Lecture Outline: The Integumentary System and Membranes

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  1. Introduction to the Integumentary System and Membranes
    1. Chapter focus: Integumentary system and membranes, as not all membranes are part of the integumentary system.
    2. The term "integument" means covering.
    3. The Integumentary System primarily includes the skin, hair, and nails.
    4. The skin is a multicellular membrane, also known as the cutaneous membrane.
    5. The skin is the largest and heaviest organ of the body.
    6. Biological Hierarchy: Cells form tissues, tissues form organs. The skin is an organ composed of different tissue types.
    7. The skin's position is primarily superficial, serving as a barrier between the body and the environment.
  2. Types of Membranes
    1. Cutaneous Membrane (Skin)
      1. The skin is a dry membrane, unlike mucous membranes.
      2. The outermost epidermis consists of dead cells.
    2. Mucous Membranes (Mucosa)
      1. Line internal hollow structures that open to the outside of the body.
      2. Examples include the entire alimentary canal (mouth to anus), vagina, and trachea.
      3. Produce mucus, a thick, viscous, and slippery fluid.
      4. Function: Lubrication (e.g., for food during mastication and deglutition) and protection.
      5. These membranes remain moist due to continuous mucus production.
    3. Serous Membranes (Serosa)
      1. Adjective of "serum," meaning a watery, thin fluid.
      2. Found in places where an organ moves considerably against its surroundings.
      3. Function: Allow for movement of structures without damage from abrasion.
      4. Each membrane is a single membrane folded back on itself, forming two layers:
        1. Visceral layer: The deeper layer, directly in contact with the organ.
        2. Parietal layer: The more superficial layer, forming a wall with the surroundings.
      5. The small space between the two layers is filled with serous fluid, produced by the membrane itself.
      6. Examples:
        1. Pericardium: Surrounds the heart.
        2. Pleura: Surrounds each lung.
      7. Analogy: A fist pushed into a partially water-filled balloon.
    4. Synovial Membranes
      1. Line fluid-filled spaces in highly movable joints (e.g., knee, ankle).
      2. The fluid-filled space is called the synovial cavity.
      3. The fluid within is synovial fluid, which has the consistency of raw egg white and is very lubricating (thicker than serous fluid).
      4. Part of the membrane is formed by hyaline cartilage on the bone surfaces, providing cushioning and slipperiness.
  3. Functions of the Integumentary System
    1. Protection from Mechanical Damage
      1. The skin is a tough, pliable, self-healing material.
      2. Toughness is due to keratin, a fibrous protein found in skin cells (soft keratin).
      3. Outer epidermal cells become cornified (hardened, horny).
      4. A layer of adipose tissue (fat cells) deep to the skin provides cushioning.
      5. Sensory receptors in the skin detect mechanical stimuli (e.g., touch, pressure).
      6. Pain receptors alert the body to potential harm.
    2. Protection from Chemical Damage
      1. The skin is resistant to moderate changes in pH (acids and bases).
    3. Protection from Micro Damage (Infection)
      1. Acts as an effective physical barrier against microorganisms.
      2. Produces the acid mantle: A thin layer of acidic and salty solution that inhibits bacterial growth.
      3. Contains phagocytes (e.g., dendritic cells) that engulf and digest foreign invaders.
      4. Historical note: Simple cuts were routinely fatal before antibiotics (e.g., penicillin in the 1940s).
    4. Protection from Ultraviolet (UV) Radiation
      1. UV radiation can damage DNA, leading to mutations and cancer.
      2. The skin produces melanin, a pigment that absorbs UV light.
      3. Melanin molecules cluster on the superficial side of the cell's nucleus, acting as a parasol.
      4. Skin cells also have mechanisms to repair DNA damage.
    5. Protection from Thermal Damage
      1. Sensory receptors in the skin detect excessively hot or cold temperatures, prompting withdrawal.
    6. Protection from Desiccation (Drying Out)
      1. Deeper body tissues must remain moist for chemical reactions (life) to occur.
      2. The skin is largely waterproof due to glycolipids secreted between epidermal cells.
      3. The epidermis, with its tightly packed cells, acts like "bricks and mortar."
    7. Thermoregulation (Maintenance of Body Temperature)
      1. A critical aspect of homeostasis.
      2. Sweating:
        1. Sweat glands in the skin produce sweat (mostly water and salt).
        2. Evaporation of sweat from the skin surface removes a significant amount of heat from the body, leading to cooling.
        3. Evaporation cools because it removes the fastest (highest energy) molecules from the liquid surface.
      3. Blood Flow Adjustment:
        1. When the body is too hot, blood flow to the skin increases (flushing), allowing heat to be lost conductively to the environment.
        2. When the body is too cold, blood flow to the skin decreases (paling), conserving heat within the body's core.
    8. Excretion (minor function)
      1. Sweat contains small amounts of urea and uric acid, which are nitrogen-containing waste products.
      2. The liver converts toxic ammonia into urea, which is primarily filtered by the kidneys.
    9. Synthesis of Vitamin D
      1. Vitamin D is unique among vitamins as the body can manufacture it (though dietary intake is also important).
      2. Process involves the skin and UV radiation:
        1. Cholesterol in skin cells is converted by UV light into cholecalciferol.
        2. Cholecalciferol travels to the liver, where it's converted to calcidiol.
        3. Calcidiol travels to the kidneys, where it's converted to calcitriol (the active form of vitamin D).
        4. Calcitriol then acts on the small intestines to enhance the absorption of calcium.
      3. Vitamin D is crucial for calcium absorption, which is vital for bone health and many other bodily functions.
  4. Anatomy of the Skin and Accessory Structures
    1. Layers of the Skin
      1. Epidermis:
        1. The most superficial layer, composed of epithelial tissue.
        2. Thinner than the dermis.
        3. Classified as a stratified squamous epithelium (many layers, flat cells at the apical/free surface).
        4. Cells are continuously produced at the basal layer and migrate superficially, eventually dying and shedding.
        5. Stratified layers (strata), from deep to superficial:
          1. Stratum Basale (Basily): Deepest layer, cuboidal cells, undergoes frequent mitosis to produce new cells.
          2. Contains two main cell types:
            1. Keratinocytes: Most abundant, produce keratin, give skin strength.
            2. Melanocytes: Produce melanin, which is transferred to keratinocytes via phagocytosis of their extensions, protecting DNA from UV.
          3. Stratum Spinosum: Cells appear "spiny" due to desmosome connections when prepared for microscopy. Keratinocytes continue producing keratin fibers.
          4. Stratum Granulosum: Cells produce granules (proteins that strengthen keratin network) and lamellar bodies (lipid-rich for waterproofing).
          5. Stratum Corneum: Outermost layer, composed of hardened, dead, flattened cells called corneocytes. It is highly stratified and waterproof.
          6. Stratum Lucidum: An additional, clear stratum found only in thick skin (palms, soles), located between the stratum granulosum and stratum corneum.
        6. Other epidermal cells:
          1. Merkel cells: Sensory receptors, superficially placed and sensitive to light touch.
          2. Dendritic cells: Immune cells that surveil the skin, engulf foreign material, and present it to white blood cells.
      2. Dermis:
        1. Deep to the epidermis, primarily composed of connective tissue.
        2. Much thicker than the epidermis.
        3. Subdivided into two layers:
          1. Papillary layer: The superficial part of the dermis, characterized by dermal papillae (finger-like projections) that interlock with the epidermis. These papillae contain capillaries that supply blood to the living epidermal cells.
          2. Reticular layer: The deeper part of the dermis, named for its dense network of collagen and elastic fibers oriented in all directions, providing strength.
        4. Contains hair follicles, glands, sensory receptors, and blood vessels.
      3. Hypodermis:
        1. Located below the dermis, but not technically part of the skin.
        2. Composed mostly of adipose tissue (fat cells).
        3. Functions as thermal insulation and cushioning.
        4. Major blood vessels are located deep in the dermis, superficial to the hypodermis, to allow for effective heat shedding when needed.
    2. Hair
      1. The visible part is the hair shaft; the living part is within the hair follicle (located in the dermis).
      2. Composed of three concentric layers from deep to superficial:
        1. Medulla (deepest).
        2. Cortex.
        3. Cuticle (most superficial): Features overlapping, shingle-like structures that help anchor the hair within the follicle.
      3. Hair is much tougher than skin due to the presence of hard keratin.
      4. The hair papilla within the follicle contains a rich blood supply for cell division and hair growth.
      5. Visible hair is dead, which is why cutting it does not cause pain.
      6. Erector Pili Muscle: Tiny muscles attached to hair follicles in the dermis.
        1. Contraction pulls the hair upright (erects pili).
        2. This action is called horripilation and can trap a thicker layer of insulating air, providing warmth.
    3. Glands in the Skin
      1. Sweat Glands:
        1. Eccrine (Merocrine) Sweat Glands: Widespread across the skin, produce watery sweat for thermoregulation via evaporation.
        2. Apocrine Sweat Glands: Located mainly in the axillae (armpits) and perineum; produce an odorless solution that bacteria break down to create body odor.
      2. Ceruminous Glands: Produce cerumen (ear wax) in the ear canal.
      3. Sebaceous Glands: Produce sebum (body oil), usually associated with hair follicles, to lubricate hair and skin.
    4. Nails
      1. Part of the integumentary system; the toughest of the three major structures.
      2. Contain significantly more hard keratin than hair.
      3. Free edge: The part of the nail that is cut.
      4. Nail bed: The skin beneath the main body of the nail.
      5. The visible nail is dead.
      6. Nail matrix: The living part of the nail, located under a fold of skin, where cell division occurs to produce new nail cells that are pushed distally.
      7. Cuticle: A hardened piece of skin near the base of the visible nail.
      8. Lunula: The white, crescent-shaped area at the base of the nail, meaning "little moon."
      9. Nails serve as important tools.