Biology Chapter 5: Integumentary System

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

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

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

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