Integ+All+Slides

Learning Objectives

  • Distinguish between the epidermis, dermis, and hypodermis.
  • Review the functions of the skin.

The Integument

  • Composed of three layers:
    • Epidermis:
    • Mostly dead, keratinized cells.
    • Avascular (lacks blood vessels).
    • Dermis:
    • Composed of live cells.
    • Vascular (rich blood supply).
    • Hypodermis: (subcutaneous layer)
    • Contains adipose and connective tissue.

Accessory Appendages

  • Pilosebaceous apparatus:
    • Hair follicles.
    • Arrector pili muscles (cause hair to stand).
    • Sebaceous (oil) glands.
  • Sudoriferous (sweat) glands.
  • Ungulae (nails).

Functions of the Integument and Appendages

  • Protection:
    • Against abrasion, friction, infection, shock absorption, UV rays.
    • Acts as a permeability barrier.
  • Regulation:
    • Thermoregulation.
    • Excretion/Absorption (sweat, oils).
    • Vitamin D synthesis (critical for calcium absorption).
    • Energy storage (adipose tissue).
  • Sensory Perception:
    • Tactile sensations (touch).
    • Temperature regulation.

Cells and Functions of the Epidermis

  • Types of Cells:
    • Keratinocytes:
    • Produce keratin, providing physical protection.
    • Replace outer skin cells every 4-6 weeks through keratinization.
    • Melanocytes:
    • Produce melanin for UV protection.
    • Transfer melanin to keratinocytes.
    • Intraepidermal Macrophages (Langerhans Cells):
    • Participate in immune responses.
    • Merkel Cells:
    • Associated with sensory receptors for tactile sensations.

Epidermal Protection

  • Keratinocytes mature by filling with keratin:
    • Forms interlocking cell layers for abrasion resistance.
    • Outer cells are constantly shed off.
  • Positive feedback loop in keratinocyte turnover:
    • Cells are generated and shed at the same rate (4-6 weeks).

UV Protection

  • Melanin:
    • Absorbs UV radiation and neutralizes free radicals.
    • Transfer to keratinocytes helps shield their DNA from UV damage.

Vitamin D Synthesis

  • Vitamin D (calcitriol):
    • Important for calcium absorption; synthesized via skin exposure to UVB light.
    • Involves conversion of 7-dehydrocholesterol (7-DHC) to Vitamin D3, which is activated by the liver and kidneys.

Sensory Functions

  • Tactile Epithelial Cells (Merkel Complex):
    • Sensitive to touch, primarily found in palms, soles, and lips.
  • Free Nerve Endings:
    • Sensitive to pain, temperature, and other stimuli.

Layers of the Epidermis

  • Stratum corneum: Most superficial, comprised of dead keratinized cells.
  • Stratum lucidum: Present in thick skin (palms/soles), clear layer.
  • Stratum granulosum: Keratinocytes undergoing apoptosis, losing metabolic activity.
  • Stratum spinosum: Keratinocytes connected by desmosomes, visible 'spines'