Overview
This lecture covers the scientific properties, structure, and composition of hair and scalp, focusing on hair anatomy, chemical composition, types, growth cycles, and key concepts crucial for cosmetologists.
Structures of Hair and Scalp
- Hair consists of two main parts: the hair root (below the skin) and the hair shaft (visible above the skin).
- The hair root includes the follicle, hair bulb, dermal papilla, arrector pili muscle, and sebaceous (oil) glands.
- Hair follicles are found all over the body except palms and soles.
- The hair shaft is composed of three layers: cuticle (outer), cortex (middle), and medulla (inner).
Chemical Composition of Hair
- Hair is about 90% protein, primarily keratin.
- Keratin is formed through keratinization as hair cells mature, fill with protein, and die.
- Five main elements in hair: carbon (51%), oxygen (21%), hydrogen (6%), nitrogen (17%), and sulfur (5%).
Protein Structure and Bonds
- Amino acids form long chains held together by peptide (end) bonds, creating polypeptide chains with a helical shape.
- The cortex contains three types of side bonds: hydrogen (broken by water/heat), salt (broken by pH changes), and disulfide (broken by chemicals).
- Hydrogen and salt bonds are weak and physical; disulfide bonds are strong and chemical.
Hair Pigment and Types
- Melanin in the cortex gives hair its color; eumelanin (brown/black) and pheomelanin (red/yellow).
- Gray and white hair have less or no melanin and may lack a medulla.
- Hair types: vellus hair (fine, unpigmented, no medulla) and terminal hair (coarse, pigmented, often with medulla).
Hair Growth and Patterns
- Hair growth cycles have three phases: anagen (growth), catagen (transition), and telogen (resting).
- Cutting or massaging the scalp does not affect hair growth rate or thickness.
- Natural wave patterns (straight, wavy, curly, coily) are genetically determined and can vary on the same head.
Key Terms & Definitions
- Trichology — the scientific study of hair, its diseases, and care.
- Keratinization — process where cells fill with keratin, die, and form the non-living hair shaft.
- Hair follicle — tube-like depression in the skin that contains the hair root.
- Cuticle — outer layer of the hair shaft.
- Cortex — middle, protein-rich layer containing pigment and influencing strength and elasticity.
- Medulla — innermost layer, often absent in fine or light hair.
- Anagen phase — growing stage of hair.
- Catagen phase — transitional stage of hair.
- Telogen phase — resting stage of hair.
Action Items / Next Steps
- Review and note all vocabulary, structures, and key processes described up to this lecture in your textbook.
- Prepare for upcoming content on hair loss, scalp conditions, and treatments in the next lecture segment.