Biology Chapter 5: Integumentary System

Epidermis

• Keratinized stratified squamous epithelium
• Composed of:
  • Keratinocytes (90%): produce fibrous protein keratin, provide protective properties of skin
  • Melanocytes (8%): produce melanin, protect from UV light
  • Dendritic cells: involved in immune response, produced in red bone marrow
  • Tactile epithelial cells: associated with nerve endings, important in sense of touch
• Thickness: 0.1 mm – 2 mm
• Layers of epidermis (strata, from deep → superficial):
  • Stratum basale: one row of mainly cuboidal or columnar keratinocytes, melanocytes and tactile cells scattered among keratinocytes
  • Stratum spinosum: 8-10 cells thick, more superficial keratinocytes are flatter
  • Stratum granulosum: 3-5 cells thick, flattened keratinocytes, nuclei and organelles degenerate, lots of keratin
  • Stratum lucidum: only in thicker skin, 3-5 cells thick, very flat, dead keratinocytes
  • Stratum corneum: 25-30 cells thick, very flat, dead keratinocytes, waterproof barrier that protects from light, heat, chemicals and invaders

Dermis

• Connective tissue with embedded structures
• Cells include: fibroblasts, macrophages
• Contains: blood vessels, nerves, glands and hair follicles
• Regions of dermis:
  • Papillary region: areolar CT with elastic fibers, projects into epidermis as dermal papillae, loops of capillaries, touch receptors
  • Reticular region: dense irregular CT with lots of collagen and some elastin, contains bits of adipose, hair follicles, nerves, oil and sweat glands

Hypodermis

• Attaches dermis to underlying structures
• Composed of: areolar and adipose tissue
• Also known as subcutaneous layer or superficial fascia
• Contains: blood vessels and nerves

Functions of Integumentary System

• Temperature regulation: evaporation of sweat decreases body temperature, changes in blood flow
• Protection: physical barrier to invasion, dehydration and UV radiation, contains cells of immune system
• Sensation: nerve cells with receptors for pain, touch, pressure, temperature
• Excretion: water and dissolved substances in sweat
• Acts as a blood reservoir: flow can be shifted to hard-working muscles if needed
• Vitamin D synthesis: epidermis makes inactive form of vitamin D upon exposure to UV

Derivatives of the Epidermis (skin appendages)

• Hair:
  • Dead, keratinized cells
  • Shaft projects from surface of skin
  • Root is beneath skin surface, surrounded by hair follicle
  • Functions: protect from light, decrease heat loss, protect eyes and nose from particles, sense of touch, hold chemical signals
• Sebaceous (oil) glands:
  • Holocrine glands
  • Ducts usually open to hair follicle
  • Secrete sebum (mix of fats, cholesterol, proteins, salts, pheromones) that coats hairs, holds moisture in skin, and inhibits bacterial growth
• Sudoriferous (sweat) glands:
  • Eccrine sweat glands:
    • On most skin
    • Ducts open to skin surface
    • Secrete sweat (water, salt, some wastes) for cooling body, excretion, and acidity protects from bacteria
  • Apocrine sweat glands:
    • In axillary, genital and anal regions
    • Ducts open to hair follicles
    • Secrete sweat, fats and proteins, may function in sexual signaling
• Nails:
  • Hard, keratinized cells
  • Nail body is visible
  • Root is under skin
  • Functions: grasp and manipulate objects, scratching