Chapter:6 Integumentary System Notes (Chapter 6)

6.1 Introduction to the Integumentary System

• Organ definition: a body structure composed of two or more different tissue types that perform specific functions.

• Skin is the largest organ in the body by weight.

• Accessory structures of the skin include: hair, nails, glands, and sensory receptors.

• The skin and its accessory organs together form the integumentary system.

• Function of the skin as a barrier: separates the internal environment from the external environment.

6.2 Skin and Its Tissues

• Membrane of the skin: the basement membrane separates the epidermis from the dermis and anchors the layers together.

• Major layers of the skin:

  • Epidermis (outer, avascular, stratified squamous epithelium).

  • Dermis (thicker, inner layer of connective tissue, containing blood vessels, smooth muscle, and nervous tissue).

  • Subcutaneous layer (hypodermis): layer beneath the dermis; composed of areolar and adipose tissues; binds skin to underlying tissues but is not part of the skin.

• Subcutaneous layer functions:

  • Insulates to conserve body heat via adipose tissue.

  • Contains major blood vessels that supply the skin.

• The epidermis: structure and turnover

  • In most areas, the epidermis has 4 layers; in thick skin, it has 5 layers.

  • Stratum basale (stratum germinativum) is the deepest layer; it contains dividing cells.

  • Basal cells divide; older cells become keratinocytes (due to keratin accumulation).

  • Keratinization: keratinocytes migrate outward, harden, dehydrate, and die as they move toward the surface.

  • The epidermis is avascular and depends on diffusion from the dermis for nutrients.

• The epidermis: layers and their roles

  • Stratum basale: innermost, dividing layer; nourished by dermal blood vessels.

  • Stratum spinosum: spine-like desmosomes contribute to strength.

  • Stratum granulosum: keratinization begins here.

  • Stratum lucidum: a thin, clear layer found only in thick skin (palms and soles).

  • Stratum corneum: outermost layer; dead, flattened, keratinized cells; continually shed from the surface.

• The epidermis’ protective role: protects against water loss, mechanical injury, chemicals, and microorganisms.

• Melanocytes and skin color

  • Melanocytes produce melanin and reside in the deepest layer of the epidermis and in the dermis.

  • Melanin protects DNA from UV radiation by shading the nucleus; higher melanin = darker skin/hair.

  • Melanin is transferred to nearby cells via cytocrine secretion.

  • Melanin types:

  • Eumelanin: brownish-black.

  • Pheomelanin: reddish-yellow (e.g., lips).

• Skin color determinants

  • All people have about the same number of melanocytes.

  • Skin color is genetically determined by the amount of melanin produced and the size/distribution of melanin granules.

  • Color results from a combination of genetic, environmental, and physiological factors.

  • Exposure to sunlight, UV light from lamps, and X-rays darken skin by increasing melanin production.

  • Circulation of blood in dermal vessels affects color: oxygen-rich blood is bright red; oxygen-poor blood can cause cyanosis (bluish tint).

  • Diet/disease can also affect color: carotene-rich foods can yellow-orange skin; jaundice can produce a bright yellow color due to liver disease.

• The dermis: structure and function

  • Binds epidermis to underlying tissues; the boundary is uneven due to epidermal ridges and dermal papillae.

  • Dermal papillae + epidermal ridges create a genetically determined pattern of friction ridges (fingerprints).

  • The dermis contains: areolar and dense connective tissue with collagen and elastic fibers arranged in a gel-like ground substance; provides toughness and elasticity.

  • Dermal blood vessels supply nutrients and regulate body temperature.

  • The dermis houses nerve fibers, sensory receptors, hair follicles, sebaceous glands, and sweat glands.

• Summary of epidermal layers in context

  • 4 layers (not including thick skin): Stratum basale, Stratum spinosum, Stratum granulosum, Stratum corneum.

  • Additional thick-skin layer: Stratum lucidum between granulosum and corneum.

6.3 Accessory Structures of the Skin: Epidermal Derivatives

• Nails

  • Protective coverings over the ends of fingers and toes.

  • Nail plate overlays the nail bed.

  • Lunula: half-moon-shaped, visible portion at base of the nail plate; indicates the nail matrix (actively growing region).

  • New nail cells are produced in the nail matrix; older cells are pushed outward and keratinize.

  • Nail keratin is harder than epidermal stratum corneum keratin.

  • Nail appearance is affected by genetics, injury, nutritional deficiencies, disease, age; nails can aid medical diagnosis.

• Hair and hair follicles

  • Hair is found in most skin regions, except for palms, soles, lips, nipples, and portions of external genitalia.

  • Hair development starts from epithelial stem cells at the base of a hair follicle.

  • Hair follicles extend into the dermis or subcutaneous layer.

  • Hair root deep in follicle ends at the hair bulb; hair matrix within the bulb contains epithelial cells nourished by dermal papilla.

  • As new cells form in the hair bulb, older cells keratinize and die, forming the hair shaft.

  • Hair is composed of dead, keratinized epithelial cells.

• Hair color and properties

  • Hair color is determined by genetics that control type/amount of melanin in epidermal melanocytes.

  • Dark hair: predominantly eumelanin; blonde/red hair: more pheomelanin.

  • Absence of melanin or aging can lead to albinism (white hair).

  • Gray hair results from a mixture of pigmented and unpigmented hair.

  • Arrector pili muscle (smooth muscle) attaches to each hair follicle; contraction causes goose bumps in response to cold or emotional states.

• Skin glands overview

  • Sebaceous glands: secrete sebum (oil) to waterproof and soften hair/skin; holocrine glands (entire cells filled with secretion are released).

  • Sweat (sudoriferous) glands: merocrine glands that secrete by exocytosis.

  • Two main types of sweat glands:

  • Eccrine glands: respond to body temperature; numerous on forehead, neck, back; secrete onto skin surface.

  • Apocrine glands: become active at puberty; respond to fear, emotional upset, pain, or sexual arousal; mainly in axilla and groin; secretion contains proteins/fats contributing to body odor; secrete into hair follicles.

• Other modified glands

  • Ceruminous glands: secrete ear wax (cerumen) in the ear canal to trap dust and pathogens.

  • Mammary glands: secrete milk to nourish a baby.

6.4 Skin Functions

• General skin functions

  • Maintains homeostasis; protective covering; barrier against substances/pathogens.

  • Waterproofs the body; slows water loss by diffusion.

  • Melanin pigment protects against UV radiation from the sun.

  • Houses sensory receptors for touch, pressure, pain, and temperature.

  • Excretes wastes; participates in chemical reactions such as synthesis of vitamin D (vital for bone and tooth development).

  • Involved in body temperature regulation via sweat gland activity and vasodilation/vasoconstriction of dermal vessels.

  • Synthesizes certain biochemicals.

• Role in body temperature regulation

  • Temperature regulation is vital for maintaining optimal metabolic reaction rates; the hypothalamus controls this process.

  • Heat production occurs in active cells (e.g., heart, skeletal muscle, liver).

  • Heat loss occurs primarily through radiation and evaporation of sweat from the skin.

• Skin’s responses to body temperature changes

  • Hyperthermia (excess heat): vasodilation of dermal vessels increases heat loss; sweating increases and evaporates to cool the surface.

  • Hypothermia (excess cooling): vasoconstriction retains core heat; sweat glands inactivate; shivering increases cellular respiration to generate heat.

• Healing of wounds (general concepts)

  • Inflammation is the normal response to injury: blood vessels dilate and become more permeable, causing redness and swelling; helps bring fluids, oxygen, nutrients, and immune cells to the site.

  • Superficial epidermal wounds heal by epithelial cell reproduction; deeper wounds extending into the dermis/subcutaneous layer require a more extensive response.

  • Phagocytic cells play a key role in cleaning debris during healing.

  • Granulation tissue forms as part of deep wound healing: new blood vessels grow into the area, accompanied by fibroblasts that produce collagen to repair the wound; vessels may be resorbed later, leaving scar tissue.

• Inflammation: characteristic signs and causes (Table 6.1 reference)

  • Redness: caused by vasodilation and increased blood flow to the area.

  • Heat: from increased blood in the area and elevated metabolic activity.

  • Swelling: from increased vascular permeability and edema (fluids in tissue spaces).

  • Pain: due to injury to neurons and increased pressure from edema.

• Healing of a deep wound (step-by-step overview)

  • Step 1: Blood clot forms, creating a scab that covers the wound; inflammation begins.

  • Step 2: Fibroblasts migrate in and secrete collagen to bind wound edges; phagocytic cells remove debris; granulation tissue forms.

  • Step 3: Blood vessels regrow into the area; granulation tissue continues to fill the wound.

  • Step 4: Regrowth of epidermis; scab may slough off; damaged dermis replaced with scar tissue (fibrosis).

  • Large wounds leave scars; granulation tissue may form a temporary elevated scar.

• Visual representation (Stage references)

  • Wound healing involves a progression from blood clot and inflammation to scab formation, granulation tissue growth, and eventual resurfacing of epidermis with possible scar formation.

Common Skin Disorders (quick reference)

• Acne (AK-nee): disease of sebaceous glands causing blackheads and pimples.

• Keloid (KEY-loyd): elevated, enlarging fibrous scar usually from an injury.

• Alopecia (al-oh-PEE-sha): hair loss, often sudden.

• Mole (mohl) / Nevus: benign pigmented skin tumor.

• Athlete’s foot (ATH-leetz foot): fungal infection (Tinea pedis) of toes/soles.

• Pediculosis: infestation with lice.

• Birthmark (BERTH-mark): congenital blemish or spot.

• Pruritus (proo-RYE-tus): itching of the skin.

• Boil (furuncle): bacterial infection of a hair follicle.

• Psoriasis (so-RYE-ah-sis): chronic red patches with silvery scales.

• Carbuncle (KAR bunk-ul): bacterial infection spreading into subcutaneous tissues.

• Pustule (PUS-tyul): pus-filled lesion.

• Cyst (sist): liquid-filled sac.

• Scabies (SKAY-beez): infestation by mites.

• Dermatitis: inflammation of the skin.

• Seborrhea (seb-oh-REE-ah): oily skin and dandruff due to sebaceous gland activity.

• Eczema (EK-zeh-mah): noncontagious skin rash with itching, blisters, scaling.

• Ulcer (UL-ser): open sore.

• Erythema (er-ih THEE-mah): reddening due to vascular dilation.

• Urticaria (ur-tee-KARE-ee-a): hives; allergic skin reaction forming red raised patches.

• Herpes (HER-peez): infectious skin disease with recurring vesicle clusters.

• Vitiligo (VIT-ih-LEE-goh): loss of melanocytes causing whitened skin areas.

• Impetigo (im-peh-TIE-go): contagious bacterial skin infection with pustules/crusts.

• Wart (wort): flesh-colored raised area from a viral infection.

6.1–6.4 Questions (for quick recall)

• 6.1: Why does the skin constitute an organ? (Answer: composed of multiple tissue types; performs specific functions.)

• 6.1: What structures make up the Integumentary System? (Answer: skin, hair, nails, glands, sensory receptors.)

• 6.2: Questions cover membranes, layers, tissues, formation of epidermis, stratum basale vs stratum corneum, stratum lucidum, tissues in dermis, and factors influencing skin color.

• 6.3: Questions cover the nail bed structure, hair formation, sebaceous gland function, and differences between merocrine and apocrine glands.

• 6.4: Questions cover functions of the skin, heat loss/conservation, inflammation response, wound healing differences between epidermal and dermal wounds, role of phagocytes, and definition of granulation.

Connections, implications, and real-world relevance

• Skin as a barrier is essential for maintaining homeostasis and protecting against pathogens, dehydration, and UV damage.

• Melanin’s protective role against UV radiation has implications for skin cancer risk, sun exposure guidelines, and cosmetic/health considerations.

• The integumentary system’s role in vitamin D synthesis links skin health to bone health and calcium metabolism.

• Understanding wound healing and inflammation has clinical relevance for treating burns, ulcers, and surgical wounds; scarring and granulation tissue are key outcomes to monitor.

• Gland function (sebaceous, eccrine, apocrine, ceruminous, mammary) has practical implications for dermatology, hygiene, and disease management (e.g., acne, hyperhidrosis).

• Common skin disorders provide a quick reference for recognizing signs that may require medical evaluation (e.g., infections, dermatitis, psoriasis, herpes, vitiligo, impetigo).

Important terms to know (glossary-style recap)

• Epidermis, Dermis, Hypodermis (Subcutaneous layer)

• Strata: basale, spinosum, granulosum, lucidum (thick skin), corneum

• Keratinocytes, Melanocytes, Melanin, Eumelanin, Pheomelanin

• Desmosomes, Basement membrane, Dermal papillae, Epidermal ridges

• Nails: nail plate, nail bed, lunula, nail matrix

• Hair follicle, Hair bulb, Hair matrix, Dermal papilla, Arrector pili

• Sebaceous glands, Sudoriferous (eccrine and apocrine) glands

• Ceruminous glands, Mammary glands

• Inflammation, Granulation tissue, Fibroblasts, Collagen, Scar (fibrosis)

• Wound healing stages: clot, scab, epithelial regeneration, granulation, scar formation

• Common disorders: acne, keloid, alopecia, mole, athlete’s foot, pediculosis, birthmark, pruritus, boil, psoriasis, carbuncle, pustule, cyst, scabies, dermatitis, seborrhea, eczema, ulcer, erythema, urticaria, herpes, vitiligo, impetigo, wart

4 layers of epidermis in most areas, 5 in thick skin.
4 major skin layers: epidermis, dermis, subcutaneous layer, plus accessory structures embedded within.