Chapter 5: The Integumentary System – Anatomy & Physiology I Study Guide

Integumentary System Overview

Functions and Characteristics of the Skin

The integumentary system is composed of the skin and its accessory structures. It serves as the body's primary barrier against environmental hazards and plays a crucial role in homeostasis.

• Protection: Shields underlying tissues and organs from mechanical shock, pathogens, and dehydration.

• Excretion: Removes salts, water, and metabolic wastes through sweat.

• Temperature Regulation: Maintains body temperature via sweat and blood flow.

• Vitamin D3 Synthesis: Converts precursor molecules to vitamin D3, essential for calcium absorption.

• Storage: Stores lipids in adipocytes within the dermis and hypodermis.

Structure of the Integumentary System

Major Components

• Cutaneous Membrane: Composed of the epidermis (stratified squamous epithelium, avascular) and dermis (dense irregular connective tissue, vascular).

• Subcutaneous Layer (Hypodermis): Not part of the cutaneous membrane; consists of areolar and adipose tissue, rich in blood vessels.

• Accessory Structures: Includes hair, nails, and four types of exocrine glands.

Skin Layering: Thick vs. Thin Skin

Skin is classified as either thick or thin based on the number of layers and location.

• Thick Skin: Found on palms and soles; contains five layers: stratum germinativum, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum.

• Thin Skin: Covers most of the body; lacks stratum lucidum and has four layers.

Epidermal Layers and Cell Types

Strata of the Epidermis

The epidermis consists of several distinct layers, each with specialized cells and functions.

• Stratum Germinativum (Basal): Deepest layer, attached to basal lamina; contains stem cells (basal cells), Merkel cells (touch receptors), and melanocytes (pigment production).

• Stratum Spinosum: 8–10 layers; contains Langerhans cells (immune defense).

• Stratum Granulosum: 3–5 layers; keratinocytes begin producing keratin.

• Stratum Lucidum: Only in thick skin; densely packed, keratin-filled cells.

• Stratum Corneum: Most superficial; 15–30 layers of dead, keratinized cells; provides water resistance.

Keratinocytes and Melanocytes

Keratinocytes are the most abundant cells in the epidermis, responsible for producing keratin. Melanocytes synthesize melanin, which protects against UV radiation.

Epidermal Ridges and Fingerprints

Epidermal ridges increase the surface area for attachment between the epidermis and dermis, forming fingerprints.

Skin Color and Pigmentation

Pigments in the Skin

• Carotene: Orange/yellow pigment from diet; converted to vitamin A.

• Melanin: Brown, yellow-brown, or black pigment produced by melanocytes; protects DNA from UV damage.

• Hemoglobin: Red pigment in blood; gives skin a reddish hue when oxygenated.

Clinical Conditions Affecting Skin Color

• Jaundice: Yellowing due to bile pigment accumulation.

• Albinism: Genetic defect preventing melanin production.

Skin Disorders and Cancers

Common Skin Disorders

• Psoriasis: Rapid cell division in stratum germinativum.

• Microbial Infections: Viral (herpes, warts), bacterial (boils, leprosy), fungal (ringworm), parasitic (scabies).

Skin Cancers

• Basal Cell Carcinoma: Malignant cancer in stratum germinativum; most common skin cancer.

• Melanoma: Cancer of melanocytes; highly aggressive.

Dermis and Subcutaneous Layer

Dermal Structure

The dermis is divided into two layers:

• Papillary Layer: Superficial; areolar tissue, capillaries, sensory neurons.

• Reticular Layer: Deep; dense irregular connective tissue, accessory organs, blood vessels, lymphatics, nerves.

Subcutaneous Layer (Hypodermis)

• Contains areolar and adipose tissue.

• Rich in blood vessels; site for injections and liposuction.

Accessory Structures

Hair

Hair is produced by hair follicles and consists of keratinized cells. It serves protective and sensory functions.

• Hair Shaft: Visible part; composed of dead, keratinized cells.

• Hair Root: Anchors hair to skin.

• Cuticle: Outer layer of shaft.

• Hair Papilla: Connective tissue with capillaries and nerves.

• Arrector Pili Muscle: Causes hair to stand up.

Hair Types and Functions

• Lanugo: Embryonic hair, shed before birth.

• Vellus Hair: Fine "peach fuzz" covering body.

• Terminal Hair: Coarse hair on scalp, eyebrows, eyelashes.

• Root Hair Plexus: Nerve network for touch sensation.

Nails

Nails are protective coverings on the dorsal surface of fingers and toes, composed of keratinized cells.

• Nail Body: Visible part covering the nail bed.

• Nail Root: Site of nail growth.

• Eponychium: Cuticle; extension of stratum corneum.

• Lunula: Crescent-shaped area near cuticle.

Glands in the Skin

• Sebaceous (Oil) Glands: Produce sebum; lubricates hair and skin, inhibits bacteria.

• Sweat (Sudoriferous) Glands:

  • Apocrine: Secrete into hair follicles; found in armpits and groin; produce odor.

  • Merocrine: Secrete directly onto skin; regulate temperature, excrete wastes.

• Mammary Glands: Produce milk.

• Ceruminous Glands: Produce ear wax (cerumen).

Repair and Regeneration of Skin

Steps in Skin Repair

1. Injury: Bleeding and inflammation; mast cells trigger immune response.

2. Scab Formation: Stratum germinativum cells migrate to injury; increased blood flow brings phagocytes.

3. Repair: Epidermal cells undermine scab; fibroblasts produce scar tissue.

4. Scab Removal: Scar tissue remains, raising epidermis.

Granulation Tissue and Keloids

• Granulation Tissue: Combination of blood clot, fibroblasts, and capillaries.

• Keloid: Thickened, shiny scar tissue.

Aging and Medical Conditions

Aging Effects on Skin

• Epidermis thins; fewer Langerhans cells.

• Reduced vitamin D3 and melanocyte activity.

• Decreased gland activity and blood supply.

• Loss of collagen and elastin; skin becomes thinner, weaker, wrinkled, and drier.

Medical Conditions

• Nail Changes: Yellow nails may indicate AIDS or thyroid issues.

• Psoriasis: Rapid stem cell division in stratum germinativum.

Key Terms and Definitions

Term	Definition
Cutaneous membrane	Skin; composed of epidermis and dermis
Melanocytes	Cells producing melanin pigment
Keratin	Protein providing strength and water resistance
Epidermal ridges	Extensions of epidermis into dermis; form fingerprints
Papillary layer	Superficial dermal layer; areolar tissue
Reticular layer	Deep dermal layer; dense irregular tissue
Elastic fibers	Provide skin elasticity
Collagen	Provides skin strength
Hair follicle	Structure producing hair
Hair root	Anchors hair to skin
Hair shaft	Visible part of hair
Hair papilla	Connective tissue at hair base
Arrector pili muscle	Muscle causing hair to stand up
Sebaceous gland	Oil-producing gland
Nail root	Site of nail growth
Nail bed	Area beneath nail body
Nail body	Visible part of nail
Nail fold	Skin folds on nail sides
Hyponychium	Area under nail free edge
Eponychium	Cuticle; extension of stratum corneum
Lunula	Crescent-shaped area near cuticle

Summary Table: Epidermal Layers

Layer	Location	Key Features
Stratum germinativum	Deepest	Stem cells, melanocytes, Merkel cells
Stratum spinosum	Above basal	Langerhans cells, 8–10 cell layers
Stratum granulosum	Middle	Keratin production, 3–5 cell layers
Stratum lucidum	Thick skin only	Clear, densely packed cells
Stratum corneum	Superficial	15–30 layers, dead keratinized cells

Important Equations

• Vitamin D3 Synthesis: $\text{7-dehydrocholesterol} \xrightarrow{UV~light} \text{cholecalciferol (Vitamin D3)} \xrightarrow{liver} \text{calcidiol} \xrightarrow{kidney} \text{calcitriol}$