Overview
This lecture covers the accessory organs of the skin—primarily hair, nails, and skin glands—focusing on their types, structure, functions, and growth.
Hair: Types and Distribution
- Hair and nails are made of hard keratin; skin is made of soft keratin.
- Humans have about 5 million hairs, with 98% on the body and ~100,000 on the head.
- Vellus hair: fine, body hair.
- Terminal hair: found on head, eyebrows, eyelashes, and after puberty in armpits, pubic areas, face, trunk, and limbs.
- Lanugo: fine, unpigmented fetal hair, seen in last three months before birth.
Hair Structure and Follicle Anatomy
- Hair follicle: tube-like depression in skin; surrounds the hair.
- Hair bulb: swelling at base where hair originates.
- Root: hair within the follicle; shaft: hair above skin surface.
- Medulla: innermost core, loosely arranged cells (thick hair only).
- Cortex: keratinized cells surrounding medulla.
- Cuticle: outermost layer.
Hair-Associated Structures & Functions
- Hair receptors: nerve fibers around follicle, sense hair movement.
- Arrector pili muscle: smooth muscle, causes hair to stand (goosebumps).
- Functions: sensation, protection from heat loss, UV rays, and entry of foreign objects (e.g., nose/ear hairs).
Hair Texture, Color, and Growth Cycle
- Straight hair: round cross-section; wavy: oval; curly: flat.
- Hair color depends on melanin type and amount: eumelanin (black/brown), pheomelanin (red/blond), lack of melanin (gray/white).
- Growth phases: anagen (growth), catagen (base dies), telogen (rest).
- Scalp hair can be in the anagen phase for 6–10 years; body hair 2–3 months.
- Lose about 50 scalp hairs daily.
Nails
- Nails are protective coverings on fingers and toes.
- Growth resembles skin and hair, via keratinocyte production.
Skin Glands
- Eccrine (merocrine) sweat glands: watery perspiration for cooling, found all over, responsible for insensible perspiration (~500 mL/day).
- Apocrine sweat glands: thick, milky secretion, found in pubic, anal, axillary, areolar regions, and male beard; may produce body odor.
- Sebaceous glands: produce sebum (oil), lubricate hair and skin.
- Ceruminous glands: produce cerumen (earwax), found in ear canal.
- Mammary glands: modified sweat glands, produce milk.
Key Terms & Definitions
- Keratin — tough structural protein in hair, nails, and skin
- Hair follicle — sheath of cells/tissue where hair grows
- Eumelanin/Pheomelanin — pigments giving hair color
- Arrector pili — muscle causing hair to stand
- Sebum — oily secretion from sebaceous glands
- Lanugo — fine fetal hair
Action Items / Next Steps
- Review diagrams of hair and gland structures.
- Read textbook section on accessory skin organs.