lecture 20

Human Anatomy and Physiology I

The Integumentary System Overview

  • Instructor: Dr. Shaun D. Cain
  • Course Code: Biol 231
  • Semester: Fall 2025
  • Institution: Eastern Oregon University

Skin Structure

  • Skin Overview

    • Accounts for 10-15% of total body weight.
    • Largest organ in the body; complex organ with many functions important for homeostasis.

  • Components of the Cutaneous Membrane (Skin):

    1. Epidermis
    • Superficial layer.
    • Composed of keratinized stratified squamous epithelium resting on basement membrane.
    1. Dermis
    • Lies deep to epidermis and the basement membrane.
    • Composed of loose connective tissue and dense irregular connective tissue.

  • Accessory Structures

    • Embedded in the cutaneous membrane include:
    • Sweat glands
    • Sebaceous glands
    • Hair
    • Nails

  • Sensory Receptors

    • Skin contains sensory receptors and arrector pili muscles (small bands of smooth muscle associated with hair).

  • Epidermis is Avascular

    • Relies on diffusion of oxygen and nutrients from blood vessels in deeper dermis; limits epidermal thickness due to lack of direct blood supply.
    • Approximately 50% of epidermal cells are too far from adequate blood supply to sustain life; therefore, the superficial layers contain only dead cells.

  • Hypodermis

    • Also known as superficial fascia or subcutaneous fat, located deep to dermis.
    • Not considered part of the skin; anchors skin to deeper structures such as muscle and bone.
    • Composed of loose connective and adipose tissues; contains abundant blood supply.

Functions of the Integumentary System

  • Overview of Functions

    • Integumentary system functions are critical for protecting underlying organs and maintaining homeostasis.

  • Protection

    • Protects from:
    • Mechanical trauma
      • Stratified squamous, keratinized epithelium provides a durable yet flexible surface that protects the body against mechanical trauma (e.g., stretching, pressure, abrasions).
    • Pathogens and Environment
      • Acts as a barrier to invasion by microorganisms; contains immune cells that eliminate pathogens before deeper tissue invasion.
      • Glands secrete antimicrobial substances (sebaceous gland secretions give skin slightly acidic pH, referred to as acid mantle, inhibiting pathogen growth).
    • Protection from environmental hazards, including ultraviolet (UV) light absorption.
    • Hydrophobic lipid-based chemicals secreted by skin repel ionic (salt) and polar covalent (water) molecules; these chemicals are crucial for maintaining water and electrolyte homeostasis under varying weather conditions.

  • Sensation

    • Enables the nervous system to perceive changes in internal and external surroundings, which is critical to homeostasis.
    • Skin is equipped with numerous sensory receptors that detect changes in the environment, including potentially harmful stimuli like heat, cold, and pain (which can lead to tissue damage).

  • Thermoregulation

    • Relies on negative feedback loops for the maintenance of stable internal body temperature.
    • Example of Feedback Loops:
    • Control of Body Temperature
      • Internal body temperature is mainly determined by muscle activity and chemical reactions (metabolism).
      • Stimulus: Body temperature increases above normal range.
      • Receptor: Thermoreceptors detect increase in skin and body temperature.
      • Control Center: Thermoregulatory center in the brain receives signals from receptors.
      • Effector/Response:
      • Blood vessels in dermis dilate to release heat.
      • Sweat glands release sweat.
      • As body temperature returns to normal, feedback mechanisms reduce effector responses.
    • When body temperature decreases below normal range, the following occurs:
      • Thermoreceptors detect decrease in skin and body temperature.
      • Blood vessels in the dermis constrict to conserve heat.

  • Excretion

    • Refers to the elimination of waste products and toxins from the body.
    • While most excretion occurs through other organs like the kidneys, skin and its accessory structures contribute significantly.

  • Vitamin D Synthesis

    • Skin plays a crucial role in synthesizing Vitamin D:
    • A modified cholesterol molecule (precursor) is converted to cholecalciferol through exposure of the epidermis to UV radiation.
    • Cholecalciferol is released into the bloodstream and modified first by the liver and then by the kidneys to form calcitriol, the active form of Vitamin D required for calcium ion absorption from the small intestine.
    • Calcium ions are critical for:
      • Nerve function
      • Muscle contraction
      • Building and maintaining bone tissue; many other physiological functions.

The Epidermis

  • Overview of Epidermis

    • The most superficial layer of the skin, composed of several cell types.

  • Keratinocytes

    • Comprising about 95% of the epidermis; have structural features that enhance strength and resistance to mechanical trauma.
    • Function: Manufacture keratin, a tough fibrous protein, linked by desmosomes (intercellular junctions that hold cells together).

  • Epidermal Strata (Layers)

    • Superficial to Deep:
    1. Stratum Corneum
      • Outermost layer, featuring several layers of dead, flattened keratinocytes with thick plasma membranes filled with keratin bundles.
      • Cells are sloughed off or exfoliated mechanically as desmosomes weaken.
    2. Stratum Lucidum
      • A narrow layer of clear, dead keratinocytes, only present in thick skin.
    3. Stratum Granulosum
      • Comprises three to five layers of cells with prominent cytoplasmic granules containing keratin bundles or lipid-based substances, secreted through exocytosis.
      • Hydrophobic lipids provide waterproofing; this function helps to maintain internal fluid and electrolyte homeostasis, leading to cell isolation and death in this layer and its superficial counterparts.
    4. Stratum Spinosum
      • The thickest layer located above stratum basale and remains close to blood supply, making it metabolically and mitotically active.
    5. Stratum Basale (Stratum Germinativum)
      • A single layer of stem cells resting on the basement membrane.
      • Closest to the dermal blood supply, it is the most metabolically and mitotically active strata responsible for Vitamin D synthesis and the replacement of dead keratinocytes from the superficial layers.

  • Study Boost: Remembering the Strata of the Epidermis

    • Memory Aid: “Brilliant Studying Gives Loads of Confidence”.
    • The “B” in “Basale” is regarded as the bottom layer.

  • Keratinocyte Life Cycle

    • Due to the physical and environmental stresses on the epidermis, keratinocytes must be continuously replaced to maintain integrity.
    • Dead keratinocytes are replenished via mitosis in stratum basale and spinosum, where blood supply is available.
    • As keratinocytes divide, they push cells above into more superficial layers.
    • It takes approximately 40-50 days for this migration from the deepest strata to stratum corneum.
    • Example Process:
      • Suppose the skin has only one row of cells in each epidermal stratum: Cell A undergoes mitosis, producing daughter cell B that enters the stratum spinosum.
      • The other daughter cell continues to divide, moving cell B into stratum granulosum, where it becomes coated in lipid-based substances, causing cell death due to loss of blood supply as it moves upward into the stratum lucidum and corneum.
      • Cell B ends its life filled with keratin and is eventually sloughed off from the skin surface.

  • Other Cells of the Epidermis

    • Dendritic (Langerhans) Cells: Located in stratum spinosum; function as phagocytes within the immune system to protect skin and deeper tissues from pathogens.
    • Merkel Cells: Oval-shaped cells scattered in the stratum basale that are sensory receptors associated with small neurons in the dermis. They detect light touch and can discriminate shapes and textures, with higher concentrations found in fingertips, lips, and at the base of hair.
    • Melanocytes: Found in stratum basale; responsible for the production of melanin, a protein pigment that ranges from orange-red to brown-black.

  • Thick and Thin Skin

    • The type of epidermis differs in various body locations to match function:
    • Thick Skin: Equivalent to the thickness of a paper towel; contains all five epidermal layers with a particularly thick stratum corneum; lacks hair follicles but has many sweat glands.
    • Thin Skin: Equivalent to the thickness of a sheet of printer paper; covers areas less subjected to mechanical stress; has only four layers (lacks stratum lucidum) with each layer appearing thinner than those in thick skin. It contains numerous hairs, sweat glands, and sebaceous glands.
    • Callus Formation: Additional layers of stratum corneum form in response to repetitive pressure, occurring in both thick and thin skin.