BIOL2401 Exam 2 Study Guide: The Integumentary System

Chapter 5: The Integumentary System

1. Components of the Integumentary Organ System

• The integumentary system is the largest organ system of the body, acting as a crucial barrier between the internal environment and the external world.
• It is principally comprised of the skin (integument), which is the primary organ.
• It also includes various accessory organs or appendages that are derivatives of the skin:
  • Hair
  • Nails
  • Sweat glands (sudoriferous glands)
  • Sebaceous glands (oil glands)

2. Functions of the Skin

• The skin performs a wide array of vital functions:
  • Protection: Acts as a physical, chemical, and biological barrier.
    • Physical barrier: Protects underlying tissues from abrasion, penetration, and impacts. The keratinized cells create a tough, durable surface.
    • Chemical barrier: Melanin protects against UV radiation. Skin secretions (acid mantle) retard bacterial growth and prevent water loss.
    • Biological barrier: Epidermal dendritic cells (Langerhans cells) and macrophages initiate immune responses, preventing pathogen entry.
  • Body Temperature Regulation:
    • Cooling: Through vasodilation of dermal blood vessels to radiate heat from the surface and evaporation of sweat produced by eccrine glands.
    • Warming: Through vasoconstriction of dermal blood vessels to conserve heat and increased metabolic activity when cold.
  • Sensation: Contains numerous sensory receptors that detect touch, pressure, pain, and temperature, enabling interaction with the environment.
    • Tactile (Meissner's) corpuscles: Detect light touch in the dermal papillae.
    • Lamellar (Pacinian) corpuscles: Detect deep pressure and vibrations in the deeper dermis and hypodermis.
    • Free nerve endings: Detect pain and temperature.
  • Metabolic Functions: Synthesizes Vitamin D when exposed to UV radiation. Vitamin D is crucial for calcium absorption in the intestine.
  • Blood Reservoir: The dermal vascular plexus can hold about $5\%$ of the body's total blood volume. In states of shock or during vigorous exercise, this blood can be shunted to other organs.
  • Excretion: Though minor, the skin excretes small amounts of nitrogenous wastes (ammonia, urea, uric acid) in sweat, as well as salt and water.

3. Layers of the Skin and Brief Comparison

• The skin is composed of two main layers:
  • Epidermis: The outermost layer.
    • Structure: Composed of keratinized stratified squamous epithelium. It is avascular, meaning it lacks blood vessels.
    • Function: Primarily protective, forming a tough, waterproof barrier.
  • Dermis: The layer underlying the epidermis.
    • Structure: Composed of strong, flexible connective tissue (fibrous connective tissue with collagen and elastic fibers). It is vascular, containing blood vessels, nerves, lymphatic vessels, and accessory structures.
    • Function: Provides structural strength, elasticity, and houses sensory receptors, blood supply, and accessory organs.
• Hypodermis (Subcutaneous Layer): While not technically part of the skin, it is closely associated with it and lies immediately deep to the dermis.
  • Structure: Composed mainly of adipose (fat) tissue and loose areolar connective tissue.
  • Function: Anchors the skin to underlying structures, insulates the body, and stores energy (fat).

4. Layers of the Epidermis and Cellular Trends

• The epidermis is composed of four to five distinct layers, or strata, depending on the body region.

  • Thin skin (covers most of the body) has four layers.
  • Thick skin (palms of hands, soles of feet) has five layers, including the stratum lucidum.

• The layers, from deep to superficial, are:

  1. Stratum Basale (Stratum Germinativum):
     • Description: Deepest epidermal layer, attached to the underlying dermis. Consists of a single row of actively dividing (mitotic) stem cells (keratinocyte stem cells).
     • Cells: Also contains melanocytes (produce melanin) and tactile (Merkel) cells (sensory touch receptors).
     • Trend: Primary site of new cell production and therefore the source of all subsequent layers.
  2. Stratum Spinosum (Prickly Layer):
     • Description: Several cell layers thick. Cells contain intermediate filaments of pre-keratin and appear spiny due to desmosomal connections that remain intact during tissue preparation.
     • Cells: Contains epidermal dendritic (Langerhans) cells, which are part of the immune system.
     • Trend: Cells begin to synthesize keratin and flatten as they are pushed upwards.
  3. Stratum Granulosum (Granular Layer):
     • Description: Thin layer, three to five cell layers thick. Keratinocytes in this layer flatten, and their nuclei and organelles disintegrate. They accumulate two types of granules:
       • Keratohyaline granules: Help to form keratin in the upper layers.
       • Lamellar granules: Contain water-resistant glycolipid, which is secreted into the extracellular space to slow water loss across the epidermis.
     • Trend: Cells begin to die and fill with keratin, becoming increasingly flattened and waterproof. This layer is crucial for the skin's barrier function.
  4. Stratum Lucidum (Clear Layer):
     • Description: A thin, transparent band present only in thick skin (palms and soles).
     • Cells: Consists of a few rows of flattened, dead keratinocytes with indistinct boundaries. Appears translucent.
     • Trend: Represents a transition zone of dead, fully keratinized cells.
  5. Stratum Corneum (Horny Layer):
     • Description: Outermost, thickest layer (up to $30$ cell layers). Consists of dead, anucleated, flattened keratinocytes (cornified cells) filled with keratin.
     • Function: Provides a durable, protective, and water-resistant outer barrier. Accounts for most of the epidermal thickness.
     • Trend: Cells are shed constantly, being replaced by cells from deeper layers. This layer protects against abrasion, penetration, and biological attacks.

• General Trends as cells move from Stratum Basale to Stratum Corneum:

  • Mitosis and Activity: Cells are actively mitotic and metabolically active in the stratum basale, gradually losing this activity as they ascend.
  • Keratinization: Cells progressively accumulate keratin, a tough, fibrous protein, becoming increasingly hardened and resistant to damage.
  • Shape Change: Cells start cuboidal, become polyhedral in spinosum, flatten in granulosum, and are completely flat and scale-like in corneum.
  • Organelle Disintegration and Death: Nuclei and organelles begin to break down in the stratum granulosum, leading to cell death by the time they reach the stratum corneum.
  • Waterproofing: Cells become increasingly waterproof due to glycolipid secretion in the stratum granulosum and the keratin within the cells.
  • Shedding: The outermost cells of the stratum corneum are continuously shed (desquamated), a process called exfoliation.

5. Melanocytes

• Identification: Melanocytes are specialized cells responsible for producing melanin, a pigment that gives color to the skin, hair, and eyes.
• Production: They produce melanin, a complex polymer derived from the amino acid tyrosine.
• Why they produce melanin: Melanin serves as a natural sunscreen, protecting the DNA of skin cells from damaging ultraviolet (UV) radiation from the sun. When exposed to UV light, melanocytes increase their production and release of melanin, leading to tanning, which is the body's protective response.
• Location: Melanocytes are primarily found in the stratum basale of the epidermis, nestled among the deepest keratinocytes.

6. Factors Affecting Skin Color

• Skin color is a phenotypic trait influenced by a combination of genetic, environmental, and physiological factors.
• Genetics:
  • Melanin Type and Amount: The primary determinant is the type and amount of melanin produced by melanocytes, not necessarily the number of melanocytes (which is roughly similar in all races).
    • Eumelanin: Produces brownish-black pigment. High levels result in darker skin and hair.
    • Pheomelanin: Produces reddish-yellow pigment. High levels, especially without counteracting eumelanin, result in red hair and fair skin (e.g., in individuals with freckles).
  • Genetic variations dictate the activity of tyrosinase (the enzyme that catalyzes melanin synthesis) and the size, number, and distribution of melanosomes (the organelles containing melanin).
• Environmental Factors:
  • Sun Exposure (UV Radiation): Exposure to ultraviolet (UV) radiation increases the activity of melanocytes, leading to increased melanin production and a temporary darkening of the skin (tanning). This is a protective mechanism against UV damage.
• Physiological Factors:
  • Dermal Blood Supply: The amount of oxygenated blood circulating through the capillaries in the dermis significantly influences skin color.
    • Pinkish hue: In fair-skinned individuals, red oxygenated hemoglobin in capillaries gives the skin a rosy glow (e.g., blushing).
    • Pallor (Blanching): Reduced blood flow (e.g., from fear, anger, anemia, low blood pressure) can make the skin appear pale.
    • Cyanosis: Poorly oxygenated blood, especially in the extremities, can give the skin a bluish tint, indicating low blood oxygen levels (e.g., in heart failure or respiratory distress).
  • Carotene: A yellow-orange pigment found in certain plant products (like carrots). It tends to accumulate in the stratum corneum and in the adipose tissue of the hypodermis. It is most noticeable in the palms and soles and can give the skin a yellowish tint if consumed in large quantities.
  • Jaundice: A yellowing of the skin and whites of the eyes caused by the accumulation of bilirubin (a byproduct of red blood cell breakdown) in the blood. This typically indicates liver dysfunction.
  • Addison's Disease: A disorder of the adrenal glands that can lead to increased production of adrenocorticotropic hormone (ACTH). ACTH can stimulate melanocytes, resulting in a bronze-like hyperpigmentation of the skin, especially in areas exposed to the sun or subject to friction.

7. Lines of Tension (Langer's Lines)

• What they are made of: Lines of tension, also known as Langer's lines or cleavage lines, are invisible (to the naked eye) patterns of collagen fibers in the reticular layer of the dermis. These bundles of collagen and elastic fibers are typically arranged in parallel along directions of tension throughout the body.
• Usefulness to plastic surgeons: Understanding and utilizing Langer's lines is crucial in surgical practices, particularly plastic and reconstructive surgery.
  • Reduced scarring: Incisions made parallel to the lines of tension tend to heal more cleanly, with less tension on the incision site. This results in less gaping of the wound edges, less scar tissue formation, and ultimately, a finer, less noticeable scar.
  • Improved healing: Wounds that run perpendicular to Langer's lines encounter more resistance, creating more tension, which can lead to larger, more prominent, and functionally compromised scars (e.g., keloids or hypertrophic scars).

8. Decubitus Ulcers

• Discussion: Decubitus ulcers, commonly known as bedsores or pressure ulcers, are localized injuries to the skin and underlying tissue, usually over a bony prominence, as a result of prolonged pressure, or pressure combined with shear or friction.
• Causes:
  • Prolonged Pressure: The primary cause. Sustained pressure on an area (e.g., from lying in one position) compresses blood vessels, leading to restricted blood flow (ischemia) to the tissues. Without adequate blood flow, cells are deprived of oxygen and nutrients, leading to tissue damage and death (necrosis).
  • Friction: Rubbing of skin against a surface (e.g., being pulled across bed linens) can strip away layers of skin, making it more vulnerable.
  • Shear: Occurs when skin remains stationary while underlying tissue shifts or moves (e.g., sliding down in a bed), causing stretching and tearing of blood vessels and capillaries.
  • Other contributing factors: Poor nutrition, dehydration, incontinence, immobility, chronic diseases (e.g., diabetes), and impaired sensation.
• Preventive Measures:
  • Frequent Repositioning: Regularly turning and repositioning individuals (e.g., every $2$ hours in bed, every $15$ minutes in a wheelchair) to relieve pressure on vulnerable areas.
  • Skin Inspection: Daily inspection of the skin, especially over bony prominences, to detect early signs of redness or skin breakdown.
  • Pressure-Relieving Devices: Use of specialized mattresses (e.g., alternating air pressure, low-air-loss), cushions, and heel protectors to distribute pressure evenly and reduce contact pressure.
  • Maintaining Skin Hygiene: Keeping the skin clean and dry, particularly in cases of incontinence, to prevent maceration (softening due to moisture) and irritation.
  • Nutrition and Hydration: Ensuring adequate intake of protein, vitamins (especially C), minerals (especially zinc), and fluids to support skin integrity and healing.
  • Minimizing Friction and Shear: Using proper lifting techniques, absorbent pads, and lubrication to prevent skin shear and friction during movement.
  • Education: Educating patients, caregivers, and healthcare providers about prevention strategies.

9. Hair

• Hair is an accessory organ of the skin, with various functions including protection (from UV rays, head trauma, and filtering particles in nose/ears) and insulation.
• Structure of Hair:
  • Hair Shaft: The part of the hair that extends above the skin surface. It consists of three concentric layers of keratinized cells:
    • Medulla: Innermost core, large cells, often absent in fine hair. Contains soft keratin.
    • Cortex: Middle layer, surrounds the medulla. Forms the bulk of the hair, consisting of several layers of flattened cells. Contains pigment granules.
    • Cuticle: Outermost layer, single layer of overlapping cells like shingles. Provides strength and keeps the inner layers tightly compacted. The most heavily keratinized region.
  • Hair Root: The part embedded in the skin.
  • Hair Follicle: A tubular invagination of the epidermis that extends into the dermis (and sometimes hypodermis).
    • Hair Bulb: The expanded deep end of the hair follicle.
    • Hair Papilla: A nipple-like bit of dermal tissue that protrudes into the hair bulb. It contains capillaries that supply nutrients to the growing hair.
    • Hair Matrix: Actively dividing cells in the hair bulb that produce the hair shaft.
  • Arrector Pili Muscle: A bundle of smooth muscle cells attached to each hair follicle and the epidermis. Contraction causes the hair to stand upright.
  • Sebaceous Gland: Usually associated with a hair follicle, secretes sebum (oil) to lubricate hair and skin.
• What causes hair to be different colors?
  • Hair color is determined by the type and amount of melanin produced by melanocytes located in the hair matrix within the hair bulb.
  • Eumelanin: Produces brown and black colors.
  • Pheomelanin: Produces red-orange and yellow colors.
  • Genetics dictate the specific ratio and concentration of these melanins. Gray hair results from a decrease in melanin production, and white hair results from a complete absence of melanin pigment and the presence of air bubbles in the medulla.
• What causes goose bumps?
  • Goose bumps (or gooseflesh) are caused by the contraction of the arrector pili muscles.
  • These are small bundles of smooth muscle cells attached to the base of each hair follicle.
  • When these muscles contract (in response to cold temperatures or emotional stimuli like fear or excitement), they pull the hair follicle upright, making the hair stand on end. This action also causes a dimpling of the skin surface, creating the characteristic goose bump appearance. In hairy animals, this action traps a layer of air close to the skin for insulation, or makes the animal appear larger to an adversary.

10. Contrast Eccrine and Apocrine Sweat Glands

• Both are types of sudoriferous (sweat) glands, but they differ in location, structure, secretion, and function.

11. Nails

• Discussion on Structure and Formation:
  • Nails are clear, protective plates of hard keratinized epidermis located on the dorsal surface of the distal end of each finger and toe.
  • Structure:
    • Nail Plate (Nail Body): The visible attached portion of the nail. It has a free edge, a nail body, and a root.
    • Free Edge: The distal, unattached portion of the nail that we trim.
    • Nail Root: The proximal part of the nail embedded in the skin.
    • Nail Bed: The deeper layers of the epidermis underlying the nail plate. It contains only the stratum basale and stratum spinosum.
    • Nail Matrix: The thickened portion of the nail bed at the proximal end, responsible for nail growth. Its cells are actively dividing keratinocytes.
    • Lunule (Lunula): The crescent-shaped white area at the base of the nail, visible through the nail plate, representing the thick nail matrix underneath.
    • Eponychium (Cuticle): The fold of skin that projects onto the nail body, providing a protective seal.
    • Hyponychium: The thickened region beneath the free edge of the nail, where dirt and debris tend to accumulate.
  • Formation: Nail formation begins at the nail matrix. Cells in the matrix divide rapidly, producing new keratinocytes. These cells become heavily keratinized with hard keratin (unlike the soft keratin of the skin), flatten, and are pushed distally over the nail bed, forming the nail plate. The nail plate itself is dead, hard keratinized cells.
• Comparison to Skin and Hair:
  • Similarities to Skin (Epidermis): Both nails and the epidermis are composed of keratinocytes, derived from epithelial tissue, and subject to continuous growth and shedding (though much slower and in plate form for nails).
  • Differences from Skin: Nails are made of hard keratin, which is tougher and more durable than the soft keratin found in the epidermis. Nails form a rigid plate, unlike the flexible, continuously shedding epidermal layers.
  • Similarities to Hair: Both nails and hair are accessory structures derived from the epidermis, and both are primarily composed of hard keratin.
  • Differences from Hair: Hair grows from a follicle and forms a shaft. Nails grow from a matrix and form a flattened plate. Their forms, locations, and specific protective functions differ.

12. Burns

• Burns are tissue damage inflicted by intense heat, electricity, radiation, or certain chemicals, all of which denature cell proteins and cause cell death.

Comparison of Burn Degrees:

Role of Accessory Organs in Burn Healing (Especially Second-Degree Burns):

• In second-degree burns, although the epidermis is destroyed, many accessory organs like hair follicles, sweat glands, and sebaceous glands typically extend deep into the dermis. If the burn does not destroy the deepest parts of these structures, their epithelial cells remain viable.
• These intact accessory structures serve as reservoirs of epithelial cells. From the surviving cells within these structures, new epidermal cells can proliferate and migrate across the wound surface to re-epithelialize the damaged area. This process significantly aids healing and reduces the need for skin grafting in many second-degree burns.

Health Problems Associated with Severe Burns:

• Severe burns (especially third-degree and extensive second-degree burns) pose several critical health threats:
  • Dehydration and Electrolyte Imbalance: The most immediate and life-threatening concern. Loss of the skin's barrier function leads to massive fluid (plasma) and electrolyte loss (e.g., sodium, potassium) from the burned surface. This can lead to renal shutdown and circulatory shock within hours.
  • Infection: Loss of the skin barrier also exposes the body to pathogens. Infection is the leading cause of death in burn victims after the initial shock stage. The burned tissue provides an ideal medium for bacterial growth.
  • Hypothermia: With extensive skin damage, the body loses its ability to regulate temperature, leading to significant heat loss and hypothermia.
  • Immunosuppression: Severe burns significantly suppress the immune system, making patients more susceptible to infections.
  • Metabolic Demands: The body's metabolic rate increases dramatically to cope with tissue repair and infection, requiring vast amounts of nutrients and energy.
  • Scarring and Contractures: Extensive third-degree burns heal with significant scarring. This can lead to contractures, which are permanent tightening of skin, muscles, tendons, or other tissues, causing joint deformities and limiting movement.
  • Psychological Trauma: Burn victims often experience severe psychological distress, anxiety, depression, and body image issues.

13. Contrast Cutaneous Carcinomas and Cutaneous Melanomas

• Skin cancers are the most common cancers of humans. Exposure to UV radiation is the most frequent cause.

Cutaneous Carcinomas:

• This category includes basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), which are the most common and generally less aggressive forms of skin cancer.
  • Basal Cell Carcinoma (BCC):
    • Origin: Arises from cells of the stratum basale (the deepest layer of the epidermis).
    • Severity: Least malignant and most common skin cancer (about $80\%$ of all cases). It grows slowly and rarely metastasizes (spreads to other parts of the body).
    • Appearance: Often manifests as a small, shiny, pearly nodule, sometimes with a central ulceration and rolled, beaded edges.
    • Who is at risk: Individuals with prolonged sun exposure, fair skin, and older age. Genetic predisposition also plays a role.
  • Squamous Cell Carcinoma (SCC):
    • Origin: Arises from the keratinocytes of the stratum spinosum (prickly layer).
    • Severity: Second most common type. More aggressive than BCC and can metastasize to lymph nodes if not caught early and treated.
    • Appearance: Often appears as a scaly, reddened papule or nodule, typically on the scalp, ears, lower lip, or hands, that can crust and ulcerate.
    • Who is at risk: Similar to BCC – chronic sun exposure, fair skin, older age, and immunosuppression are significant risk factors.

Cutaneous Melanomas (Malignant Melanoma):

• Origin: Arises from melanocytes, the pigment-producing cells of the skin.
• Severity: This is the most dangerous and aggressive type of skin cancer. It is highly metastatic, meaning it spreads rapidly to other body parts, and is often resistant to chemotherapy. If not detected and treated early, it has a high mortality rate.
• Appearance: Often appears as a spreading brown or black patch (sometimes red or blue), and can develop from existing moles. The ABCDE rule is used for detection:
  • A - Asymmetry: The two halves of the spot do not match.
  • B - Border Irregularity: The edges are notched, blurred, or ragged.
  • C - Color Variation: The spot has multiple colors (blacks, browns, tans, blues, reds).
  • D - Diameter: The spot is larger than $6 \text{ mm}$ (the size of a pencil eraser).
  • E - Evolving: The mole or spot changes in size, shape, color, or other characteristics (e.g., itching, bleeding) over time.
• Who is at risk:
  • UV Exposure: Especially intense, intermittent sun exposure leading to sunburns (particularly during childhood).
  • Genetics: Family history of melanoma, presence of atypical moles (dysplastic nevi).
  • Phenotype: Fair skin, light hair/eyes, many moles, and a history of severe sunburns.
  • Immune Suppression: Weakened immune system.

14. Identification of Terms

• Alopecia: Hair loss; baldness, which can be partial or complete. Can be caused by genetics, aging, hormonal changes, medical conditions, or medication.
• Pruritus: Severe itching of the skin. It can be a symptom of various skin conditions (like eczema, hives) or underlying systemic diseases.
• Pustule: A small, circumscribed, elevated lesion of the skin that contains pus; often seen in acne or impetigo.
• Vesicle: A small, fluid-filled blister on the skin, typically smaller than $5 \text{ mm}$ in diameter. Examples include blisters from shingles or herpes simplex.
• Boil (Furuncle): A deep-seated, painful infection of a single hair follicle and surrounding tissue, usually caused by Staphylococcus aureus bacteria. It forms a tender, red, swollen lesion with a puss-filled center.
• Cyst: A closed sac or capsule-like structure that is typically fluid-filled, air-filled, or semi-solid. Cysts can occur anywhere in the body and vary in size.
• Wart: A small, benign, rough growth on the skin, caused by infection with the human papillomavirus (HPV). Warts can appear in various shapes and sizes on different parts of the body.
• Ulcer: An open sore on an external or internal surface of the body, caused by a break in the skin or mucous membrane that fails to heal. It results from tissue loss, often due to poor circulation, pressure, or infection (e.g., decubitus ulcers, peptic ulcers).
• Urticaria (Hives): A skin rash characterized by raised, itchy, red welts (wheals) that appear suddenly. It is often triggered by an allergic reaction to food, medicine, insect bites, or other environmental factors.