outermost and thinnest layer, composed of stratified squamous epithelium. Contains the stratum germinativum (deepest layer, site of cell reproduction and melanin production) and stratum corneum (outermost layer of keratin-filled cells).

• Dermis: Deeper and thicker layer, composed largely of connective tissue. Contains dermal papillae, collagenous and elastic fibers, nerve endings, muscle fibers, hair follicles, sweat and sebaceous glands, and blood vessels.

Skin Color and Variations

Skin color is determined by the presence of pigment cells called melanocytes, which produce melanin. Variations in skin color can indicate physiological or pathological changes.

• Pink flush: Increased blood volume or oxygen.

• Cyanosis: Bluish-gray color due to decreased blood oxygen.

• Vitiligo: Patchy light skin areas from loss of melanocytes.

• Albinism: Hereditary condition with partial or total lack of melanin.

• Freckles: Small, flat macules; normal pigment variation.

Dermal-Epidermal Junction

The dermal-epidermal junction is a specialized area of contact between the epidermis and dermis, providing support. Weakening or destruction can cause blisters and, if widespread, serious infection.

Thick and Thin Skin

Skin can be classified as thick or thin based on structure and function.

• Thick skin: Has parallel friction ridges and no hairs (e.g., palms, soles).

• Thin skin: Has irregular, shallow grooves and hair.

Dermal Features and Birthmarks

The dermis contains elastic fibers, which decrease with age, contributing to wrinkles. Birthmarks result from malformation of dermal blood vessels.

• Strawberry hemangioma: Mass of dilated dermal blood vessels, resembling a strawberry.

• Port-wine stain: Flat, red or purple mark present at birth.

• Stork bite: Dilation of dermal capillaries at the nape of the neck in newborns.

Subcutaneous Tissue (Hypodermis)

The subcutaneous tissue, or hypodermis, is not a layer of the skin but connects the skin to underlying structures. It contains loose fibrous and adipose tissues and is a site for subcutaneous injections.

Appendages of the Skin

Hair

Hair is produced by epidermal tubelike structures called hair follicles. Growth begins from the hair papilla, and the visible part is called the shaft. Hair is most dense on the scalp, eyelids, and eyebrows.

• Lanugo: Soft hair of fetus and newborn.

• Alopecia: Hair loss.

• Arrector pili: Specialized smooth muscle causing "goose pimples" and hair to stand up.

Nails

Nails are produced by epidermal cells over the terminal ends of fingers and toes. The visible part is the nail body, and the root is hidden by the cuticle. The lunula is the crescent-shaped area nearest the root.

• Nail bed: May change color with blood flow.

• Normal variations: Longitudinal ridges in light-skinned individuals, pigmented bands in dark-skinned individuals.

• Abnormal variations: Onycholysis (separation of nail from bed), pitting (common in psoriasis).

Receptors

Specialized nerve endings in the skin make it possible for the skin to act as a sense organ.

• Tactile (Meissner) corpuscle: Detects light touch.

• Lamellar (Pacini) corpuscle: Detects pressure.

Skin Glands

The skin contains sweat (sudoriferous) glands and sebaceous (oil) glands.

• Eccrine sweat glands: Most numerous, produce sweat for heat regulation and waste elimination.

• Apocrine sweat glands: Found in axilla and genitalia, secrete thicker, milky secretion; function begins at puberty.

• Sebaceous glands: Secrete oil (sebum) for hair and skin lubrication; secretion increases during adolescence and is regulated by sex hormones. Blockage can cause acne vulgaris.

Functions of the Skin

Protection

The skin acts as the first line of defense against infection, UV rays, chemicals, cuts, and tears. Bruising can cause discoloration as blood released from damaged vessels breaks down.

Temperature Regulation

The skin releases heat through sweat secretion and regulation of blood flow close to the surface. It can release up to 3000 calories of body heat per day.

Sensation

The skin functions as a sense organ, detecting light touch, pressure, pain, heat, and cold through specialized receptors.

Excretion

The skin regulates the amount and chemical content of sweat, excreting waste products such as uric acid, ammonia, and urea.

Synthesis of Vitamin D

UV light from the sun causes skin cells to produce a substance that forms vitamin D, which is then transported to the liver and kidneys for activation.

Conditions of the Skin

Lesions

Skin lesions are measurable variations from normal structure and can be elevated, flat, or depressed.

• Elevated lesions: Papule, plaque, vesicle, pustule, crust, wheal (hive).

• Flat lesions: Macule.

• Depressed lesions: Excoriation, ulcer, fissure.

Skin Repair

The skin can repair itself after injury, involving blood clotting, cell growth, and fiber production.

Burns

Classification and Severity

Burns are classified by depth and number of tissue layers involved, total body surface area affected, and type of homeostatic mechanisms damaged.

• First-degree (partial-thickness): Only surface layers of epidermis involved.

• Second-degree (partial-thickness): Involve deep epidermal layers and upper dermis.

• Third-degree (full-thickness): Complete destruction of epidermis and dermis; may involve underlying muscle and bone (fourth-degree).

Rule of Nines

The "rule of nines" is used to estimate body surface area affected by burns in adults. The body is divided into 11 areas of 9% each, with an additional 1% around the genitals.

Skin Infections

Common Skin Infections

• Impetigo: Highly contagious bacterial infection.

• Tinea: Fungal infection (mycosis) of the skin.

• Warts: Benign neoplasm caused by papillomavirus.

• Boils: Staphylococcal infection in hair follicles.

• Scabies: Parasitic infection.

Vascular and Inflammatory Skin Conditions

Common Disorders

• Decubitus ulcers (bedsores): Develop from pressure slowing blood flow.

• Urticaria (hives): Red lesions caused by fluid loss from blood vessels.

• Scleroderma: Autoimmune disorder characterized by hardening of the skin.

• Psoriasis: Chronic inflammatory condition with scaly plaques.

• Eczema: Common inflammatory condition with papules, vesicles, and crusts.

Skin Cancer

Types of Skin Cancer

• Squamous cell carcinoma: Slow-growing malignant tumor of the epidermis; may metastasize if untreated.

• Basal cell carcinoma: Begins in cells at the base of the epidermis; rarely spreads.

• Melanoma: Malignancy in a nevus (mole); most serious type.

• Kaposi sarcoma: Characterized by purple lesions; associated with AIDS and immune deficiencies.

The most important causative factor in common skin cancers is exposure to sunlight.

Review Questions

1. Which of these membranes is classified as a connective tissue membrane? Answer: Synovial

2. Identify the two main layers of the skin. Answer: Epidermis, dermis

3. The term _____ is used to describe a condition characterized by patchy looking areas of light skin resulting from the loss of epidermal melanocytes. Answer: Vitiligo

4. Hair growth begins from a small, cap-shaped cluster of cells called the: Answer: Hair papilla

5. Which of these would NOT be considered a function of the skin? Answer: Transportation

6. A burn that causes minor discomfort and some reddening of the skin is classified as a _____ burn. Answer: First-degree

7. Which skin disorder is characterized by silvery white, scale-like plaques that may remain fixed on the skin for months? Answer: Psoriasis

8. What is the most serious form of skin cancer? Answer: Melanoma