Integumentary System Anatomy and Physiology

• Integumentary System, Chapter 5, Second Edition.
• Authored by Erin C. Amerman
• PowerPoint Lectures created by Suzanne Pundt, University of Texas at Tyler

Overview of the Integumentary System

• Skin accounts for 10-15% of total body weight.
• The skin is the largest organ in the body and is a complex organ with many functions important for homeostasis.
• The cutaneous membrane (skin) has two main components: epidermis and dermis.
• Epidermis is the superficial layer, consisting of keratinized stratified squamous epithelium resting on a basement membrane.
• Dermis is deep to the epidermis and basement membrane, made of loose connective tissue and dense irregular connective tissue.
• Accessory structures are embedded in the cutaneous membrane: sweat glands, sebaceous glands, hair, and nails
• Sensory receptors and arrector pili muscles are contained within the skin
• Epidermis is avascular, relying on diffusion of oxygen and nutrients from blood vessels in the dermis, which limits epidermal thickness.
• About 50% of epidermal cells are too far from an adequate blood supply to sustain life.
• The superficial layers are made up entirely of dead cells.
• Hypodermis is deep to the dermis, consisting of superficial fascia or subcutaneous fat.
• Hypodermis anchors skin to deeper structures like muscle and bone and is not part of the skin
• Hypodermis is made of loose connective and adipose tissues and has an abundant blood supply.

Cellulite

• Cellulite is the dimpled or "orange peel" appearance of skin when collagen bands form around adipose tissue in the hypodermis.
• Cellulite develops in the thighs, hips, and gluteal area and is influenced by genetics, gender, amount and distribution of adipose tissue, and age.
• Cellulite is a normal condition, not a disorder.
• The only proven way to minimize cellulite is a healthy diet and regular exercise.

Functions of the Integumentary System (1 of 12)

• Integumentary system functions are critical for protecting underlying organs or maintaining homeostasis.
• Integumentary system protects from mechanical trauma, pathogens, and environment in several ways.
• Stratified squamous, keratinized epithelium provides a durable surface for protection from mechanical trauma (stretching, pressure, or abrasions).
• The integumentary system provides a continuous barrier to the invasion of microorganisms or pathogens.
• It contains cells of the immune system that destroy pathogens before they invade deeper tissues..

Functions of the Integumentary System (2 of 12)

• Antimicrobial substances and acidic pH (acid mantle) on the surface of the skin inhibit the growth of many pathogens.
• The integumentary system provides protection from environmental hazards, including absorption of UV light before deeper tissues are damaged.
• It secretes hydrophobic lipid-based chemicals that repel ionic (salt) and polar covalent (water) molecules.
• The integumentary system keeps both salt and water molecules from leaving and entering the body via skin and is critical for maintaining water and electrolyte homeostasis in a wide range of weather conditions

Functions of the Integumentary System (3 of 12)

• Sensation enables the nervous system to perceive changes in the body's internal or external surroundings.
• Sensation is critical to homeostasis.
• The skin has numerous sensory receptors that detect changes in internal and/or external environment.
• Receptors detect harmful stimuli (heat, cold, and pain), which could lead to tissue damage.

Functions of the Integumentary System (4-6 of 12)

• Thermoregulation relies on negative feedback loops for the maintenance of a stable internal temperature.
• Internal body temperature is determined mostly by muscle activity and chemical reactions (metabolism).
• When body temperature rises above normal: sensory receptors (thermoreceptors) detect the increase in temperature in skin and internal body fluids.
• The control center in the hypothalamus acts as a thermostat (thermoregulatory center), receiving input from thermoreceptors, processing, and responding to inputs.
• The control center stimulates sweating; sweat glands are stimulated to release sweat, and water carries heat when it evaporates.
• The control center also stimulates cutaneous vasodilation.
• Dermal blood vessels widen (dilate), and increased blood flow through vessels increases heat radiated from the body.
• Body temperature returns to normal range and cooling mechanisms decline by negative feedback.
• Thermoreceptors stop signaling the hypothalamus, thus ending the control center responses.
• Sweating and vasodilation ends.

Functions of the Integumentary System (8-9 of 12)

• When body temperature drops below normal, thermoreceptors detect it and relay the information to the thermoregulatory center in the hypothalamus.
• Blood vessels in the dermis narrow (vasoconstrict) to reduce blood flow, which limits heat lost to the environment.
• Vasoconstriction redirects blood flow to deeper tissues and conserves heat.
• When body temperature returns back to normal range, thermoreceptors stop signaling the hypothalamus, ending heat conservation, a closed feedback loop.

Functions of the Integumentary System (11-12 of 12)

• Excretion is the elimination of waste products and toxins from the body, mostly through other organs like the kidneys.
• Skin and accessory structures make a small but significant contribution to excretion.
• Skin plays a critical role in vitamin D synthesis.
• A modified cholesterol molecule (precursor) is converted to cholecalciferol by exposure of the epidermis to UV radiation.
• Cholecalciferol is released into the blood and modified first by the liver, then by the kidneys, to form calcitriol: the active form of vitamin D.
• Calcitriol is required for calcium ion absorption from the small intestine.
• Calcium ion is critical for building and maintaining bone tissue, as well as nerve function, muscle contraction, and many other physiological functions.

The Epidermis

• Epidermis is the most superficial layer and is composed of several cell types.
• Keratinocytes make up about 95% of the epidermis.
• Structural features make epidermis stronger and less susceptible to mechanical trauma.
• Keratinocytes manufacture keratin, a tough fibrous protein, and are linked together by desmosomes, intercellular junctions that hold cells together.

The Epidermis Structa (Layers)

• Epidermal strata (layers) are structurally distinct from deep to superficial.
• Stratum basale (stratum germinativum) is a single layer of stem cells resting on a basement membrane.
• The stratum basale is closest to the dermal blood supply and is the most metabolically and mitotically active strata.
• The stratum basale is responsible for Vitamin D synthesis and replacement of dead keratinocytes (lost from superficial layers).
• Stratum spinosum is the thickest layer, on top of the stratum basale, and still close to blood supply, making it metabolically and mitotically active.

The Epidermis Strata Continued

• Stratum granulosum has three to five layers of cells, and prominent cytoplasmic granules with keratin.
• Hydrophobic lipids provide waterproofing, maintaining electrolyte homeostasis, leading to cell isolation and death in layers.
• Stratum lucidum is a narrow layer of clear, dead keratinocytes found only in thick skin.
• Stratum corneum is the outermost layer with several layers of dead, flattened keratinocytes with thickened plasma membranes.
• Keratinocytes are filled with keratin bundles and Sloughed off or exfoliated mechanically as desmosomes holding cells together are lost

The Life Cycle of Keratinocytes

• The location and functions of the epidermis are subject to physical and environmental stress.
• The stratum corneum continuously sheds dead cells that are replaced to maintain the integrity of the epidermis.
• Dead keratinocytes are replaced by mitosis in the stratum basale and spinosum, where blood supply available.
• Keratinocytes in deeper strata divide and push cells above them into more superficial layers.
• Keratinocytes begin life either in the stratum basale or stratum spinosum and start to pass through each epidermal layer to be shed form the stratum corneum.
• Migration from the deepest to the surface of the stratum normally takes 40 to 50 days.
• Cell B is now quite far from blood supply and coated with lipid-based substance, causing the cell to die
• Stem cells continue to divide, pushing cell B further away from blood supply into stratum lucidum and stratum corneum
• Cell B is now dead and filled with keratin and will eventually be sloughed off skin surface

Other Epidermis Cells

• Dendritic (Langerhans) cells are located in the stratum spinosum.
• Dendritic cells are phagocytes of the immune system and protect skin and deeper tissues from pathogens.
• Merkel cells are oval cells scattered throughout the stratum basale and are sensory receptors associated with small neurons in the dermis.
• Merkel cells detect light touch and discriminate shapes and textures.
• They are most numerous in regions specialized for touch: fingertips, lips, and at the base of hairs
• Melanocytes are in the stratum basale and produce melanin (protein pigment ranging from orange-red to brown-black).

Thick and Thin Skin

• The form of the epidermis in various parts of the body differs to match function.
• Palms of the hand and sole of the foot are subjected to a great deal of mechanical stress.
• Other regions of the skin are not subjected to as much stress and are covered with thin and thick skin.
• Thick skin is as thick as paper towel, has all five epidermal layers and very thick stratum corneum, no hair follicles, and many sweat glands.
• Thin skin covers areas of body not subjected to as much mechanical stress and only as thick as printer paper.
• Thin skin has only four layers (no stratum lucidum), and each layer is thinner than the those of thick skin with numerous hairs, sweat glands, and sebaceous glands.
• A callus is caused by repetitive pressure causing additional layers of the stratum corneum, which form in either thick or thin skin.

Explore the structure and function of the integumentary system. Questions cover the epidermis, dermis, hypodermis, and accessory structures. Learn about diffusion, tissue types, and the impact of damage to different components which affects functions of the skin.