Module 5: The Integumentary System - Comprehensive Notes

Module 5: The Integumentary System

Components of the Skin

• Consists of two distinct regions:

  • Epidermis: The superficial region made of epithelial tissue.

  • Dermis: Underlies the epidermis and is primarily composed of fibrous connective tissue.

• Hypodermis:

  • Not technically part of the skin, but shares several functions.

  • It is the subcutaneous layer located deep to the skin.

  • Mainly composed of adipose tissue, which helps absorb shock and insulates the body.

  • Anchors the skin to underlying structures, mostly to muscles.

Elements and Cells of the Epidermis

• The epidermis is characterized as keratinized, stratified squamous epithelium.

• Consists of four main cell types:

  • Keratinocytes:

    • Comprise the majority of the epidermis.

    • Tightly connected by desmosomes.

    • Primarily responsible for producing the fibrous protein keratin.

  • Melanocytes:

    • Account for about $10-25\%$ of the cells in the deepest epidermal layers.

    • Produce the pigment melanin, which is packaged into melanosomes.

    • Melanin protects the apical surface of keratinocyte nuclei from harmful UV damage.

  • Dendritic (Langerhans) Cells:

    • These are macrophages that act as key activators of the immune system.

  • Tactile (Merkel) Cells:

    • Function as sensory touch receptors.

• Consists of five distinct layers (from superficial to deep):

  • Stratum corneum

  • Stratum lucidum (found only in thick skin)

  • Stratum granulosum

  • Stratum spinosum

  • Stratum basale

Layers of the Epidermis

• Stratum Basale:

  • The deepest epidermal layer, also known as the basal layer or stratum germinativum.

  • Firmly attached to the dermis.

  • Composed of a single row of actively mitotic stem cells.

  • These stem cells produce two daughter cells:

    • One cell remains in the stratum basale as a new stem cell.

    • The other cell embarks on a journey from the basal layer to the surface (stratum corneum).

    • This journey typically takes $25-45$ days.

    • The cell dies as it moves towards the surface.

  • Melanocytes make up $10-25\%$ of this layer.

• Stratum Spinosum:

  • Known as the "prickly layer" and is several layers thick.

  • Cells contain a web-like system of intermediate prekeratin filaments, which are attached to desmosomes.

  • This layer is abundant in melanosomes and dendritic cells.

• Stratum Granulosum:

  • A thin layer, typically about $4-6$ cell layers in depth, named for its granular appearance.

  • Cells undergo gradual changes in appearance:

    • Cells flatten.

    • Nuclei and organelles disintegrate.

    • Keratinization begins, where cells accumulate keratohyaline granules (which help form keratin in upper layers).

    • Cells also accumulate lamellar granules, whose water-resistant glycolipids slow water loss.

  • Cells above this layer are too far from the dermal capillaries to receive sufficient nutrients and thus begin to die.

• Stratum Lucidum:

  • Known as the "clear layer."

  • Found exclusively in thick skin, such as palms, fingertips, and soles of the feet.

  • A thin, translucent band situated superficial to the stratum granulosum.

  • Consists of a few rows of flat, dead keratinocytes.

• Stratum Corneum:

  • Referred to as the "horny layer."

  • Composed of $20-30$ rows of dead, flat, anucleated, keratinized membranous sacs.

  • Represents about $3/4$ of the total epidermal thickness.

  • Despite being dead, these cells retain crucial functions:

    • Protect deeper cells from water loss and the external environment.

    • Provide protection against abrasion and penetration.

    • Act as a barrier against biological, chemical, and physical agents.

Cell Differentiation in the Epidermis

• Cells undergo a specialized form of apoptosis as they transition from the stratum basale to the stratum corneum.

• During this process:

  • The nucleus and organelles break down.

  • The plasma membrane thickens.

• This differentiation allows old cells to slough off effectively.

• Approximately $50,000$ epidermal cells are shed every minute.

Components and Layers of the Dermis

• The dermis is a strong and flexible connective tissue layer.

• Composed of fibroblasts, macrophages, mast cells, and other white blood cells.

• Fibers in the matrix (corresponding to the "hide" used for leather) bind the body together.

• Contains nerve fibers, as well as blood and lymphatic vessels.

• Houses epidermal hair follicles, as well as oil (sebaceous) and sweat (sudoriferous) glands.

• Composed of two layers:

  • Papillary layer

  • Reticular layer

Layers of the Dermis: Papillary Layer

• Composed of areolar connective tissue with collagen and elastic fibers, including blood vessels.

• It is a loose tissue.

• Dermal papillae:

  • Superficial, peg-like projections.

  • Most contain capillary loops, providing nutrients to the epidermis.

  • Some contain Meissner's corpuscles (sensory touch receptors).

  • Some contain free nerve endings (pain receptors).

  • In thick skin, they lie on top of dermal ridges which cause epidermal ridges.

  • Collectively, these ridges are called friction ridges.

    • Enhance gripping ability.

    • Contribute to the sense of touch.

    • The unique pattern of these ridges forms our fingerprints.

Layers of the Dermis: Reticular Layer

• Comprises about $80\%$ of the dermis thickness.

• Consists of dense fibrous connective tissue.

• Elastic fibers provide stretch-recoil properties to the skin.

• Contains collagen fibers:

  • Provide strength and resiliency.

  • Bind water, keeping the skin hydrated.

  • Create cleavage lines (or tension lines) because collagen fibers run parallel to the skin surface.

    • These lines are externally invisible but are crucial for surgeons; incisions made parallel to cleavage lines will gap less and heal more readily.

Skin Markings

• Flexure Lines:

  • Dermal folds occurring at or near joints.

  • The dermis is tightly secured to deeper structures in these areas.

  • Since the skin cannot slide easily for joint movement, deep creases are formed.

  • Visible on the hands, wrists, fingers, soles, and toes.

• Striae:

  • Silvery-white scars, also known as stretch marks.

  • Caused by extreme stretching of the skin, leading to dermal tears.

• Blister:

  • A fluid-filled pocket that separates the epidermal and dermal layers.

  • Results from acute, short-term trauma to the skin.

Pigments Contributing to Skin Coloration

• Melanin:

  • The only pigment produced directly in the skin.

  • Exists in two major forms: reddish-yellow (pheomelanin) and brownish-black (eumelanin).

  • Differences in skin color are due to the amount and form of melanin produced.

  • Produced by melanocytes; the relative number of melanocytes is similar in all people.

  • Melanin migrates to keratinocytes to form "pigment shields" for the nuclei.

  • Local accumulations of melanin are found in freckles and pigmented moles.

  • Sun exposure stimulates melanin production.

  • Sunspots (tinea versicolor) are a fungal infection, not related to melanin.

• Carotene:

  • A yellow to orange pigment, most evident in the palms and soles.

  • Accumulates in the stratum corneum and hypodermis.

  • Can be converted to Vitamin A, which is essential for vision and epidermal health.

• Hemoglobin:

  • The main protein in red blood cells, responsible for oxygen transport.

  • Imparts the pinkish hue seen in fair skin due to its oxygenated state in dermal capillaries.

Using Skin Coloration to Make a Diagnosis

• Cyanosis: Bluish skin color, attributed to low oxygenation of hemoglobin.

• Erythema: Redness of the skin, caused by increased blood flow, potentially indicating fever, hypertension, inflammation, or allergy.

• Pallor: Blanching or paleness of the skin, which can be a sign of anemia, low blood pressure, fear, or anger.

• Jaundice: A yellow cast to the skin, symptomatic of liver disorder due to increased bilirubin concentration.

• Bronzing: A metallic bronze appearance, indicative of inadequate steroid hormones in Addison's Disease (chronic renal insufficiency).

• Bruises: Visible purplish, black, or blue marks caused by clotted blood beneath the skin.

Common Issues Associated with the Skin

• Moles:

  • Caused by the clustering of melanocytes.

  • Contributing factors include genetics, sun exposure, hormonal changes, age, and trauma.

  • Usually benign, but can potentially transform into melanoma.

• Vitiligo:

  • An autoimmune disorder resulting in the destruction of melanocytes.

  • Results in depigmentation, leading to lighter colored patches on the skin.

  • The exact cause is unknown.

• Acne:

  • Often due to over-productive sebaceous glands, leading to skin inflammation.

  • Many underlying causes.

• Eczema:

  • A common skin disorder presenting as a red, flaky rash.

  • Exact cause is unknown, but likely influenced by genetics or environmental factors such as irritants, allergens, hormonal changes, or bacterial infections.

Derivatives of the Skin: Hair

• Composed of dead, keratinized cells of hard keratin.

• Hard keratin is more durable than the soft keratin found in the skin.

• Hair is not found on the palms, soles, lips, nipples, and portions of the external genitalia.

• Multiple functions:

  • Warn of insects on the skin.

  • Detect physical trauma.

  • Prevent heat loss.

  • Protect from sunlight.

• Consists of multiple pigments:

  • Melanins (yellow, rust, brown, and black) are responsible for various hair colors.

  • Trichosiderin is the pigment found in red hair.

  • Gray/white hair indicates a decreased melanin production combined with an increased presence of air bubbles in the hair shaft.

Derivatives of the Skin: Hair Follicles

• Extend from the epidermal surface down to the dermis.

• Have a two-layered wall, partly dermal and partly epidermal.

• Hair Bulb:

  • The expanded deep end of the hair follicle.

  • Contains the hair follicle receptor (root hair plexus), which are sensory nerve endings serving as touch receptors.

  • Contains the hair matrix, an actively dividing area responsible for hair growth.

• Arrector Pili:

  • A smooth muscle attached to the hair follicle.

  • Its contraction is responsible for "goose bumps."

• Hair Papilla:

  • Dermal tissue containing blood supply to nourish the hair matrix.

Types and Growth of Hair

• Vellus hair:

  • Pale, fine body hair found on children and adult females.

• Terminal hair:

  • Coarse, long hair found in eyebrows and on the scalp.

  • At puberty, it appears in the axillary and pubic regions in both sexes.

  • Also appears on the face and neck of males.

• Hair growth is influenced by nutrition and hormones.

• Hair follicles cycle between active and regressive phases of growth.

• Average hair growth is about $2.25$ mm per week.

• An average person loses about $90$ scalp hairs daily.

Hair Thinning and Baldness

• Alopecia:

  • General hair thinning experienced by both sexes after the age of $40$.

• True (Frank) Baldness:

  • A genetically determined and sex-influenced condition.

  • Male pattern baldness is specifically caused by the follicular response to dihydrotestosterone (DHT).

  • Treatments include minoxidil (Rogaine) and finasteride (Propecia).

Derivatives of the Skin: Nails

• Scalelike modifications of the epidermis.

• Provide a protective cover for the distal, dorsal surface of the fingers and toes.

• Composed of hard keratin.

• Contain a nail matrix that extends, similar to hair growth, dictating nail length.

Derivatives of the Skin: Sweat Glands (Sudoriferous Glands)

• Present on all skin surfaces except the nipples and parts of the external genitalia.

• Humans have approximately $3$ million sweat glands per person.

• Contain myoepithelial cells that contract upon nervous system stimulation to force sweat into ducts.

• Two main types:

  • Eccrine (merocrine) sweat glands

  • Apocrine sweat glands

Eccrine Sweat Glands

• The most numerous type of sweat gland.

• Abundant on the palms, soles, and forehead.

• Ducts connect directly to pores on the skin surface.

• Primary function is thermoregulation, regulated by the sympathetic nervous system.

• Their secretion is known as sweat.

• Sweat is primarily composed of water, with some salts, vitamin C, antibodies, dermicidin (a microbe-killing peptide), and metabolic wastes.

Apocrine Sweat Glands

• Confined to the axillary (armpit) and anogenital areas.

• Secrete sweat along with fatty substances and proteins.

• The secretion is viscous, milky, and yellowish in color.

• Initially odorless, but bacterial interaction with the secretion leads to characteristic body odor.

• Larger than eccrine sweat glands.

• Ducts empty into hair follicles.

• Begin functioning at puberty.

• Their specific function is unknown but is most likely related to acting as a sexual scent gland.

• Modified apocrine glands include:

  • Ceruminous glands: Line the external ear canal and secrete cerumen (earwax).

  • Mammary glands: Specialized glands that secrete milk.

Derivatives of the Skin: Sebaceous (Oil) Glands

• Widely distributed across the body, though absent in the thick skin of the palms and soles.

• Most develop from and secrete their product into hair follicles.

• Relatively inactive until puberty.

• Stimulated by hormones, particularly androgens.

• Secrete sebum:

  • An oily holocrine secretion.

  • Possesses bactericidal properties, helping to inhibit bacterial growth on the skin.

  • Softens and lubricates both hair and skin.

Functions of the Integumentary System

• Protection

• Body temperature regulation

• Cutaneous sensation

• Metabolic functions

• Blood reservoir

• Excretion

Functions: Protection (Barriers)

• Chemical Barriers:

  • The low pH environment of the skin surface (acid mantle) represses bacterial growth.

  • Sebum and defensins (antimicrobial peptides) actively kill bacteria.

  • Melanin provides defense against UV radiation damage.

• Physical Barriers:

  • Flat, dead cells of the stratum corneum are surrounded by lipids.

  • Keratin and glycolipids block most water and water-soluble substances.

  • Skin penetration is very limited, primarily allowing lipid-soluble substances, organic solvents, salts of heavy metals, plant oleoresins (like poison ivy), and some drugs/drug agents.

• Biological Barriers:

  • Dendritic cells and macrophages in the skin engulf foreign antigens and present them to other white blood cells, activating the immune response.

  • DNA itself can absorb some UV radiation and convert it into harmless heat, acting as a built-in sunscreen.

Functions: Thermoregulation

• Under environmental temperatures below $31-32 \text{ } ^\circ C$, an individual loses about $500$ mL of fluid a day through insensible perspiration (unnoticeable sweat diffusion).

• If the body temperature rises, dilation of dermal blood vessels and increased sweat gland activity cools the body through sensible perspiration (noticeable sweating).

• Under cold external environmental stresses:

  • Dermal blood vessels constrict, shunting blood away from the skin surface.

  • Skin temperature drops, which slows passive heat loss from the body.

Functions: Others

• Cutaneous Sensations:

  • Specialized cutaneous sensory receptors in the skin can detect temperature, touch, and pain, allowing us to interact with our environment.

• Metabolic Functions:

  • Involves the synthesis of vitamin D precursor (cholesterol-derived) and collagenase.

  • Plays a role in the chemical conversion of carcinogens and the activation of certain hormones.

• Blood Reservoir:

  • The dermal vascular supply can hold up to $5\%$ of the body's total blood volume, which can be redirected to other organs if needed.

• Excretion:

  • The integumentary system aids in the excretion of nitrogenous wastes and salts via sweat.

Skin Cancer

• Most skin tumors are benign (non-cancerous) and do not spread (metastasize).

• Numerous risk factors:

  • Overexposure to UV radiation is a primary cause.

  • Frequent irritation of the skin.

  • Some skin lotions contain enzymes that can help to repair damaged DNA.

• Three major types of skin cancer:

  • Basal cell carcinoma

  • Squamous cell carcinoma

  • Melanoma

Basal Cell Carcinoma

• The most common and least malignant type of skin cancer.

• Involves proliferation of stratum basale cells, which slowly invade the dermis and hypodermis.

• Typically cured by surgical excision in $99\%$ of cases.

Squamous Cell Carcinoma

• The second most common type of skin cancer.

• Involves keratinocytes of the stratum spinosum.

• Usually presents as a scaly, reddened papule, often found on the scalp, ears, lower lip, and hands.

• Can metastasize if not treated.

• Generally has a good prognosis if treated by radiation therapy and surgical removal.

Melanoma

• Cancer of the melanocytes.

• The most dangerous type of skin cancer due to its high propensity to metastasize.

• Treated by wide surgical excision, often accompanied by immunotherapy.

• Key to survival is early detection, often guided by the ABCD rule:

  • A: Asymmetry: The two sides of the pigmented area do not match.

  • B: Border irregularity: The border exhibits indentations or is notched.

  • C: Color: Contains several colors (e.g., black, brown, tan, and sometimes red or blue).

  • D: Diameter: The diameter is larger than $6$ mm.

Burns

• Defined as tissue damage caused by heat, electricity, radiation, or certain chemicals.

• Damage occurs by denaturing proteins and killing cells.

• Immediate threat:

  • Dehydration and electrolyte imbalance, which can lead to renal shutdown and circulatory shock.

• Burn evaluation:

  • The Rule of Nines is used to estimate the volume of fluid loss, which is critical for treatment.

Classifying the Severity of Burns

• Partial-thickness burns:

  • First-degree burns: Involve only epidermal damage. Characterized by localized redness, edema (swelling), and pain.

  • Second-degree burns: Involve epidermal and upper dermal damage. Result in the appearance of blisters, along with redness, edema, and pain.

• Full-thickness burns:

  • Third-degree burns: Involve the entire thickness of the skin (epidermis and dermis, and sometimes hypodermis).

  • The skin often has a gray-white, cherry red, or blackened coloration.

  • These burns are typically not painful initially because nerve endings have been destroyed.

  • Significant swelling is present.

  • Skin grafting is usually necessary for healing.

Treatment of Burns

• Burns are considered critical if:

  • >25\% of the body has second-degree burns.

  • >10\% of the body has third-degree burns.

  • Third-degree burns are located on the face, hands, or feet.

• Treatment includes:

  • Debridement (surgical removal) of burned skin.

  • Administration of antibiotics to prevent infection.

  • Application of temporary coverings to protect the wound.

  • Skin grafts to replace lost skin, especially for third-degree burns.

The Skin as We Develop and Age

• Fetal Development:

  • The epidermis develops from the ectoderm.

  • The dermis and hypodermis develop from the mesoderm.

• Infancy to Adulthood:

  • Skin thickens and accumulates more subcutaneous fat.

  • Sweat and sebaceous gland activity increases during puberty.

  • The cumulative effects of environmental assaults (e.g., sun exposure) become noticeable after the age of $30$.

  • Scaling and dermatitis become more common.

• Aging Skin:

  • Epidermal replacement slows down, making the skin thinner, drier, and itchier due to decreased sebaceous gland activity.

  • Subcutaneous fat and elasticity decrease, leading to cold intolerance and wrinkles.

  • Increased risk of skin cancer due to decreased numbers of melanocytes and dendritic cells.

  • Hair thinning is also common.

• To delay the effects of aging on the skin, it is recommended to practice:

  • UV protection.

  • Good nutrition.

  • Adequate fluid intake.

  • Good hygiene practices.