Chapter 05 Integumentary System Notes (Skin, Hair, Nails, Glands, Burns, and Related Topics)
Skin and Integumentary System Overview
The skin is composed of two main layers: the epidermis (superficial) and the dermis (deep). The integumentary system includes the skin and accessory structures such as hair, glands, and nails.
Key skin functions:
Barrier protection against water loss, chemicals, microorganisms, and UV radiation.
Vitamin D production during sun exposure via photolysis of pro-vitamin D3 in the epidermis.
Sensory reception (touch, pressure, pain, temperature).
Excretion and secretion via sweat and sebaceous secretions.
Temperature regulation through blood flow and sweating.
The skin contains multiple structures and tissues that work together for protection, sensation, and homeostasis.
Epidermis: Structure and Functions
The epidermis is a stratified squamous epithelium; it gives rise to hair, nails, and glands.
The epidermis contains keratinocytes and several other cell types (melanocytes, Langerhans cells, etc.).
Keratin and keratinization: Keratin is a fibrous, insoluble protein that strengthens epidermal cells and protects against damage and stress. Keratin monomers assemble into intermediate filaments to form tough, unmineralized epidermal appendages.
Keratin is extremely insoluble in water and organic solvents.
Two forms exist: primitive (softer) keratin and harder keratin found in sauropsids (reptiles/birds) with some notes that birds are reptiles in some texts.
Spider silk has been described as keratin-like in some contexts, though this may have evolved independently.
Epidermal cellular organization and life cycle:
The epidermis has 5 major strata (layers) from deep to superficial: basale, spinosum, granulosum, lucidum, corneum.
Desmosomes are intercellular junctions that function like spot welds between cells.
The stratum granulosum is moribund (dying state, apoptosis near death). The stratum corneum and stratum lucidum are composed of dead cells.
Layer-by-layer overview (from deep to superficial):
1) Stratum basale: Deepest layer; a single layer of cuboidal/columnar cells attached to the basement membrane; produces cells for the superficial strata; melanocytes reside here and synthesise melanin, contributing to UV protection.
2) Stratum spinosum: 8–10 layers of many-sided cells; synthesizes keratin fibers; lamellar bodies form inside keratinocytes.
3) Stratum granulosum: 2–5 layers of flattened, diamond-shaped cells; produces keratohyalin granules; lamellar bodies release lipids from cells; cells die (moribund).
4) Stratum lucidum: 3–5 layers of dead, transparent cells; present in thick skin; disperses keratohyalin around keratin fibers.
5) Stratum corneum: 25+ layers of dead squamous cells protected by keratin and lipids; provides structural strength and prevents water loss; most superficial layer; sloughing-off provides abrasion resistance.Lipids and cellular envelopes:
Intercellular lipids and lamellar bodies contribute to the lipid-rich barrier that reduces water loss.
Movement and life cycle notes:
The basal layer continuously produces cells that move superficially and eventually become part of the dead corneum.
Special notes:
Thick skin (palms and soles) has a thick epidermis; the thickest combination of dermis and epidermis is found between the scapulae.
Melanocytes in the basale layer produce melanin, which protects against UV light. Melanin is then transferred to keratinocytes.
Dermis: Structure, Nerve Endings, and Appendages
The dermis lies beneath the epidermis and provides structural strength and elasticity through connective tissue.
Two major dermal layers:
Papillary layer: Loose connective tissue with dermal papillae projecting toward the epidermis; brings blood vessels close to the epidermis; dermal papillae form fingerprints and footprints.
Reticular layer: Dense irregular connective tissue with a fibrous network of collagen and elastic fibers; forms the main fibrous layer of the dermis; strong in multiple directions; cleavage (Langer) lines influence surgical incisions.
Dermal appendages and notable features:
Hair follicles, sebaceous glands, and sweat glands (eccrine and apocrine).
Blood vessels and nerves run through the dermis; hemodynamics (blood flow) respond to temperature and injury.
Sensory receptors and mechanoreceptors:
Pacinian corpuscles: mechanoreceptors sensitive to vibration and pressure; important for proprioception.
Meissner’s corpuscles (tactile) and Ruffini end organs (continuous touch/pressure).
Free nerve endings detect pain, tickle, temperature, and itch.
Hair and arrector pili:
Arrector pili: smooth muscle attached to hair follicles; contracts to make hairs stand up (goose bumps).
Glands within the dermis:
Sebaceous glands produce sebum (oil) to keep skin from flaking and contribute to skin protection.
Eccrine (sudoriferous) glands are distributed all over the body and secrete a watery sweat composed of water, NaCl, and nitrogenous wastes; sweat is part of the integumentary excretory system.
Apocrine sweat glands are concentrated in the groin and axillary regions; secretion is associated with body odor due to bacterial metabolism of secretions; apocrine glands also secrete oil that promotes bacterial growth.
Dermal–epidermal junction:
Basement membrane anchors the epidermis to the dermis.
The dermis also contains subcutaneous tissue (hypodermis) that lies beneath and provides padding, insulation, and energy storage through adipose tissue; not technically part of the skin but intimately associated.
Subcutaneous Tissue (Hypodermis)
Loose connective tissue with abundant adipose tissue.
Not part of the skin proper but attaches the dermis to underlying structures.
Provides energy storage, insulation, and padding; contains blood vessels and nerves that supply the dermis.
Hair: Structure, Growth, and Forensics
Hair structure components:
Hair shaft (above skin surface): medulla, cortex, cuticle.
Hair root (below skin surface): within the hair follicle; hair bulb at the base houses hair papilla and matrix.
Hair follicle: root sheath (dermal and epithelial components) that anchors the hair; matrix is the growth zone.
Dermal root sheath and other internal/external epithelial root sheaths.
Arrector pili (smooth muscle) attached to the hair follicle.
Growth and anatomy:
Matrix (growth zone) and hair papilla drive hair production.
Hair shaft: medulla, cortex, cuticle structure; shaft is what you shave; follicle is the foundation; root lies between shaft and bulb.
Hair types:
Lanugo: fine fetal hair; present in development; disappears at birth or is replaced by vellus hair.
Vellus hair: fine, pale hair covering much of the body.
Terminal hair: coarser, pigmented hair (found in scalp, eyebrows, etc.).
Pigmentation:
Melanocytes in the hair follicle produce melanin; melanosomes move into the keratinocyte processes and transfer pigment to the hair matrix and shaft.
Forensics and DNA in hair:
Two parts to hair from which DNA can be extracted: 1) hair follicle (nuclear DNA and mtDNA can be isolated) and 2) hair shaft (usually contains mtDNA and may contain very little nuclear material).
Melanin and UV damage:
Melanin production = genetic; higher melanin protects against UV damage; excessive sun exposure can lead to sunburn, epidermal/dermal damage, peeling, and cancer risk.
Nails: Anatomy and Features
Nail structures:
Nail body, nail bed, free edge, nail groove, nail folds, cuticle (eponychium), lunula, nail root, nail matrix, hyponychium.
Cuticle note:
The cuticle (eponychium) is an extension of the stratum corneum from the proximal nail fold.
Nail growth and protection:
The matrix in the nail root is where nail growth occurs; the nail plate travels over the nail bed as it grows.
Glands, Secretions, and Skin Excretory Role
Sebaceous glands: oil production to keep skin from flaking; contribute to skin barrier and hair lubrication.
Eccrine (sudoriferous) sweat glands: widespread across the body; secrete sweat composed primarily of water, NaCl, and nitrogenous wastes; part of excretory system; sweat is produced to regulate temperature and electrolyte balance.
Apocrine sweat glands: located mainly in the groin and axillary regions; secretion is stimulated by nervousness; can contribute to body odor due to bacterial breakdown of secretions; oils secreted can promote bacterial growth; antiperspirants reduce sweating, deodorants mask odor.
Integument as an excretory organ: sweat contributes to excretion of wastes alongside other organs.
Keratin and Keratinization: Structural Proteins of the Skin
Keratin definition and role:
Keratin is a fibrous, structural protein (scleroprotein) that protects epithelial cells from damage and stress.
Keratin monomers assemble into bundles forming intermediate filaments; keratinized tissues are tough and protective.
Types and distribution:
Primitive, softer keratins vs harder keratins found in sauropsids (reptiles/birds).
Keratinization fortifies tissues such as horns (cattle, rhinos) and osteoderms in armadillos; keratinized tissue toughness is among the strongest biological materials alongside chitin.
Relationship to other structures:
Found in scales, hair, nails, feathers, horns, claws, and hooves; outer layer of vertebrate skin is keratinous.
Miscellaneous notes:
Some references claim spider silk is keratin-like, though this may be an independent venous evolution.
Vitamin D Synthesis in the Skin
UV exposure converts a skin precursor into cholecalciferol (vitamin D3).
Pathway (simplified):
In liver: vitamin D3 is converted to calcidiol; in kidney: calcidiol is converted to calcitriol (active vitamin D).
Calcitriol is the active hormone form that regulates calcium and phosphate metabolism crucial for bone health.
Additional notes: Vitamin D3 is present in certain fish and eggs and can be synthesized commercially from lanolin; the kidney completes the final activation.
Skin Cancer: Basal, Squamous, and Melanoma
Basal cell carcinoma (BCC): starts in the stratum basale; slow-growing; least lethal among the three major skin cancers.
Squamous cell carcinoma (SCC): starts in the stratum spinosum; more aggressive than BCC.
Melanoma: starts in melanocytes; may begin as a mole; can metastasize; most lethal among the three; highest risk with UV exposure.
Carcinoma definition: a cancer that originates in epithelial tissue.
Practical note: Early recognition and prognosis depend on the type and depth of invasion; melanoma has the highest potential for metastasis.
Burns: Degrees, Management, and Systemic Effects
Burn severity is categorized by depth:
First-degree: partial-thickness, involves only the epidermis.
Second-degree: partial-thickness, involves epidermis and part of the dermis.
Third-degree (full-thickness): all layers of skin, may involve underlying tissues.
The burn scale (illustrated by Figure 05.14) is used to assess burn severity.
Treatment and supportive care:
Intravenous fluids to maintain perfusion and prevent shock.
High-protein, high-calorie diet to support healing and metabolic needs.
Antimicrobials to prevent infection.
Debridement of dead tissue.
Skin grafts for wound closure and functional restoration.
Systemic and organ-level responses to burns (multisystem impact):
Cardiovascular/Lymphatic/Immune: decreased blood volume, edema, shock; increased capillary permeability; risk of infection; potential for immune suppression over time.
Nervous: pain in partial-thickness burns; brain centers reset body temperature during metabolic stress.
Muscular/Skeletal: hypermetabolic state; loss of muscle mass; potential bone marrow responses (e.g., increased red blood cell production in red bone marrow).
Digestive: decreased blood flow can damage intestinal lining and liver; risk of bacterial translocation and systemic infection; liver releases clotting factors in response to injury.
Urinary: reduced urine production due to low blood volume; potential kidney injury from poor perfusion.
Respiratory: edema can obstruct airways; tachypnea from hypermetabolic state.
Endocrine: stress-related release of epinephrine and norepinephrine increases metabolic rate and body temperature.
Common complications and signs:
Edema, shock, infection risk, and systemic inflammatory response.
Integumentary Diseases and Conditions (Table 5.2 Overview)
Birthmarks: congenital disorders of the dermal capillaries.
Ringworm: fungal infection causing patchy scaling and inflammation.
Eczema and dermatitis: inflammatory skin conditions due to allergy, infection, poor circulation, or chemical/environmental exposure.
Frostbite: tissue freezing injury (case studies in the chapter).
Psoriasis: chronic skin disease with thickened epidermis and silvery scales; may bleed if scales are scraped.
Vitiligo: patches of skin with loss of melanocytes.
Alopecia areata: autoimmune hair loss.
Skin cancer: includes BCC, SCC, and melanoma.
Impetigo: bacterial infection with small pus-filled blisters forming crusts.
Erysipelas: swollen patches caused by Streptococcus pyogenes.
Decubitus ulcers (pressure ulcers): occur from prolonged pressure in bedridden or immobile individuals.
Acne: common inflammatory skin condition related to bacteria and sebum.
Viral infections: measles (rubeola), rubella, chickenpox (varicella), shingles (reactivation of varicella), cold sores (herpes simplex I), genital herpes (herpes simplex II).
Note: COVID-19 discussed in the context of skin-related inflammation.
Forensics, DNA, and Hair Analysis Notes
Two DNA sources from hair:
Hair follicle: contains cellular DNA (nuclear DNA).
Hair shaft: typically contains mitochondrial DNA (mtDNA) and may have little nuclear DNA.
DNA extraction and forensic relevance depend on the presence of root tissue in the sample.
Patterned Skin Incisions and Surgical Considerations
Surgeons cut parallel to pattern bands in connective tissue to minimize scarring; cutting perpendicular to these lines can increase scarring.
These patterns are referred to as Langer’s lines (or natural cleavage lines).
Greek and Latin Roots: Language Primer (Exam Hint)
The course emphasizes Greek and Latin roots used in anatomical terms; these roots will appear on exams and through clinical practice.
Immunology of Skin Grafts
Skin grafts may require immunosuppression to prevent rejection when donor tissue is not MHC-matched with the recipient.
Major Histocompatibility Complex (MHC) is critical in graft acceptance or rejection.
The Cuticle and Nail Anatomy (Repeated Note)
The cuticle (eponychium) is an extension of the stratum corneum from the proximal nail fold.
Inguinal Hernias: Basic Concept (From Later Slides)
An inguinal hernia is the abnormal exit of tissue or an organ (commonly bowel) through the wall of the cavity in which it normally resides.
Inguinal canals are passages in the anterior abdominal wall through which spermatic cords (in males) and the round ligament of the uterus (in females) pass; hernias typically present as bulges in the groin.
Lifting heavy objects is a risk factor for inguinal hernias.
Notes and Practical Reminders
The skin and its appendages (hair, nails, glands) play a role in temperature regulation, protection, and sensory perception.
Healthy skin requires adequate nutrition, hydration, and protection from excessive UV exposure.
For burns, prompt medical evaluation is essential, and management focuses on fluid balance, infection prevention, and wound closure.
Forensic DNA from hair can be informative but depends on sample quality and which part of the hair is available.
Surgical incisions work best when aligned with natural skin tension lines to minimize scar visibility.
Many Greek/Latin roots underpin anatomical terms; familiarity with these roots speeds learning and clinical communication.
Key Equations and Notable Formulas
Hydrogen peroxide detoxification (a common ROS pathway in skin):
Vitamin D3 synthesis pathway (simplified):
Epidermal layer counts (for quick reference):
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