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