Chapter 5: The Integumentary System

Module 5.1 Overview of the Integumentary System

The integumentary system is the body's largest organ system, primarily composed of the skin and its accessory structures. It serves as the first line of defense against the external environment and plays vital roles in protection, sensation, thermoregulation, excretion, and synthesis of vitamin D.

• Cutaneous Membrane: The skin itself, consisting of two main layers: the epidermis (superficial) and the dermis (deep).

• Hypodermis: Also called the subcutaneous layer, located beneath the dermis; composed mainly of adipose tissue and not technically part of the skin.

• Accessory Structures: Include sweat glands (produce sweat for thermoregulation), sebaceous glands (secrete sebum for lubrication), hair (provides protection and sensation), and nails (protect the tips of fingers and toes).

Functions of the Integumentary System:

• Protection: Acts as a physical barrier, prevents water loss, and protects against pathogens and UV radiation.

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

• Thermoregulation: Regulates body temperature through sweat production and blood flow adjustments.

• Excretion: Removes waste products such as urea and salts through sweat.

• Vitamin D Synthesis: Skin cells produce cholecalciferol (vitamin D3) when exposed to UV light, essential for calcium homeostasis.

Module 5.2 The Epidermis

The epidermis is the outermost layer of the skin, composed of stratified squamous epithelium. It provides a waterproof barrier and creates our skin tone.

• Five Layers of Thick Skin (from deep to superficial):

  1. Stratum basale: Single row of stem cells; site of mitosis; contains melanocytes and Merkel cells; cells are alive.

  2. Stratum spinosum: Several layers of keratinocytes; contains dendritic (Langerhans) cells; cells are alive.

  3. Stratum granulosum: 3-5 layers; cells begin to die; contain keratohyalin granules.

  4. Stratum lucidum: Present only in thick skin (palms, soles); thin, clear layer of dead keratinocytes.

  5. Stratum corneum: 20-30 layers of dead, flattened, keratinized cells; provides protection.

• Cell Types:

  • Keratinocytes: Most abundant; produce keratin for strength and waterproofing.

  • Dendritic (Langerhans) cells: Immune cells that detect pathogens.

  • Merkel cells: Sensory receptors for touch.

  • Melanocytes: Produce melanin pigment for UV protection.

• Keratinocyte Life Cycle: Cells are produced in the stratum basale, migrate upward, undergo keratinization, and eventually die and slough off in the stratum corneum.

• Thick vs. Thin Skin: Thick skin (palms, soles) has all five layers and no hair; thin skin (rest of body) lacks stratum lucidum and has hair follicles.

Module 5.3 The Dermis

The dermis is a connective tissue layer beneath the epidermis, providing structural support and housing blood vessels, nerves, and accessory structures.

• Layers of the Dermis:

  • Papillary Layer: Superficial; made of loose areolar connective tissue; contains dermal papillae and tactile corpuscles (touch receptors).

  • Reticular Layer: Deep; made of dense irregular connective tissue; contains lamellated (Pacinian) corpuscles (pressure receptors), hair follicles, and glands.

• Functions of the Dermis: Provides strength and elasticity, houses sensory receptors, and supplies nutrients to the epidermis.

• Dermal Papillae: Projections that increase surface area for nutrient exchange and form the basis for fingerprints.

• Skin Markings:

  • Epidermal ridges: Form fingerprints; improve grip.

  • Tension (cleavage) lines: Patterns of collagen fibers; important for surgical incisions.

  • Flexure lines: Skin folds at joints.

Module 5.4 Skin Pigmentation

Skin color is determined by the presence of several pigments, primarily melanin, carotene, and hemoglobin. These pigments also play roles in protection and health indication.

• Melanin: Produced by melanocytes in the stratum basale; packaged into melanosomes and transferred to keratinocytes. Melanin absorbs UV radiation, protecting DNA from damage.

• Distribution: All humans have similar numbers of melanocytes; skin color differences are due to melanin production and distribution, not cell number.

• Carotene: Yellow-orange pigment from diet; accumulates in stratum corneum and fat.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue, especially in light-skinned individuals.

• Pathological Color Changes:

  • Erythema: Redness due to increased blood flow (e.g., inflammation).

  • Pallor: Paleness from decreased blood flow or anemia.

  • Cyanosis: Bluish color from low oxygen levels.

Module 5.5 Accessory Structures of the Integument: Hair, Nails, and Glands

Accessory structures of the skin include hair, nails, and various glands, each with specialized functions.

• Hair:

  • Structure: Hair root (embedded in skin), hair shaft (projects above surface), hair follicle (surrounds root), hair bulb (base of follicle), arrector pili muscle (smooth muscle causing hair to stand).

  • Growth: Two phases: growth (anagen) and resting (telogen). Hair length depends on duration of growth phase.

  • Types: Lanugo (fetal hair), vellus (fine, unpigmented), terminal (thick, pigmented).

• Nails:

  • Structure: Nail matrix (growth area), nail bed (underlying skin), eponychium (cuticle), hyponychium (under free edge), nail plate (visible part), nail root (embedded), lunula (white crescent), free edge (extends past finger).

  • Growth: Occurs at the nail matrix.

• Glands:

  • Eccrine sweat glands: Widely distributed; secrete watery sweat onto skin surface for cooling.

  • Apocrine sweat glands: Found in axillae, groin; secrete into hair follicles; become active at puberty.

  • Ceruminous glands: In ear canal; produce cerumen (earwax).

  • Mammary glands: In breasts; produce milk.

  • Sebaceous glands: Associated with hair follicles; secrete oily sebum for lubrication.

• Secretions:

  • Sweat: Mostly water, salts, and waste; cools body.

  • Cerumen: Earwax; protects ear canal.

  • Milk: Nutrient-rich secretion for infants.

  • Sebum: Oily substance; lubricates and waterproofs skin and hair.

Module 5.6 Pathology of the Skin

The skin is susceptible to various injuries and diseases, including burns and cancers. Understanding their classification and severity is essential for treatment and prognosis.

• Burns:

  • First-degree: Damage to epidermis only; redness (erythema), mild pain, no blisters.

  • Second-degree: Damage to epidermis and part of dermis; redness, pain, blisters.

  • Third-degree: Damage through dermis; may appear white, charred, or leathery; no pain (nerve endings destroyed).

  • Rule of Nines: Method to estimate body surface area affected by burns; body divided into regions representing 9% (or multiples) of total area.

• Skin Cancer:

  • Definitions: Cancer is uncontrolled cell growth; tumor is a mass of abnormal cells; metastasize means to spread; carcinoma is a cancer of epithelial origin.

  • Types:

    • Basal cell carcinoma: Most common; arises from stratum basale; rarely metastasizes; good prognosis.

    • Squamous cell carcinoma: From keratinocytes of stratum spinosum; may metastasize; good prognosis if caught early.

    • Malignant melanoma: From melanocytes; least common but most dangerous; high risk of metastasis; requires aggressive treatment.

Example: A patient with burns covering the anterior and posterior surfaces of both legs (each leg = 18%) would have 36% total body surface area burned, according to the rule of nines.

Additional info: The above notes expand on the learning objectives by providing definitions, examples, and context for each major topic in the integumentary system, suitable for college-level Anatomy & Physiology students.