Introduction to the Integumentary System
The integument is the largest system of the body
16% of body weight
1.5 to 2 m2 in area
The integument is made up of two parts
Cutaneous membrane (skin)
Accessory structures

The cutaneous membrane has two components
Outer epidermis
Superficial epithelium (epithelial tissues)
Inner dermis
Connective tissues

Accessory Structures
Originate in the dermis
Extend through the epidermis to skin surface
Hair
Nails
Multicellular exocrine glands

Connections
Cardiovascular system
Blood vessels in the dermis
Nervous system
Sensory receptors for pain, touch, and temperature

Subcutaneous layer (superficial fascia or hypodermis)
Loose connective tissue
Below the dermis
Location of hypodermic injections

Functions of Skin
Protects underlying tissues and organs
Excretes salts, water, and organic wastes (glands)
Maintains body temperature (insulation and evaporation)
Synthesizes vitamin D3
Stores lipids
Detects touch, pressure, pain, and temperature
Epidermis
Epidermis is
Avascular stratified squamous epithelium
Nutrients and oxygen diffuse from capillaries in the dermis

Cells of the Epidermis
Keratinocytes
Contain large amounts of keratin
The most abundant cells in the epidermis

Thin Skin
Covers most of the body
Has four layers of keratinocytes
Thick Skin
Covers the palms of the hands and soles of the feet
Has five layers of keratinocytes

Structures of the Epidermis
The five strata of keratinocytes in thick skin
From basal lamina to free surface
Stratum germinativum
Stratum spinosum
Stratum granulosum
Stratum lucidum
Stratum corneum
Stratum Germinativum
The “germinative layer”
Has many germinative (stem) cells or basal cells
Is attached to basal lamina by hemidesmosomes
Forms a strong bond between epidermis and dermis
Forms epidermal ridges (e.g., fingerprints)
Dermal papillae (tiny mounds)
Increase the area of basal lamina
Strengthen attachment between epidermis and dermis
Specialized Cells of Stratum Germinativum
Merkel cells
Found in hairless skin
Respond to touch (trigger nervous system)
Melanocytes
Contain the pigment melanin
Scattered throughout stratum germinativum

Stratum Spinosum
The “spiny layer”
Produced by division of stratum germinativum
Eight to ten layers of keratinocytes bound by desmosomes
Cells shrink until cytoskeletons stick out (spiny)
Continue to divide, increasing thickness of epithelium
Contain dendritic (Langerhans) cells, active in immune response
Stratum Granulosum
The “grainy layer”
Stops dividing, starts producing
Keratin:
a tough, fibrous protein makes up hair and nails
Keratohyalin : dense granules cross-link keratin fibers

Cells of Stratum Granulosum
Produce protein fibers
Dehydrate and die
Create tightly interlocked layer of keratin surrounded by keratohyalin

Stratum Lucidum
The “clear layer”
Found only in thick skin
Covers stratum granulosum
Stratum Corneum
The “horn layer”
Exposed surface of skin
15 to 30 layers of keratinized cells
Water resistant
Shed and replaced every 2 weeks

Keratinization
The formation of a layer of dead, protective cells filled with keratin
Occurs on all exposed skin surfaces except eyes
Skin life cycle
It takes 15–30 days for a cell to move from stratum germinativum to stratum corneum
Perspiration
Insensible perspiration
Interstitial fluid lost by evaporation through the stratum corneum
Sensible perspiration
Water excreted by sweat glands
Dehydration results: from damage to stratum corneum (e.g., burns and blisters [insensible perspiration]) from immersion in hypertonic solution (e.g., seawater [osmosis])
Hydration
Results from immersion in hypotonic solution (e.g., freshwater [osmosis])
Causes swelling of epithelial cells, evident on the palms and soles
Skin Color
Skin color is influenced by
Two pigments
Carotene:
orange-yellow pigment found in orange vegetables accumulates in