Lecture Notes: Structures and Functions of Hair

Overview of Hair Structure

• Hair is composed of dead keratinized cells.
• Absent in areas: palms, soles, lips, nipples, portions of external genitalia.
• Functions include:
  • Sensory detection (e.g., insects).
  • Protection from physical trauma.
  • Reducing heat loss.
  • Shielding skin from sunlight.

Structure of Hair

• Hair (Pili): Flexible strands of dead keratinized cells.
• Similar to the stratum corneum in skin but tubular in shape.
• Produced by Hair Follicles:
  • Contain hard keratin (tougher than the soft keratin in skin).

Regions of Hair

• Shaft: Part extending above the skin; keratinization is complete.
• Root: Part within the skin/scalp; keratinization is still occurring.
• Parts of the Hair Shaft:
  • Medulla: Central core of large cells and air spaces.
  • Cortex: Several layers of flattened cells surrounding the medulla.
  • Cuticle: Outer layer of overlapping single cells.

Hair Pigmentation

• Created by melanocytes in the hair follicle.
• Combination of melanin types (yellow, rust, brown, black) determines hair color.
  • Red hair contains additional pigment, pheomelanin.
  • Gray/white hair from decreased melanin production, air bubbles replace melanin.

Hair Follicle Structure

• Components:
  • Peripheral connective tissue sheath (from dermis).
  • Epithelial root sheath (from epidermis).
  • Hair Matrix: Actively dividing area producing new hair cells.
• Hair Papilla: Dermal tissue extension supplying nutrients to hair matrix.
• Sensory Components:
  • Hair follicle receptors (root hair plexus)—sensory nerve endings.

Types of Hair

• Vellus Hair: Pale, fine body hair (e.g., children, adult females, peach fuzz).
• Terminal Hair: Coarse, long hair (e.g., scalp, eyebrows, during puberty).

Hair Growth and Health

• Influences: nutrition and hormones.
• Follicle cycles between active and regressive phases.
• Normal loss: ~90 scalp hairs/day.
• Growth rate: ~2.25 mm/week.

Hair Loss Conditions

• Alopecia: Age-related thinning after 40 years.
• True Baldness: Genetic and sex-linked (male pattern baldness).
  • Caused by dihydrotestosterone (DHT) affecting follicle response.
• Induced Hair Thinning:
  • Factors: high fever, surgery, emotional trauma, certain drugs (e.g., antidepressants, blood thinners, steroids, chemotherapy).
  • Alopecia Areata: Autoimmune attack on follicles.
  • Some hair loss reversible; permanent loss due to scars or burns.