Chapter 5: The Integumentary System – Structure, Function, and Clinical Relevance

The Integumentary System

Overview

The integumentary system is the body's largest organ system, primarily responsible for protection and homeostasis. It consists of the skin and its associated structures.

• Components: Skin, hair, nails, sweat glands, sebaceous (oil) glands

• Main Functions: Protection, temperature regulation, sensation, metabolic functions, excretion

Structure of the Skin

Skin Regions

The skin is composed of two main layers and an underlying layer:

• Epidermis: Superficial, keratinized stratified squamous epithelium

• Dermis: Deep, strong, flexible connective tissue

• Hypodermis (Superficial Fascia): Not part of skin, mostly adipose tissue, anchors skin to underlying structures

Cells of the Epidermis

The epidermis contains four main cell types:

• Keratinocytes: Produce keratin, a protein that gives skin its strength and waterproofing

• Melanocytes: Spider-shaped cells that produce melanin pigment, which protects against UV radiation

• Dendritic (Langerhans) Cells: Immune cells that help protect against pathogens

• Tactile (Merkel) Cells: Sensory cells associated with touch

Layers of the Epidermis

The epidermis is organized into distinct layers (strata). Thick skin (palms, soles) has five layers; thin skin has four.

• 1. Stratum basale (basal layer): Deepest, single row of stem cells, site of mitosis, contains melanocytes

• 2. Stratum spinosum (prickly layer): Several cell layers thick, contains keratinocytes, melanosomes, dendritic cells

• 3. Stratum granulosum (granular layer): 4-6 cell layers, cells flatten, keratinization begins, cells above this layer die

• 4. Stratum lucidum (clear layer): Only in thick skin, thin translucent band of dead keratinocytes

• 5. Stratum corneum (horny layer): 20-30 rows of dead, keratinized cells, provides barrier function

Dermis

The dermis is a strong, flexible connective tissue layer containing fibers, blood vessels, nerves, and glands.

• Layers: Papillary (superficial), Reticular (deep)

• Structures: Epidermal hair follicles, sweat and oil glands

Papillary Layer

• Superficial, loose connective tissue

• Dermal papillae: finger-like projections containing capillaries and sensory receptors (e.g., Meissner's corpuscles)

• Friction ridges: form fingerprints

Reticular Layer

• Makes up ~80% of dermal thickness

• Dense irregular connective tissue, collagen and elastic fibers

• Cutaneous plexus: network of blood vessels

• Adipose cells

• Cleavage (tension) lines: Formed by collagen fibers, important for surgical incisions

• Flexure lines: Dermal folds at joints (palms, fingers)

Clinical Relevance: Skin Markings

• Striae: Stretch marks from extreme stretching

• Blisters: Fluid-filled pockets from acute trauma

Skin Color

Pigments Contributing to Skin Color

• Melanin: Only pigment made in skin, ranges from yellow to brown to black, protects against UV

• Carotene: Yellow to orange pigment, most obvious in palms and soles

• Hemoglobin: Red pigment in blood, visible in fair skin

Clinical Imbalances in Skin Color

• Cyanosis: Blue skin color due to low oxygen

• Erythema: Redness due to increased blood flow

• Pallor: Pale skin due to decreased blood flow

• Jaundice: Yellow skin due to bilirubin accumulation

• Bronzing: Metallic appearance, often due to adrenal disease

• Bruises: Black and blue marks from blood under skin

Glands of the Skin

Sudoriferous (Sweat) Glands

• Present on almost all skin surfaces

• Types: Eccrine (merocrine) and apocrine

• Contain myoepithelial cells to force sweat into ducts

Eccrine (Merocrine) Sweat Glands

• Most numerous, abundant on palms, soles, forehead

• Ducts connect to pores

• Function: Thermoregulation

• Secretion: 99% water, some salts

Apocrine Sweat Glands

• Axillary and anogenital areas

• Begin functioning at puberty

• Secrete viscous, milky/yellowish sweat

• Modified apocrine glands: Ceruminous glands (earwax), Mammary glands (milk)

Sebaceous (Oil) Glands

• Widely distributed, usually associated with hair follicles

• Secrete sebum (oil), lubricates skin and hair

• Relatively inactive until puberty, stimulated by hormones

Functions of the Skin

Main Functions

• Protection: Physical, chemical, and biological barriers

• Body temperature regulation

• Cutaneous sensations: Sensory receptors for touch, pain, temperature

• Metabolic functions: Synthesis of vitamin D

• Blood reservoir: Stores blood in dermal vessels

• Excretion: Removal of wastes via sweat

Protection: Barriers

• Chemical barrier: Skin secretes chemicals (sebum, defensins, acid mantle, melanin)

• Physical barrier: Stratum corneum, dead keratinized cells

• Biological barrier: Phagocytic cells (dendritic cells, macrophages), DNA absorbs UV radiation

Water Loss

• Insensible water loss: Continuous, unnoticed evaporation

• Sensible water loss: Noticeable sweating

• Cold environments slow passive heat loss

Cutaneous Sensations

• Receptors in skin detect touch, pressure, pain, and temperature

• Exteroceptors: Respond to stimuli outside the body

• Free nerve endings sense pain

Summary Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Single row of stem cells, mitosis, melanocytes
Stratum spinosum	Above basale	Several layers, keratinocytes, dendritic cells
Stratum granulosum	Middle	Keratinization begins, cells flatten
Stratum lucidum	Only in thick skin	Clear, dead keratinocytes
Stratum corneum	Superficial	20-30 layers of dead, keratinized cells

Summary Table: Skin Pigments

Pigment	Color	Source	Function
Melanin	Yellow to brown to black	Produced by melanocytes	UV protection
Carotene	Yellow to orange	Diet (vegetables)	Converted to vitamin A
Hemoglobin	Red	Blood	Oxygen transport

Key Equations

• Water Loss Equation:

$\text{Total Water Loss} = \text{Insensible Water Loss} + \text{Sensible Water Loss}$

Additional info: Some details, such as the function of the acid mantle and the role of vitamin D synthesis, were expanded for academic completeness.