Integumentary System: Structure, Function, and Accessory Organs

Integumentary System Overview

Introduction

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 environmental hazards and plays a vital role in maintaining homeostasis.

• Components: Skin, hair, glands, nails

• Main functions: Protection, thermoregulation, waste excretion, lipid synthesis/storage, metabolic functions, and sensation

Functions of the Integumentary System

Major Roles

• Protection: The skin provides three types of barriers:

  • Physical barrier: Prevents entry of pathogens and minimizes mechanical injury

  • Chemical barrier: Secretions (e.g., sebum, sweat) and melanin protect against bacteria and UV radiation

  • Biological barrier: Immune cells in the skin initiate immune responses

• Thermoregulation: Regulates body temperature via sweat production and blood flow

• Waste excretion: Removes metabolic wastes through sweat

• Lipid synthesis/storage: Stores energy and insulates the body

• Metabolic functions: Synthesizes vitamin D when exposed to sunlight

• Cutaneous sensation: Detects touch, pressure, pain, and temperature

Structure of the Skin

Layers of the Cutaneous Membrane

• Epidermis: Outermost layer, composed of keratinized stratified squamous epithelium

• Dermis: Middle layer, made of connective tissue, richly supplied with nerves and blood vessels

  • Papillary layer: Areolar connective tissue with collagen and elastic fibers; contains dermal papillae

  • Reticular layer: Dense irregular connective tissue with adipose cells and thick collagen fibers

• Hypodermis (subcutaneous layer): Superficial fascia composed mostly of adipose tissue; anchors skin to underlying structures (technically not part of the skin)

Epidermis

Layers and Cell Types

The epidermis consists of up to five distinct layers, each with specialized cells and functions:

• Stratum corneum (SC): 15–30 layers of dead keratinocytes (cornified cells)

• Stratum lucidum (SL): Only present in thick skin (e.g., palms, soles)

• Stratum granulosum (SG): 3–5 layers where keratinization begins

• Stratum spinosum (SS): Several layers of keratinocytes connected by desmosomes

• Stratum basale (SB): Attached to basal lamina; contains basal cells (stem cells), melanocytes (produce melanin), and Merkel cells (sensitive to touch)

Dermis and Hypodermis

Connective Tissue Layers

• Dermis:

  • Contains fibroblasts and leukocytes

  • Richly supplied with nerves and blood vessels

  • Papillary layer: Areolar CT with collagen and elastic fibers; contains dermal papillae

  • Reticular layer: Dense irregular CT with adipose cells and thick collagen fibers

• Hypodermis:

  • Superficial fascia composed mostly of adipose tissue

  • Loosely anchors skin to underlying structures

  • Technically not part of the skin

Skin Cancer

Types and Recognition

• General causes: Uncontrolled cell division due to DNA damage (often from UV exposure)

• Major types:

  • Basal cell carcinoma

  • Squamous cell carcinoma

  • Malignant melanoma

• ABCDE rule for melanoma:

  • Asymmetry

  • Border irregularity

  • Color variation

  • Diameter > 6mm

  • Evolving shape/size/color

	Basal cell carcinoma	Squamous cell carcinoma	Malignant melanoma
Cells/layers involved	Basal cells (stratum basale)	Keratinocytes (stratum spinosum)	Melanocytes (stratum basale)
How dangerous is it?	Least dangerous, rarely metastasizes	Moderately dangerous, can metastasize	Most dangerous, high metastatic potential
How common is it?	Most common	Second most common	Least common

Accessory Structures

Hair

• Structure: Flexible strands of mostly dead, keratinized cells

• Regions: Shaft, root, bulb

• Hair papilla: Carries blood vessels and nerves into the bulb

• Hair matrix: Site of mitosis, just above papilla

• Arrector pili: Bands of smooth muscle causing hair to raise ("goosebumps")

• Hair follicle: Structure from papilla to skin surface

• Types:

  • Vellus hair: Fine, pale hair

  • Terminal hair: Coarse, pigmented hair

Skin Glands

• Sebaceous glands: Holocrine glands that secrete sebum (oil) into hair follicles

  • Softens hair, slows water loss, antibacterial/antifungal properties

  • Stimulated by androgens, more active at puberty

  • Some are not associated with hair follicles (face, chest, nipples, external genitalia)

• Sweat glands: Contain myoepithelial cells to help discharge secretions

  • Eccrine (merocrine) sweat glands: Release secretions onto skin surface; 99% water with salts, metabolic wastes, antimicrobial peptides

  • Apocrine sweat glands: Release secretions into hair follicles (axillae, nipples, groin); contain fatty substances and proteins, produce odor when decomposed by bacteria; non-functional until puberty

• Modified sweat glands:

  • Ceruminous glands: Form cerumen (earwax)

  • Mammary glands: Produce milk

Nails

• Structure: Scale-like modification of stratum corneum on dorsal surface of distal fingers/toes

• Nail body: Covers nail bed

• Nail matrix: Under nail root, responsible for growth

• Lunula: Visible part of nail matrix

• Eponychium: Cuticle

Summary Table: Layers of the Epidermis

Layer	Main Features
Stratum corneum	Dead, keratinized cells; protection
Stratum lucidum	Clear layer; only in thick skin
Stratum granulosum	Keratinization begins
Stratum spinosum	Desmosome connections; keratinocytes
Stratum basale	Stem cells, melanocytes, Merkel cells

Key Equations

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

Additional info:

• Some details about skin cancer and hair types were inferred from standard academic sources.

• Tables were expanded for clarity and completeness.