Accessory Structures of the Skin - Summary

Accessory Structures of the Skin

  • Accessory structures include hair, nails, sweat glands, and sebaceous glands.

  • Originate from the epidermis and extend into the dermis/hypodermis.

Hair

  • Functions: protection, sensory input, thermoregulation.

  • Types:

    • Lanugo: unpigmented hair in fetus and newborns.

    • Vellus: short, pale, fine hair over the body; pigmented.

    • Terminal: dark, coarse hair on scalp, brows, legs, axillae, perineum after puberty; pigmented.

  • Composition:

    • Keratinized cells from hair follicle.

    • Hair shaft: projects above skin surface.

    • Hair root: penetrates dermis.

    • Hair bulb: deep in the dermis.

    • Hair matrix: mitotically active basal cells.

  • Hair Shaft Layers:

    • Medulla

    • Cortex

    • Cuticle

  • Arrector pili muscle contraction makes hair erect.

  • Growth: new cells at hair bulb push hair shaft towards surface.

  • Colour: melanin from melanocytes in hair papilla; different types of melanin cause different colors; melanin production decreases with age.

Nails

  • Hard, keratinized epidermal cells on dorsal surfaces of fingers and toes.

  • Parts:

    • Free edge

    • Transparent nail body (plate) with whitish lunula at base

    • Nail root embedded in skin fold

Glands

  • Sebaceous (oil) glands and sudoriferous (sweat) glands in the reticular region of the dermis.

Sweat Glands (Sudoriferous)

  • Most numerous on palms and soles.

    • Eccrine glands: respond to temperature and stress; secrete sweat for thermoregulation and waste excretion (urea).

    • Apocrine (odoriferous) glands: in axilla and genital area; respond to stress; secrete pheromones; react with bacteria to cause body odor.

    • Ceruminous glands: apocrine glands in ear canal; secrete cerumen to prevent drying and trap substances.

Sebaceous Glands

  • Produce sebum to lubricate and protect skin and hair.

  • Sebum:

    • Has antibacterial properties

    • Prevents water loss

Skin and Homeostasis

  • Functions: protection, temperature regulation, sensory reception, water balance, vitamin and hormone synthesis, absorption.

  • Skin as barrier against microbes, viruses, and water/fluid loss.

  • Melanocytes protect against UV radiation.

Skin & Other Systems

  • Immune System: first line of defense with oil-secreting glands and immune cells.

  • Synthesis: synthesizes melanin, carotenes and vitamin D.

  • Thermoregulation: Brain sends nerve impulses to the skin to increase or decrease the body's heat loss.

    • Hair traps warmth when standing up.

    • Glands secrete sweat for evaporation.

    • Capillaries open to cool, close to conserve heat.

  • Circulatory System: blood vessels transport O2 and nutrients to the skin and remove CO2 and wastes.

  • Endocrine System: circulating hormones communicate between skin and body (steroid, thyroid, sex, growth hormones, epidermal growth factor).

Permeability of Skin

  • Selectively permeable to fat-soluble substances (vitamins A, D, E, K, steroid hormones).

  • Transdermal patches deliver drugs through epidermis to blood vessels in the dermis (small, lipid-soluble molecules like estrogen, nitroglycerin, nicotine).