Integumentary System – Comprehensive Module 1 Notes
Overview of the Integumentary System (Module 1)
The integumentary system includes the skin and its accessory structures (hair, nails, glands) and sensory receptors.
Skin accounts for about of a person’s total body weight and is the largest organ in the body.
The skin is a cutaneous membrane with two main components:
Epidermis: outer layer of keratinized stratified squamous epithelium.
Dermis: deep to the epidermis; mainly loose connective tissue (superficial) and dense irregular connective tissue (deeper).
The hypodermis (subcutaneous layer or superficial fascia) is deep to the dermis. It is not part of the skin proper but anchors the skin to muscles and bones. It is made of loose connective tissue and adipose tissue and has an abundant blood supply to nourish the skin.
Accessory structures and components of the skin include sweat glands, sebaceous glands, hair, nails, sensory receptors, and erector pili muscles.
The epidermis is avascular and is nourished by diffusion of oxygen and nutrients from dermal blood vessels. This limits epidermal thickness.
About of the epidermis consists of dead cells because they are too far from the dermis to receive nutrients.
Cell types in the epidermis include keratinocytes (most numerous), Langerhans (dendritic) cells, Merkel cells, and melanocytes.
The Epidermis: Structure, Cells, and Layers
Main functions of keratinocytes:
Produce keratin, a tough protein that increases resistance to mechanical trauma.
Connected by desmosomes (like buttons or rivets) to allow pliability without tearing.
Epidermal layers (from deep to superficial): stratum basale (germinativum), stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum.
Stratum basale (basal layer): a single layer of stem cells resting on the basement membrane; closest to blood supply; highly metabolically and mitotically active; involved in vitamin D synthesis and replacement of dead keratinocytes in superficial layers.
Stratum spinosum (spiny layer): thickest layer; still close to blood supply; metabolically active in its region.
Stratum granulosum (granular layer): 3–5 cell layers thick; cells contain prominent cytoplasmic granules filled with keratin bundles and lipid-rich substances that are secreted via exocytosis; lipids provide waterproofing to maintain fluid and electrolyte homeostasis; this lipid-rich barrier can cause isolation and death of cells in this and superficial layers.
Stratum lucidum (clear layer): narrow, found only in thick skin; composed of clear, dead keratinocytes.
Stratum corneum (outermost): several layers of dead, flattened keratinocytes with thick plasma membranes, filled with keratin; cells are exfoliated as they detach from neighboring cells.
Mnemonic for strata: "Brilliant studying gives loads of confidence" -> Basale, Spinosum, Granulosum, Lucidum, Corneum; superficial vs deep cue: basale = bottom/base.
All thick skin contains five epidermal layers and has a very thick stratum corneum but no hair follicles; it has many sweat glands.
Thin skin has four layers (no stratum lucidum) and contains numerous hairs, sweat glands, and sebaceous glands.
Epidermal cell migration timeline: from stratum basale to stratum corneum typically takes .
Deep to superficial cell life cycle example: a cell in basale divides; one daughter moves upward (like a stack of pancakes) and progressively loses access to nutrients as it moves away from the blood supply, ultimately becoming part of the stratum corneum.
Other epidermal cells and their roles:
Langerhans (dendritic) cells: phagocytes in the stratum spinosum; immune defense.
Merkel cells: associated with sensory receptors and small neurons in the dermis; detect light touch; abundant in highly sensitive areas (fingertips, lips, base of hairs).
Melanocytes: located in the stratum basale; produce melanin and transfer it to keratinocytes via melanosomes; melanin protects DNA from UV radiation.
Epidermal pigment and its functional significance:
Melanin: ranges from orange-red to brown-black; produced by melanocytes in the stratum basale; melanosomes are transferred to keratinocytes; melanin shields keratinocyte DNA from UV damage; melanin production increases with UV exposure (immediate oxidation of existing melanin causes quick darkening; a delayed secondary response occurs within ~72 hours and lasts longer).
Melanin also affects vitamin D synthesis: UV exposure influences production of vitamin D; less UV exposure can reduce vitamin D synthesis, affecting calcium absorption.
Factors influencing skin color include the number of melanocytes (virtually identical across people) and enzyme activity/type of melanin produced; distribution of melanin can vary by region (pigmentation is often patchy, e.g., hands/feet lighter than face/arms).
Common variations:
Freckle: localized increased pigmentation.
Nevus (mole): local proliferation of melanocytes causing pigmented spots.
Albinism: tyrosinase deficiency; inability to synthesize melanin -> pinkish skin and very light hair; greatly increases UV damage risk.
Minor pigments: carotene (yellow-orange from diet; accumulates in stratum corneum) and hemoglobin (gives color via blood flow in the dermis).
Skin color changes related to blood flow:
Arrhythmia (erythema): increased dermal blood flow; redness during exercise, fever, embarrassment, trauma.
Pallor: decreased dermal blood flow; pale look, more apparent in light-skinned individuals; may occur with cold exposure or nervous/endocrine changes.
Cyanosis: low oxygen levels in hemoglobin; bluish tint signaling potential medical concern; e.g., choking or severe respiratory insufficiency.
The Dermis: Structure, Layers, and Functions
The dermis lies deep to the epidermis and is highly vascular; it provides the blood supply to the epidermis, contains sensory receptors, and anchors the epidermis to underlying tissues.
Two distinct layers within the dermis:
Papillary layer (superficial): composed of loose connective tissue; contains dermal papillae that project toward the epidermis; capillary loops enable diffusion of oxygen and nutrients into the dermal interstitium and into epidermal cells; contains tactile (Meissner’s) corpuscles for light touch; site of dermal ridges that create epidermal ridges (fingerprints) with sweat pores that leave a film when touching surfaces.
Reticular layer (deeper): dense irregular connective tissue; provides tensile strength via abundant collagen fibers; elastic fibers allow skin to stretch and recoil; rich in proteoglycans that draw water into the ground substance to keep skin hydrated; houses most accessory structures (blood vessels, sweat glands, hair follicles, sebaceous glands, adipose tissue).
Skin markings arise from interactions between the dermis and epidermis; dermal ridges create epidermal ridges (fingerprints) and traction patterns on thick, stress-prone areas like palms and soles; prints have loops, arches, and whorls and are unique to individuals; sweat pores along ridges leave a fingerprint.
Cleavage (tension) lines in the reticular layer influence surgical incisions: parallel incisions heal faster with less scarring; transverse (perpendicular) incisions cross collagen fibers and tend to scar more.
Wrinkles: aging-related thinning and degeneration of collagen, elastin, proteoglycans, and adipose tissue; UV exposure and smoking accelerate wrinkling; preventive or cosmetic approaches include:
Botulinum toxin injections (Botox) to paralyze facial muscles and smooth wrinkles.
Dermal fillers (adipose tissue, collagen, or proteoglycans) to plump wrinkles.
Topical anti-wrinkle products (often limited efficacy).
Skin pigmentation and UV exposure:
Melanin production increases with UV exposure, producing tanning; UVA and UVB both contribute to DNA damage and tanning, though UVA is more associated with photoaging.
Tanning salons promote unsafe tanning; no level of tanning is truly safe.
Other minor pigments:
Carotene (diet-derived) accumulates in the stratum corneum; can tint skin yellow-orange.
Hemoglobin in red blood cells affects skin color via dermal blood flow; the deeper the blood flow sits near the epidermis, the more visible the color; changes in color can aid in diagnosis of disease states.
Skin markings and aging implications:
Flexor lines around joints; wrinkles and lines appear with aging and repetitive movement.
Exposure to UV light accelerates aging and increases skin cancer risk.
The Hypodermis (Subcutaneous Layer)
Located deep to the dermis; not technically part of the skin but essential for anchoring the skin to underlying tissues (muscle and bone).
Composed mainly of loose connective tissue and adipose tissue; contains an abundant blood supply to nourish the skin.
Hair and Nails: Accessory Structures
Hair:
Found over most of the body except thick skin areas (palms, soles, lips, portions of external genitalia).
Functions: provides protection (eyes, nose), UV protection for scalp, and a sensory role via small nerve endings.
Structure: shaft (above skin; keratinized) and root (below skin; embedded in a follicle).
Hair bulb and hair papilla at the base contain blood vessels; matrix contains actively dividing keratinocytes for growth.
Epithelial root sheath anchors the follicle to the dermis; around it are dermal root sheath components.
Arrector pili muscles attach around the hair follicle; when they contract, hair stands erect (goosebumps).
Hair growth rate: typically ; growth occurs in cycles with two main phases:
Growth (anagen) phase: mitosis occurs; hair lengthens; duration depends on location, can last from about 1 month up to 6 years.
Resting (telogen) phase: mitosis ends; hair follicle shortens; hair is shed and a new growth cycle begins.
Hair types:
Lanugo: fine, non-pigmented hair covering the fetus; usually shed before birth.
Terminal hair: thick, pigmented hair found on the scalp and around the eyes; generally replaces vellus hair after puberty.
Vellus hair: thin, non-pigmented, softer hair found elsewhere on the body.
Hair color: determined by melanin produced in the hair matrix; blond hair has little melanin; brown/black hair has more melanin; red hair contains iron-containing pigment.
Aging: melanocyte activity decreases with age -> hair grays/whites.
Nails:
Hard accessory structures at the ends of digits; composed of stratified squamous epithelium filled with hard keratin.
Nail plate is the visible part; nail bed underneath has a nail body and nail root (under the skin) where the matrix actively divides to grow nails.
Nail folds surround and reinforce the nail plate: proximal nail fold, cuticle (epithelium; stratum corneum), medial and lateral nail folds.
Distal (free) edge of the nail attaches to the nail bed via the hyponychium.
Nail growth rate is about ; toenails grow more slowly.
Nails lack melanocytes and are typically translucent, except at the base near the cuticle where the lunula (little moon) is visible.
Primary function: protection; can aid in grasping and manipulating small objects.
Glands and Other Skin Structures
Glands:
Sweat glands (sudiferous) and sebaceous glands are the two major types; both derived from epithelial cells in the dermis and secrete via exocytosis (merocrine secretion).
Sweat glands (four types in total) differ in structure and secretions; eccrine (merocrine) glands are the most widespread and produce a watery secretion that helps cool the body.
Sebaceous glands secrete sebum (oil) to keep skin pliable and moisturized; typically associated with hair follicles.
The importance of topical medications: some drugs are safe to apply to the skin but toxic if ingested. The epidermal barrier affects absorption based on molecule polarity:
Polar/charged molecules have limited diffusion through the epidermis and tend to remain local when used topically.
Nonpolar molecules diffuse more readily and can reach systemic circulation; this explains topical efficacy of some hormones and patches, but also potential systemic toxicity from toxins that can cross the epidermis.
Practical takeaways:
Wearing gloves when handling toxic chemicals is recommended to avoid absorption through the skin.
Functions of the Integumentary System and Homeostasis
Main protective function: shields underlying tissues from mechanical trauma, pathogens, and environmental exposure.
Sensation: skin contains receptors that detect changes in the internal and external environment, contributing to homeostasis.
Thermoregulation: negative feedback loops regulate body temperature via vasodilation/vasoconstriction and sweating; see below for more detail.
Excretion: small amounts of waste products and toxins can be excreted via sweat (not a primary organ like kidneys).
Vitamin D synthesis: epidermal cells convert inactive vitamin D to its active form; active vitamin D is essential for calcium absorption in the intestines, which supports bone health and muscle contraction.
Negative feedback in thermoregulation (example):
When body temperature rises above the normal range:
Thermoreceptors in the skin detect this and send signals to the brain (control center).
The brain responds by promoting dermal vasodilation (increasing blood flow to the surface) and sweating.
Evaporation of sweat and heat loss via the dilated vessels bring body temperature back toward normal; once normal temperature is reached, the response stops.
When body temperature falls below the normal range:
Thermoreceptors detect cooling and signal the brain.
The brain responds by causing vasoconstriction (reducing heat loss) and inducing shivering to generate heat.
Heat is conserved and produced until body temperature returns to normal, then the response ceases.
Practical and Ethical Considerations
Claims about cellulite treatments: current evidence suggests no cure from TV or store products; the most effective approach to minimizing appearance is a healthy diet and regular exercise, with recognition that complete elimination is unlikely.
Cosmetic interventions for wrinkles (Botox, fillers) provide temporary improvement but do not reverse aging; sun protection and hydration help delay wrinkle formation.
UV exposure risks: both UVA and UVB contribute to skin aging and cancer risk; tanning has no truly safe level and should be avoided; sunscreen and protective behaviors are advised.
Summary Connections to Core Principles
Structure–function relationship: epidermal layers and keratinocyte differentiation provide protection; dermal layers provide mechanical support and housing for nerves, vessels, and glands.
Homeostasis and feedback: thermoregulation illustrates negative feedback and integration of nervous and integumentary systems.
Replacement and regeneration: basal layer continuously replenishes superficial layers; migration time and mitotic activity reflect tissue maintenance.
Real-world relevance: UV exposure, vitamin D synthesis, and cosmetic considerations connect anatomy to health outcomes and daily life.
Quick Reference: Key Terms and Concepts
Epidermis: avascular outer layer; keratinized stratified squamous epithelium.
Dermis: vascular layer with papillary (loose) and reticular (dense irregular) regions; houses nerves, vessels, follicles, glands.
Hypodermis: loose connective tissue and adipose tissue beneath the dermis; anchors skin.
Keratinocytes: primary epidermal cells; produce keratin; connected by desmosomes.
Melanocytes: synthesize melanin in stratum basale; melanin transfer to keratinocytes
Melanosome: pigment-containing vesicle exported to keratinocytes.
Langerhans cells: epidermal dendritic cells; immune defense.
Merkel cells: tactile cells associated with nerves; light touch perception.
Strata (basale, spinosum, granulosum, lucidum, corneum): order from deep to superficial; functions include regeneration, barrier formation, waterproofing, and protection.
Dermal papillae: peg-like projections that increase surface area for exchange and anchor epidermis; form fingerprints with sweat pores.
Cleavage (tension) lines: collagen fiber orientation affects healing and scar formation.
Hair zones: shaft, root, hair bulb, papilla, matrix, epithelial root sheath, dermal root sheath.
Hair growth cycle: anagen (growth) and telogen (resting); rate ; typical growth time varies by location.
Nails: nail plate, bed, root/matrix; growth rate ; lunula visible at base.
Glands: sweat glands (four types) and sebaceous glands; secretion by exocytosis; sweat helps cooling; sebum provides lubrication.
Pigments: melanin (UV protection), carotene (diet-derived), hemoglobin (blood flow affects color).
Common color changes: erythema (arrhythmia), pallor, cyanosis.
Albinism: tyrosinase deficiency leading to lack of melanin.
Vitamin D: synthesis in epidermis; essential for calcium absorption and bone/muscle function.
Aging and cosmetics: wrinkles linked to diminished collagen/elastin and UV exposure; prevention via sun protection and hydration.