Integumentary System: Structure and Function (Chapter 5 Study Notes)

Integumentary System

Overview

The integumentary system, primarily composed of the skin, is the body's largest organ system and serves as a protective barrier between the internal environment and the external world. It consists of multiple layers and associated structures that perform vital physiological functions.

Primary Functions of the Skin

Protection

The skin provides several types of protection to the body:

• Chemical Protection: Sweat and oily secretions (sebum) contain antimicrobial substances that kill bacteria. Melanin pigment prevents damage caused by ultraviolet (UV) light.

• Physical Protection: The skin acts as a physical barrier that prevents the entrance of bacteria and other pathogens.

• Biological Protection: Dendritic cells (Langerhans cells) in the skin stimulate immune responses, while macrophages destroy large-sized invaders.

Temperature Regulation

The skin helps regulate body temperature through the process of perspiration and blood flow:

• Insensible Perspiration: Unnoticeable sweat loss due to evaporation. This helps maintain baseline body temperature and prevents dehydration.

• Sensible Perspiration: Noticeable sweat loss that occurs when excess heat needs to be lost by the body, such as during exercise or high environmental temperatures.

• Vasoconstriction: In cold environments, blood vessels constrict, pulling blood away from the skin to conserve heat. This can lower skin temperature and help maintain core body temperature.

Sensation & Metabolic Functions

• Sensation: The skin contains sensory receptors that respond to external stimuli. These include nociceptors (pain), thermoreceptors (temperature), and mechanoreceptors (touch and pressure).

• Metabolic Functions: The skin is involved in vitamin D production when exposed to sunlight. Vitamin D is essential for calcium absorption and bone health.

Blood Reservoir & Excretion

• Blood Reservoir: Up to 5% of the body's total blood volume can be found in the skin and can be redirected by the central nervous system as needed.

• Excretion: The skin allows for the elimination of certain nitrogenous wastes (urea, ammonia, uric acid) through sweat.

Structure of the Integument (Skin)

Layers of the Skin

• Epidermis: The avascular, outermost portion of the skin, composed primarily of epithelial cells.

• Dermis: The vascularized layer beneath the epidermis, making up most of the skin's mass and providing strength and flexibility.

Associated Layer: Hypodermis

The hypodermis (subcutaneous layer) lies deep to the dermis and is not considered a true part of the integument. It is composed mostly of adipose tissue and serves several important functions:

• Storage: Provides an easy-to-access energy source for the body.

• Protection/Shock Absorption: Prevents physical trauma to underlying tissues.

• Insulation: Reduces excessive heat loss.

• Anchor: Holds the skin to underlying muscle tissue while remaining flexible.

Microscopic Structure of the Skin

Epidermis

The epidermis is composed of several cell types and layers:

• Keratinocytes: Produce the protein keratin, which gives the epidermis its protective qualities. These cells reproduce mitotically from the bottom up, and millions are lost daily.

• Melanocytes: Produce the pigment melanin in organelles called melanosomes. Melanin is transferred to keratinocytes and migrates to the "sunny side" of the cell to protect against UV damage.

• Dendritic Cells (Langerhans Cells): Immune cells that help activate the immune system.

• Tactile Cells (Merkel Cells): Sensory receptors located at the epidermal-dermal junction, allowing perception of touch.

Layers of the Epidermis (from deep to superficial)

• Stratum Basale: Innermost layer containing stem cells. Cell division occurs here, which is essential for skin regeneration. Contains mostly keratinocytes and some melanocytes.

• Stratum Spinosum: Contains pre-keratin protein and thick bundles of filaments for tension resistance. Dendritic cells are most abundant here.

• Stratum Granulosum: Contains two types of granules: keratohyaline (helps form keratin) and lamellar granules (contain water-resistant glycolipids).

• Stratum Lucidum: Found only in thick skin (e.g., palms, soles). Cells in this layer are dead.

• Stratum Corneum: Outermost layer, composed of dead cells. Makes up most of the epidermal thickness. Lipids between cells help waterproof the skin, and keratin inside cells protects from friction and abrasion.

Dermis

The dermis is a strong and flexible connective tissue layer containing fibroblasts, macrophages, and a rich matrix. It is divided into two regions:

• Papillary Dermis: Composed of areolar connective tissue. Contains projections called dermal papillae that have pain receptors and tactile corpuscles. These projections form friction ridges (commonly known as fingerprints), which are genetically determined.

• Reticular Dermis: Composed of dense irregular connective tissue. Contains cleavage lines (from separation of collagen fibers) and flexure lines (where the skin is tightly anchored at joints, forming visible creases).

Skin Color

Pigments Determining Skin Color

• Melanin: A dark-colored polymer synthesized in melanocytes by the enzyme tyrosinase. The amount of melanin determines skin pigmentation and increases with sun exposure.

• Carotene: A yellow-orange pigment that accumulates in the stratum corneum and adipose tissue. Used by the body to produce vitamin A, which is important for epidermal health.

• Hemoglobin: A pink/red pigment whose color comes from blood supply in the dermis.

Homeostatic Imbalances of Skin Color

• Jaundice: Yellowing of the skin due to liver failure.

• Albinism/Melanism: Genetic mutations affecting melanin production.

• Cyanosis: Bluish discoloration due to low hemoglobin.

• Argyria: Grayish discoloration due to silver exposure.

Skin Appendages

Hair

Hair is a filamentous structure composed of dead, hard-keratinized cells. It serves sensory and protective functions.

• Root: Part of the hair embedded in the skin.

• Shaft: Part of the hair projecting out of the skin.

• Layers of Hair:

  • Medulla: Central core composed of large cells (absent in fine hairs).

  • Cortex: Several layers of flattened cells.

  • Cuticle: Outermost layer, heavily keratinized, with cells stacked like roof shingles for protection.

• Associated Structures:

  • Hair Follicle: Fold extending from epidermis into dermis; contains the matrix that produces new hair cells.

  • Root Hair Plexus: Nerve endings at the base of the follicle for sensation.

  • Hair Papilla: Provides capillaries to nourish the hair.

  • Arrector Pili: Smooth muscle attached to the follicle; contraction causes hair to stand upright (goosebumps).

• Hair Types:

  • Vellus Hair: Thin, fine hairs.

  • Terminal Hair: Thick, coarse hairs, usually darker.

• Hair Color: Determined by melanin deposition; red hair results from pheomelanin, while gray/white hair is due to decreased melanin and air bubbles in the cortex.

• Balding: Hair loss is influenced by genetics and hormones; follicles may respond to androgens by shrinking and producing vellus hairs.

Nails

Nails are hard keratin structures composed of a root, nail plate, and free edge. The nail matrix is responsible for nail growth. Nails protect the distal portions of fingers and toes and contribute to dexterity.

Skin Glands

• Eccrine Sweat Glands: Most numerous, especially on palms and forehead. Secrete watery sweat directly to the skin surface for temperature regulation.

• Apocrine Sweat Glands: Located in axillary and anogenital areas; secrete sweat into hair follicles, which contains fatty substances and proteins.

• Modified Apocrine Glands: Include ceruminous glands (earwax) and mammary glands (milk production).

• Sebaceous Glands: Oil glands that secrete sebum, a lipid-based substance released into hair follicles. Sebum lubricates skin and hair, prevents water loss, and inhibits bacterial growth.

Homeostatic Imbalances: Skin Cancer and Burns

Skin Cancer

• Basal Cell Carcinoma: Cancer of cells in the stratum basale, usually caused by excess UV exposure. Most common but least malignant; slow-growing and rarely metastasizes.

• Squamous Cell Carcinoma: Cancer of cells in the stratum spinosum, also linked to UV exposure. Faster growing and can metastasize if untreated; appears as scaly, red lesions.

• Melanoma: Cancer of melanocytes, often arising from pre-existing moles. Least common but most deadly; metastasizes quickly and is resistant to chemotherapy.

Burns

• First-Degree Burn: Painful, reddened skin with inflammation; no scarring involved in healing (e.g., sunburn).

• Second-Degree Burn: Pain, redness, and fluid-filled blisters; takes longer to heal and may involve scarring.

• Third-Degree Burn: Full-thickness burns requiring intensive treatment (IV fluids, skin grafts, antibiotics); scarring and fibrosis occur during healing.

Table: Layers of the Skin and Their Main Features

Layer	Main Cell Types	Key Features
Stratum Basale	Keratinocytes, Melanocytes	Stem cell division, melanin production
Stratum Spinosum	Keratinocytes, Dendritic cells	Pre-keratin filaments, immune defense
Stratum Granulosum	Keratinocytes	Keratohyaline and lamellar granules, water resistance
Stratum Lucidum	Dead keratinocytes	Present only in thick skin
Stratum Corneum	Dead keratinocytes	Protection, waterproofing, friction resistance

Key Equations

• Vitamin D Synthesis: $\text{7-dehydrocholesterol} + UV \rightarrow \text{Cholecalciferol (Vitamin D}_3)$

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for college-level Anatomy & Physiology students.