Structure and Function of the Skin: Epidermis, Dermis, Hypodermis, and Skin Appendages

Skin Structure and Function

Overview of the Skin

The skin is the largest organ of the human body, providing protection, sensation, and regulation of body temperature. It consists of three main layers: the epidermis, dermis, and hypodermis. Each layer has distinct cell types and functions essential for maintaining homeostasis.

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

• Dermis: Middle layer, made of connective tissue, contains blood vessels, nerves, and glands.

• Hypodermis: Deepest layer, primarily adipose tissue, connects skin to underlying structures.

Epidermis

Cell Types in the Epidermis

The epidermis is a keratinized stratified squamous epithelium with a wavy basement membrane. It contains several specialized cell types:

• Keratinocytes (≈90%): Produce keratin, a fibrous protein that provides protective properties to the skin.

• Melanocytes (≈8%): Produce melanin pigment, which protects against UV light. Melanocytes have projections that transfer melanin to keratinocytes.

• Dendritic cells (Langerhans cells): Part of the immune system, important in immune response and defense against pathogens.

• Tactile epithelial cells (Merkel cells): Associated with nerve endings, important for the sense of touch.

Layers of the Epidermis (Strata)

The epidermis is organized into distinct layers, from deep to superficial:

• Stratum basale (basal layer): Single row of cuboidal/columnar keratinocytes; site of cell division. Contains melanocytes and some tactile cells.

• Stratum spinosum: 8–10 cell layers; keratinocytes with spiny projections; some cell division occurs.

• Stratum granulosum: 3–5 cell layers; flattened keratinocytes; contains keratohyaline granules (strengthen keratin filaments) and lamellar granules (form waterproof barrier).

• Stratum lucidum: Only in thick skin (palms, soles); 3–5 layers of clear, dead keratinocytes.

• Stratum corneum: 25–30 layers of dead, flat keratinocytes; provides waterproof barrier and protection.

Key Structures in the Epidermis

• Keratohyaline granules: Bundle keratin filaments, strengthening the cell.

• Lamellar granules: Contain lipids, form a waterproof barrier to prevent water loss.

Dermis

Structure and Regions of the Dermis

The dermis is a connective tissue layer beneath the epidermis, containing fibroblasts, macrophages, blood vessels, nerves, glands, and hair follicles. It is divided into two regions:

• Papillary region: Thinner, superficial; contains areolar connective tissue with elastic fibers. Forms dermal papillae (projections into epidermis), which contain capillary loops and touch receptors, and cause epidermal ridges (fingerprints).

• Reticular region: Deeper, thicker; dense irregular connective tissue with collagen and elastin. Contains hair follicles, nerves, glands, and adipose tissue.

Hypodermis

Structure and Function

The hypodermis (subcutaneous layer) attaches the dermis to underlying structures. It consists mainly of areolar and adipose tissue, contains blood vessels and nerves, and serves as insulation and energy storage.

• Also known as superficial fascia.

• Thickness varies by body region.

• Superficial fascia term is used when referring to muscle.

Functions of the Skin

Major Functions

• Temperature regulation: Evaporation of sweat cools the body; blood flow changes help conserve or release heat.

• Protection: Physical barrier against invasion, dehydration, and UV radiation; contains immune cells.

• Sensation: Nerve cells detect pain, touch, pressure, and temperature.

• Excretion: Sweat removes water and dissolved substances.

• Blood reservoir: Skin can shift blood flow to muscles when needed.

• Vitamin D synthesis: Epidermis produces vitamin D precursor upon UV exposure, aiding calcium absorption.

Derivatives of the Epidermis (Skin Appendages)

Hair

Hair is composed of dead, keratinized cells and serves protective and sensory functions.

• Shaft: Projects above the skin surface.

• Root: Beneath the skin surface, surrounded by the hair follicle.

• Root plexus: Nerve endings around the follicle, contribute to sensation.

• Arrector pili: Smooth muscle attached to hair follicle; contracts to make hair "stand up" (goosebumps).

• Functions: Protection, heat loss reduction, sensory input, filtering particles, and chemical signaling (pheromones).

Glands

Skin contains several types of glands with distinct functions:

• Sebaceous (oil) glands: Holocrine glands that secrete sebum (mix of fats, cholesterol, proteins, salts, pheromones) into hair follicles. Sebum coats hair, inhibits bacterial growth, and keeps skin moist.

• Sudoriferous (sweat) glands:

  • Eccrine sweat glands: Most common; secrete water, salts, and wastes directly onto skin surface. Function in cooling and excretion; sweat acidity protects against bacteria.

  • Apocrine sweat glands: Found in axillary/genital regions; secrete sweat, fats, and proteins into hair follicles. Contribute to body odor and pheromone signaling.

Nails

Nails are hard, keratinized cells that protect the distal phalanges and aid in grasping and manipulating objects.

• Nail body: Visible part of the nail.

• Nail root: Embedded under the skin.

• Functions: Protection, grasping, scratching.

Table: Comparison of Epidermal Layers

Layer	Cell Type	Main Features
Stratum basale	Cuboidal/columnar keratinocytes, melanocytes, tactile cells	Cell division, melanin production
Stratum spinosum	Keratinocytes, dendritic cells	Spiny projections, some cell division
Stratum granulosum	Flattened keratinocytes	Keratohyaline and lamellar granules, waterproof barrier
Stratum lucidum	Dead keratinocytes	Only in thick skin, clear layer
Stratum corneum	Dead, flat keratinocytes	Protection, waterproofing

Key Terms and Definitions

• Keratin: A tough, fibrous protein that provides strength and waterproofing to the skin.

• Melanin: Pigment produced by melanocytes, protects against UV radiation.

• Holocrine gland: Gland whose cells release their product by rupturing.

• Merocrine secretion: Exocytosis-based secretion, as in eccrine sweat glands.

• Apocrine secretion: Secretion involving loss of part of the cell, as in apocrine sweat glands.

Example: Vitamin D Synthesis Equation

Upon exposure to UV light, the epidermis converts 7-dehydrocholesterol to vitamin D3 (cholecalciferol):

$ext{7-dehydrocholesterol} ightarrow[ ext{UV light}] ext{cholecalciferol (vitamin D}_3 ext{)}$

Additional info:

• Some details about cell types and functions were expanded for clarity.

• Table entries and definitions were inferred and supplemented for completeness.