Integumentary System - Chapter 6

Integumentary System - Chapter 6 (Dr. Christiné Rigsby, Ph.D.)

Introduction (Page 2)

  • Integumentary system: Consists of the skin and its accessory organs, which include:
    • Hair
    • Nails
    • Cutaneous glands
    • Sensory receptors
  • Physical Exam Significance: Inspection of the skin, hair, and nails is a crucial part of a physical examination.
  • Vulnerability of Skin: The skin is the most vulnerable organ, constantly exposed to radiation, trauma, infection, and injurious chemicals. Consequently, it receives more medical treatment than any other organ system.
  • Dermatology: The scientific study and medical treatment of the integumentary system.

Structure of the Skin (Pages 3-5)

  • The integumentary system includes various structures visible in figures, such as:
    • Sensory nerve fibers
    • Apocrine sweat gland
    • Piloerector muscle
    • Lamellar (pacinian) corpuscle (pressure receptor)
    • Hair bulb
    • Motor nerve fibers
    • Cutaneous blood vessels
    • Hypodermis (subcutaneous fat)
    • Epidermis
    • Merocrine sweat gland
    • Hair receptor
    • Dermal papilla
    • Blood capillaries
    • Hair follicle
    • Sebaceous gland
    • Hairs
    • Sweat pores
    • Dermis
    • Tactile corpuscle (touch receptor)
  • Layered Structure: Sectional views show the distinct layers: Epidermis, Papillary Dermis, Reticular Dermis. Key epidermal layers from superficial to deep are Stratum Corneum, Stratum Lucidum, Stratum Granulosum, Stratum Spinosum, and Stratum Basale.
  • Dermal-Epidermal Junction: Relationship among epidermal ridges, dermal papillae, and epidermal pegs, which increase surface area and enhance grip, contribute to fingerprints and house tactile corpuscles.

The Skin and Subcutaneous Tissue (Pages 6-7)

  • Largest Organ: The body's largest and heaviest organ, also known as the cutaneous membrane.
    • Covers an area of 1.51.5 to 2.0extm22.0 ext{ m}^2.
    • Accounts for 7−15%7-15\% of body weight.
    • Most skin is 11 to 2extmm2 ext{ mm} thick.
  • Two Primary Layers:
    • Epidermis: The superficial layer, composed of stratified squamous epithelium.
    • Dermis: The deeper layer, a connective tissue layer.
  • Subcutaneous Layer (Hypodermis): Another connective tissue layer located below the dermis.
  • Thick Skin: Found on the palms and soles, and corresponding surfaces of fingers and toes.
    • Has sweat glands but no hair follicles or sebaceous (oil) glands.
    • Epidermis is approximately 0.5extmm0.5 ext{ mm} thick.
  • Thin Skin: Covers the rest of the body.
    • Epidermis is about 0.1extmm0.1 ext{ mm} thick.
    • Possesses hair follicles, sebaceous glands, and sweat glands.

Functions of the Skin (Page 8)

  1. Resistance to Trauma and Infection:
    • Keratin: Provides structural toughness to the epidermis.
    • Acid Mantle: A protective acidic film on the skin surface that inhibits bacterial growth.
  2. Other Barrier Functions:
    • Waterproofing: Prevents excessive water loss from the body.
    • UV Radiation: Shields underlying tissues from harmful ultraviolet rays.
    • Harmful Chemicals: Acts as a barrier against various injurious chemicals.
  3. Vitamin D Synthesis:
    • The skin performs the first step in Vitamin D synthesis upon exposure to UV light.
    • The liver and kidneys complete the conversion process to its active form (calcitriol).
  4. Sensation: The skin is our most extensive sense organ, containing various receptors for touch, pressure, temperature, and pain.
  5. Thermoregulation:
    • Thermoreceptors: Detect changes in body temperature.
    • Vasoconstriction/Vasodilation: Regulates blood flow to the skin, aiding in heat retention or dissipation.
  6. Nonverbal Communication: Skin features like acne, birthmarks, or scars, and expressions can convey information.
  7. Transdermal Absorption: Allows for the administration of certain drugs steadily through thin skin via adhesive patches.

The Epidermis (Page 9)

  • Description: A keratinized, stratified squamous epithelium.
  • Composition: Consists of dead cells at the surface that are packed with a tough protein called keratin.
  • Vascularity: Lacks blood vessels, relying on the diffusion of nutrients from the underlying connective tissue (dermis).
  • Innervation: Contains sparse nerve endings primarily for touch and pain sensation.

Cells of the Epidermis (Pages 10-12)

There are five types of cells found in the epidermis:

  1. Stem Cells:
    • Undifferentiated cells that give rise to keratinocytes.
    • Located in the deepest layer of the epidermis, specifically the stratum basale.
  2. Keratinocytes:
    • The great majority of epidermal cells.
    • Responsible for synthesizing keratin, the tough fibrous protein.
  3. Melanocytes:
    • Occur only in the stratum basale.
    • Synthesize the pigment melanin, which shields DNA from ultraviolet radiation.
    • Possess branched processes that spread among keratinocytes to distribute melanin.
  4. Tactile (Merkel) Cells:
    • Located in the basal layer of the epidermis.
    • Function as touch receptor cells, associated with dermal nerve fibers.
  5. Dendritic (Intraepidermal Macrophage or Langerhans) Cells:
    • Macrophages originating in the bone marrow that guard against pathogens.
    • Primarily found in the stratum spinosum and stratum granulosum.
    • Their role is to stand guard against toxins, microbes, and other pathogens that penetrate the skin.

Layers of the Epidermis (Pages 13-14)

From superficial to deep, the layers of the epidermis must be known in order:

  1. Stratum Corneum (Surface)
  2. Stratum Lucidum (Only in thick skin)
  3. Stratum Granulosum
  4. Stratum Spinosum
  5. Stratum Germinativum (Basale) (Basal layer)

Stratum Basale (Page 15)

  • Location: Innermost layer of the epidermis, also known as the stratum germinativum.
  • Epidermal Ridges: Forms epidermal ridges that extend into the dermis. The contours of the skin, including genetically determined fingerprints (which do not change), follow these ridge patterns. Ridges increase surface area and friction, improving grip.
  • Cell Types:
    • Basal Cells (Germinative Cells): Stem cells that undergo continuous mitotic divisions.
    • Merkel Cells: Tactile cells found on skin surfaces that lack hair; sensitive to touch.
    • Melanocytes: Produce melanin, which creates brown tones in the skin.
  • Cellular Junctions:
    • Desmosomes: Connect keratinocytes within the layer.
    • Hemidesmosomes: Anchor cells to the basement membrane, connecting the epidermis to the dermis.
  • Mitotic Rate: Keratinocytes in this layer undergo mitotic divisions approximately every 1919 days.

Stratum Spinosum (Page 16)

  • Location: Superficial to the stratum basale.
  • Cell Layers: Consists of 8−108-10 layers of keratinocytes.
  • Cell Origin: Cells migrate from the stratum basale as that layer divides.
  • Mitosis: Only the deep layers of the stratum spinosum undergo mitosis.
  • Keratin Production: Cells in this layer begin to produce abundant keratin, a tough, fibrous protein that is also a component of hair and nails. As keratin develops, cells grow thinner and flatter, contributing to the water barrier.
  • Langerhans (Dendritic) Cells: Cells of the immune system found in this layer. They stimulate defense mechanisms against microorganisms and skin cancers.

Stratum Granulosum (Page 17)

  • Location: Superficial to the stratum spinosum.
  • Cell Layers: Composed of 2−52-5 layers of non-dividing keratinocytes.
  • Keratohyalin Granules: Contains protein granules of keratohyalin, which accumulate in the cytoplasm.
  • Cellular Degeneration: In the superficial layers, cellular organelles degenerate, leaving behind keratin fibers and keratohyalin.
  • Lamellar Granules: These granules help to form the water-repellent barrier of the skin.

Stratum Lucidum and Corneum (Page 18)

Stratum Lucidum
  • Location: ONLY found in thick skin (e.g., fingertips, palms, and soles).
  • Appearance: Covers the stratum granulosum and appears transparent due to the loss of cellular organelles and dispersal of keratohyalin.
Stratum Corneum
  • Location: The exposed surface layer in both thick and thin skin.
  • Composition: Consists of 15−3015-30 layers of keratinized cells.
  • Keratinization (Cornification): The process of forming protective cells filled with keratin.
  • Keratin Types: Distinguishes between soft keratin (found in the stratum corneum of the skin) and hard keratin (found in hair and nails).

Stratum Corneum Details (Page 19)

  • Cell Transit Time: Takes 15−3015-30 days for a cell to move through all epidermal layers to reach the stratum corneum.
  • Cell Shedding: Cells are shed in large groups or sheets, a process called exfoliation.
  • Protective Layer: The dead cells in this layer provide a protective barrier for the live cells beneath.
  • Water Resistance: This layer is water-resistant but not waterproof.
  • Insensible Perspiration (Transpiration): Water continuously evaporates from the surface, resulting in a loss of approximately 500extmL500 ext{ mL} of water per day.
  • Sensible Perspiration: Water produced by actively secreting sweat glands.
  • Damage to Stratum Corneum:
    • Causes increased water loss because its barrier function is compromised.
    • The rate of insensible perspiration rises dramatically.
    • This is a significant consequence of severe burns and xerosis (dry skin).

Epidermal Layers and Keratinization Process (Page 20)

Summary of cell journey and changes:

  1. Stratum Basale: Cells divide by mitosis; newly formed cells migrate superficially.
  2. Stratum Spinosum: Keratin fibers and lamellar bodies accumulate.
  3. Stratum Granulosum: Keratohyalin granules accumulate, a hard protein envelope forms beneath the plasma membrane, lamellar bodies release lipids; cells begin to die.
  4. Stratum Lucidum: (Only in thick skin) Cells are dead and contain dispersed keratohyalin.
  5. Stratum Corneum: Dead cells have a hard protein envelope, contain keratin, and are surrounded by lipids.

Water Barrier and Exfoliation (Page 21)

  • Epidermal Water Barrier: Forms between the stratum granulosum and stratum spinosum.
  • Components of the Water Barrier:
    • Lipids secreted by keratinocytes.
    • Tight junctions between keratinocytes.
    • A thick layer of insoluble protein on the inner surfaces of the keratinocyte plasma membranes.
  • Significance: Critical for retaining water in the body and preventing dehydration.
  • Cell Death: Cells above the water barrier quickly die because they are cut off from nutrients below.
  • Exfoliation: Dead cells continuously exfoliate as dander.
  • Dandruff: Clumps of dander stuck together by sebum (oil).

Epidermis and Vitamin D (Page 22)

  • Vitamin D (Cholecalciferol) Synthesis:
    • Synthesized by cells in the stratum spinosum or stratum germinativum when exposed to UV light (sunlight).
    • It is a cholesterol-related steroid.
  • Activation Process:
    • The liver converts vitamin D into an intermediate form.
    • The kidneys then use this intermediate to produce calcitriol, the active form of Vitamin D.
  • Importance of Calcitriol:
    • Essential for the normal absorption of calcium and phosphorus by the small intestine.
    • Required for proper bone maintenance and growth.
    • Deficiency can lead to rickets, a condition of bone deformities.

The Dermis (Pages 23-24)

  • Description: A connective tissue layer situated beneath the epidermis.
  • Thickness: Ranges from 0.2extmm0.2 ext{ mm} (eyelids) to 4extmm4 ext{ mm} (palms, soles).
  • Composition: Primarily composed of collagen, with additional elastic fibers, reticular fibers, and fibroblasts.
  • Vascularity and Innervation: Well-supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings.
  • Accessory Organs: Hair follicles and nail roots are embedded within the dermis.
  • Piloerector Muscles: Smooth muscle associated with hair follicles. These muscles contract in response to stimuli such as cold, fear, and touch, causing hair to stand erect (producing