Module 5.1 Overview of the Integumentary System

Skin Structure

The skin is the largest organ in the body, serving as a complex barrier with multiple vital functions. It is composed of several layers and associated structures that contribute to its protective and regulatory roles.

• Cutaneous Membrane: The skin, also called the cutaneous membrane, has two main components:

  • Epidermis: The superficial layer, composed of stratified squamous epithelium.

  • Dermis: The deeper layer, made of connective tissue.

• Accessory Structures: Embedded in the skin are glands (sebaceous and sweat), hair, and nails.

• Sensory Receptors: The skin contains sensory receptors and arrector pili muscles (small bands of smooth muscle associated with hair follicles).

• Blood Supply: The epidermis is avascular and relies on diffusion from blood vessels in the dermis for oxygen and nutrients, limiting its thickness.

• Cellular Composition: About 50% of epidermal cells are too far from blood supply to survive; the superficial layers are made up of dead cells.

• Hypodermis: Also called the superficial fascia or subcutaneous fat, this layer lies deep to the dermis and consists of loose connective and adipose tissue, providing insulation and energy storage.

Functions of the Integumentary System

The integumentary system protects underlying organs and maintains homeostasis through several key functions:

• Protection: The skin acts as a barrier against mechanical injury, pathogens, and UV radiation. Glands secrete acidic substances to inhibit pathogen growth, and the skin repels water and ions to maintain fluid and electrolyte balance.

• Sensation: Sensory receptors detect changes in the internal and external environment, such as heat, cold, and pain.

• Thermoregulation: The skin helps regulate body temperature through sweat production and blood flow adjustments.

• Excretion: Sweat glands excrete waste products such as salts and urea.

• Vitamin D Synthesis: The skin synthesizes vitamin D when exposed to UV light, which is essential for calcium absorption in the intestines.

Additional info: Calcium ions are critical for bone health, muscle contraction, nerve transmission, and blood clotting.

Module 5.2 The Epidermis

Cell Types in the Epidermis

• Keratinocytes: Make up about 95% of the epidermis. They produce keratin, a protein that strengthens the skin and makes it less susceptible to mechanical trauma.

  • Structural Features:

    1. Keratin filaments provide mechanical strength.

    2. Desmosomes tightly join cells, increasing resistance to stress.

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

• Merkel Cells: Sensory cells that detect light touch, found in specialized regions like fingertips and lips.

• Melanocytes: Produce melanin, the pigment responsible for skin color.

Layers of the Epidermis

Keratinocytes are organized into five distinct layers (from deep to superficial):

1. Stratum basale (germinativum): Single layer of stem cells on the basement membrane; high mitotic activity and vitamin D synthesis.

2. Stratum spinosum: Several layers thick; cells are still close to blood supply and are metabolically active.

3. Stratum granulosum: Middle three to five layers; cells contain keratohyalin granules and lamellar bodies, which secrete lipids for waterproofing.

4. Stratum lucidum: Thin, clear layer found only in thick skin (palms, soles).

5. Stratum corneum: Outermost layer; several layers of dead, flattened keratinocytes.

Additional info: The keratinocyte life cycle involves continuous shedding and replacement, taking about 40-50 days.

Thick vs. Thin Skin

• Thick Skin: Found on palms and soles; contains all five epidermal layers and a thick stratum corneum.

• Thin Skin: Covers the rest of the body; lacks stratum lucidum and has a thinner stratum corneum.

Module 5.3 The Dermis

Structure of the Dermis

The dermis is a connective tissue layer beneath the epidermis, providing structural support and housing blood vessels, nerves, and accessory structures.

• Papillary Layer: Thin, superficial layer of loose connective tissue. Contains dermal papillae, which project into the epidermis and contain capillaries and tactile (Meissner) corpuscles for light touch sensation.

• Reticular Layer: Thicker, deeper layer of dense irregular connective tissue. Contains collagen bundles (for strength), elastic fibers (for flexibility), and lamellated (Pacinian) corpuscles (for pressure/vibration).

• Embedded Structures: Blood vessels, sweat glands, hair follicles, sebaceous glands, and adipose tissue.

Functions of Dermal Components

• Collagen Bundles: Provide tensile strength and prevent overstretching.

• Elastic Fibers: Allow skin to return to its original shape after stretching.

• Proteoglycans: Retain water, keeping the skin hydrated.

Skin Markings

• Dermal Ridges: Found in thick skin (palms, soles); form fingerprints and enhance grip.

• Reticular Layer Markings: Responsible for lines of tension (cleavage lines) and flexure lines.

Module 5.4 Skin Pigmentation

Melanin

• Production: Melanin is produced by melanocytes in the stratum basale from the amino acid tyrosine, catalyzed by the enzyme tyrosinase.

• Functions:

  • Protects DNA from UV radiation.

  • Contributes to skin color.

  • Absorbs UV light, reducing risk of DNA mutations.

• UV Exposure: Increases melanin synthesis, leading to tanning. Immediate effects include darkening; delayed effects involve increased melanin production.

• Distribution: Varies by body region and individual; fewer melanocytes on palms/soles.

Other Pigments Affecting Skin Color

• Carotene: Yellow-orange pigment from diet, accumulates in stratum corneum.

• Hemoglobin: Red pigment in blood, gives skin a pinkish hue when oxygenated.

Skin Color as a Diagnostic Tool

• Erythema: Redness due to increased blood flow (e.g., inflammation, fever).

• Pallor: Paleness due to decreased blood flow (e.g., shock, anemia).

• Cyanosis: Bluish color due to low oxygen levels (e.g., respiratory distress).

Module 5.5 Accessory Structures of the Integument: Hair, Nails, and Glands

Hair

• Functions:

  • Protection (e.g., scalp from UV, eyelashes from debris).

  • Sensory input (hair is associated with sensory neurons).

  • Limited role in thermoregulation in humans.

• Structure: Hair consists of the shaft (above skin) and root (embedded in dermis). Both are made of keratinized epithelial cells.

• Root Sheath: The epithelial root sheath is an infolding of the epidermis; the dermal root sheath is connective tissue.

• Hair Layers:

  • Medulla: Soft core (only in thick hair).

  • Cortex: Several layers of keratinocytes with hard keratin.

  • Cuticle: Single layer of overlapping keratinocytes.

• Arrector Pili Muscle: Small smooth muscle that causes hair to stand up (goosebumps) in response to cold or fear.

• Growth Cycle: Hair grows in cycles: growth stage (active mitosis) and resting stage (no growth).

Nails and Glands

• Nails: Hard structures of keratinized cells protecting the tips of fingers and toes.

• Glands: Include sweat glands (eccrine and apocrine), sebaceous glands (produce sebum), and specialized glands (e.g., mammary, ceruminous).

Summary Table: Layers of the Epidermis

Key Equations and Concepts

• Vitamin D Synthesis:

• Thermoregulation: Heat loss via sweat evaporation and vasodilation; heat conservation via vasoconstriction and reduced sweat.

Example: Clinical Application

• Burns: Damage to the skin can disrupt its protective barrier, leading to fluid loss, infection, and impaired thermoregulation.

• Jaundice: Yellowing of the skin due to bilirubin accumulation, often indicating liver dysfunction.