Integumentary System: Structure and Function of the Skin, Hair, and Nails

5.1 The Skin Consists of Two Layers: The Epidermis and Dermis

Overview of Skin Structure

The skin is the largest organ of the human body and consists of two main layers: the epidermis and the dermis. Beneath these layers lies the subcutaneous tissue (hypodermis), which is not technically part of the skin but supports its functions.

• Epidermis: The outermost layer, composed primarily of keratinized stratified squamous epithelium. It provides a protective barrier against environmental hazards.

• Dermis: The deeper, thicker layer made of connective tissue. It houses blood vessels, nerves, hair follicles, and glands.

• Subcutaneous tissue (Hypodermis): Consists mainly of adipose (fat) tissue and areolar connective tissue, providing insulation and cushioning.

Vascularization: The dermis is vascularized, while the epidermis is avascular and receives nutrients via diffusion from the dermis.

Key Point: The subcutaneous tissue is primarily composed of adipose tissue, which stores fat and anchors the skin to underlying structures.

5.2 The Epidermis: A Keratinized Stratified Squamous Epithelium

Composition and Layers of the Epidermis

The epidermis is made up of keratinized stratified squamous epithelium. It is organized into several distinct layers, each with specialized functions.

• Stratum basale: The deepest layer; site of mitosis and new cell production. Contains melanocytes and tactile cells.

• Stratum spinosum: Several layers of keratinocytes; contains dendritic (Langerhans) cells for immune defense.

• Stratum granulosum: Cells accumulate keratohyalin granules; keratinization begins here.

• Stratum lucidum: Present only in thick skin (palms, soles); thin, translucent layer.

• Stratum corneum: Outermost layer; consists of dead, keratinized cells that are continuously shed.

Thick vs. Thin Skin: Thick skin (palms, soles) has all five layers, while thin skin lacks the stratum lucidum.

Cell Types: The main cell type is the keratinocyte, which produces keratin, a protein that strengthens the skin. Melanocytes produce melanin, and dendritic cells provide immune protection.

5.3 The Dermis: Papillary and Reticular Layers

Structure and Function of the Dermis

The dermis is a strong, flexible connective tissue layer beneath the epidermis. It is divided into two regions:

• Papillary layer: Thin, superficial layer made of areolar connective tissue. Contains dermal papillae, which form fingerprints and increase surface area for nutrient exchange.

• Reticular layer: Thicker, deeper layer composed of dense irregular connective tissue. Provides strength and elasticity to the skin.

Connective Tissue Types: The papillary layer contains loose areolar connective tissue, while the reticular layer contains dense irregular connective tissue.

Fingerprints: Friction ridges formed by dermal papillae create fingerprints, which enhance grip.

5.4 Melanin, Carotene, and Hemoglobin Determine Skin Color

Factors Affecting Skin Color

Skin color is determined by three main pigments:

• Melanin: Produced by melanocytes in the stratum basale; provides a natural sunscreen by absorbing UV radiation. Responsible for brown to black hues.

• Carotene: A yellow-orange pigment from plant sources (e.g., carrots); accumulates in the stratum corneum and subcutaneous fat.

• Hemoglobin: The red pigment in blood; gives a pinkish hue to fair skin due to the transparency of the epidermis.

Clinical Significance: Changes in skin color can indicate disease states:

Changes to skin color	Possible cause
Blueness (cyanosis)	Low oxygenation of hemoglobin
Redness (erythema)	Fever, hypertension, inflammation, allergy
Brown or black patches	Hormonal imbalance, bruising
White (pallor)	Anemia, low blood pressure, fear, anger
Yellow (jaundice)	Liver disorder (bilirubin accumulation)

5.5 Hair: Structure, Function, and Color

Hair Follicle Anatomy and Function

Hair is composed of dead, keratinized cells and grows from follicles in the dermis. Each hair follicle has several parts:

• Hair bulb: The base of the follicle where hair growth occurs.

• Arrector pili muscle: Smooth muscle that contracts to cause "goosebumps" and elevate the hair.

• Hair root plexus: Sensory nerve endings that detect hair movement.

Hair Color: Determined by the amount and type of melanin produced by melanocytes in the hair bulb. More eumelanin results in black or brown hair; pheomelanin produces red or blonde hair.

Hair Growth: Hair grows in cycles: anagen (growth), catagen (regression), and telogen (resting). With age, terminal hairs may be replaced by vellus (fine) hairs.

5.6 Nails: Scale-like Modifications of the Epidermis

Nail Structure and Growth

Nails are protective coverings on the dorsal surface of the fingers and toes. They are composed of hard keratin.

• Nail plate: The visible, hard part of the nail.

• Nail bed: The skin beneath the nail plate.

• Lunula: The whitish, crescent-shaped area at the base of the nail.

• Nail matrix: The region responsible for nail growth.

• Cuticle (eponychium): The fold of skin at the base of the nail.

Appearance: The lunula appears white due to the thickened underlying matrix obscuring blood vessels.

5.7 Sweat and Oil Glands: Structure, Location, and Function

Types of Skin Glands

The skin contains two main types of exocrine glands: sweat (sudoriferous) glands and sebaceous (oil) glands.

Description	Eccrine Sweat Glands	Sebaceous Glands
Particularly abundant in the palms, soles, and forehead	X
Function to lubricate the skin and hairs		X
Involved in thermoregulation	X
Secrete sebum		X
Also called oil glands		X
Duct typically ends in the upper parts of hair follicles		X
Secrete sweat, a hypotonic filtrate of blood	X
Duct ends at the skin surface	X
Also called sudoriferous glands	X
Found everywhere except the palms and soles		X

Eccrine vs. Apocrine Sweat Glands:

• Eccrine glands: Widely distributed, especially on palms, soles, and forehead; secrete watery sweat for thermoregulation.

• Apocrine glands: Found in axillary and anogenital areas; secrete a thicker, milky sweat that may produce body odor when broken down by bacteria.

Similarities: Both are types of sweat glands and help in excretion and temperature regulation.

Differences: Eccrine glands open directly onto the skin surface, while apocrine glands open into hair follicles and become active at puberty.

5.8 Functions of the Skin

Protective and Regulatory Roles

The skin performs multiple vital functions:

• Barrier function: Protects against mechanical, chemical, and biological threats.

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

• Sensory reception: Contains receptors for touch, pain, temperature, and pressure.

• Metabolic functions: Synthesizes vitamin D, which is essential for calcium absorption in the digestive tract.

• Excretion: Eliminates wastes such as urea and salts through sweat.

Types of Barriers:

• Chemical: Acid mantle, defensins, melanin

• Physical: Intact epidermis, keratin

• Biological: Dendritic cells, dermal macrophages

Cutaneous Receptors:

• Free nerve endings: Detect pain and temperature

• Tactile (Meissner's) corpuscles: Detect light touch

• Lamellar (Pacinian) corpuscles: Detect deep pressure and vibration

• Hair follicle receptors: Detect hair movement

Vitamin D Synthesis: The skin initiates the synthesis of vitamin D when exposed to UV light, which is necessary for calcium absorption in the intestines.