Integumentary System: Skin and Accessory Structures (Video)

Epidermis

• Outermost layer of skin; no blood vessels (avascular).

• Nourished by diffusion from the dermis.

• Made of cells in layers (strata).

Epidermal Layers (deep to superficial)

• Stratum basale (germinitivum): Deepest layer; new cells are born here (high mitotic activity); cells begin to fill with keratin.

• Stratum spinosum: Some cell division; looks spiny due to connections (desmosomes).

• Stratum granulosum: Cells die; granules of keratohyalin form.

• Stratum lucidum: Clear, thin layer; only in thick skin (palms, soles).

• Stratum corneum: Topmost layer; dead, flattened, keratin-filled cells; acts as a protective barrier.

Epidermal Cells

• Keratinocytes: Main cells; produce keratin for strength.

• Melanocytes: Produce melanin (pigment); transfer melanin to keratinocytes to protect from UV.

• Langerhans’ cells: Immune system cells.

• Merkel’s cells: Detect light touch and pressure.

• Desquamation: Outer cells regularly shed as deeper cells divide and move up.

• Keratinization: Process where cells fill with keratin on their way to the surface, forming a tough layer.

Dermis

• Layer beneath the epidermis.

• Papillary layer (outer, ${\sim}1/5$ ): Loose connective tissue; contains blood vessels, nerve endings (Meissner’s-touch, free nerve endings-pain), and structures for fingerprints.

• Reticular layer (inner, ${\sim}4/5$ ): Dense connective tissue; has collagen and elastic fibers; contains hair follicles, glands, and heat sensors.

• Provides strength, nourishment to epidermis, and houses nerves and blood vessels.

Subcutaneous Tissue (Hypodermis)

• Not technically part of the skin.

• Made of loose connective tissue and fat tissue.

• Functions: Stores energy, insulates, cushions; contains major blood vessels and nerves for skin.

Skin Color

• Mainly due to:

  • Melanin: Brown/black pigment from melanocytes; protects from UV light.

  • Carotene: Yellow-orange pigment.

  • Blood flow: Amount of blood in skin capillaries (e.g., flushing, blue tint called cyanosis).

• Melanin production and distribution vary, but the number of melanocytes is similar across individuals.

Hair: Structure and Growth

• Components: Hair shaft (visible part), root (below skin), hair bulb (base), hair follicle (surrounds root).

• Layers: Medulla (inner), Cortex, Cuticle (outer).

• Growth: Grows from the matrix in the hair bulb. Average rate is about
  $0.3\,\text{mm/day}$.

• Hair Cycle: Involves growth (anagen), regression (catagen), and resting (telogen) phases.

  • Active growth:
    $2{-}5\,\text{years}$; new cells are added, and hair elongates.

  • Hair is eventually shed, and a new cycle begins.

• Arrector pili muscle: Small muscle that makes hair stand up (goosebumps).

• Hair color: Determined by the type and amount of melanin.

Accessory Skin Structures: Glands

• Sebaceous glands: Produce oily sebum; lubricates hair and skin; has antibacterial properties; usually opens into hair follicles.

• Sweat (sudoriferous) glands:

  • Eccrine (merocrine): Most common; found on palms, soles; regulate body temperature by releasing watery sweat directly onto skin surface.

  • Apocrine: Found in armpits, groin, nipples; active at puberty; release thicker fluid that bacteria break down, causing odor.

• Ceruminous glands: In ear canal; produce earwax (cerumen) to protect the ear.

• Mammary glands: Modified apocrine glands that produce milk.

Nails

• Made of hard keratin; grow continuously.

• Parts: Nail body, eponychium (cuticle), hyponychium (under free edge), nail matrix (where growth occurs).

• Fingernails grow about
  $0.5{-}1.2\,\text{mm/day}$.

Physiology of the Integumentary System

• Protection: Barrier against injury, chemicals, germs, and UV light.

• Sensation: Detects touch, pressure, temperature, pain.

• Thermoregulation: Controls body temperature (sweating to cool, changing blood flow to warm/cool).

• Vitamin D production: Starts in the skin when exposed to UV light, essential for calcium absorption.

• Excretion: Small amounts of waste (urea, salts) are removed in sweat.

• Immunity: Contains immune cells; prevents pathogens from entering.

Aging and Sunlight Effects

• Aging: Skin becomes thinner, less elastic (wrinkles), drier, reduced blood flow. Age spots may appear.

• Sun exposure: Speeds up aging, damages skin cells.

Skin Injuries: Burns and Grafts

• Burns: Classified by depth:

  • First-degree: Only epidermis (e.g., sunburn).

  • Second-degree: Epidermis and part of dermis (blisters).

  • Third-degree: All layers, including subcutaneous tissue (severe).

• Grafts: Healthy skin (split-thickness) or artificial skin used to replace burned or damaged areas.

Cleavage (Tension) Lines

• Natural lines in the skin where collagen and elastic fibers are aligned.

• Incisions made parallel to these lines heal better with less scarring.

• Overstretching skin (e.g., pregnancy) can cause stretch marks (striae).

Quick Reference Formulas and Numbers

• Hair growth rate:
  $0.3\,\text{mm/day}$.

• Epidermal layers: 5 (Stratum basale, spinosum, granulosum, lucidum [thick skin only], corneum).