Study Notes on Hair, Nails, and Glands

Chapter Five - Accessory Structures of the Skin

Hair

• Composition: Hair is composed of dead keratinized cells similar to skin, but it contains a tougher form of keratin known as hard keratin due to additional chemical bonds called disulfide bridges.

Types of Hair

1. Lanugo

   • Definition: Fine, downy, unpigmented hair that develops in the fetus during the last three months of gestation.
   • Characteristics: Normally not visible in full-term infants; may appear in slightly premature babies and will naturally fall off.
   • Visual Comparison: May resemble a "little wolf baby" if present.

2. Vellus Hair

   • Definition: Fine hair found on the bodies of children and women.
   • Function: Replaces lanugo at birth.

3. Terminal Hair

   • Definition: Thicker, coarser hair found on the scalp, eyebrows, eyelashes, and, post-puberty, underarms and pubic areas.
   • Development: In males, terminal hair replaces some vellus hair on the face and body post-puberty.
   • Example: Contrast between male and female leg hair (male having terminal hair and female having vellus hair).

Structure of Hair

• Hair Bulb: Swelling at the base of the hair containing living cells (matrix), associated with mitosis for new cell generation.
• Root: Portion of the hair located beneath the skin within the follicle.
• Shaft: The visible part of the hair above the skin.
• Piloerector Muscles: Smooth muscles attached to hair follicles, causing hair to stand upright (goosebumps) in response to cold or fear.

Layers of Hair

1. Medulla: Innermost layer that contains loosely arranged cells and air spaces.
2. Cortex: Bulk of the hair comprised of several layers of elongated keratinized cells.
3. Cuticle: Outermost layer visible above the skin, appearing like scales in microscopic examination.

Hair Receptors

• Function: Nerves surrounding each hair follicle that detect movement or presence of parasites.

Hair Texture and Color

• Texture: Determined by the shape of the hair shaft:
  • Straight: Round shaft
  • Curly: Oval shaft
  • Very curly: Flat shaft.
• Color: Dependent on melanin type:
  • Eumelanin: Responsible for brown or black hair.
  • Pheomelanin: Found in blonde or red hair, lighter color due to sulfur bonding.
  • Gray Hair: Occurs when melanocytes stop producing melanin.
  • White Hair: Results from air spaces in the medulla.

Hair Cycle Phases

1. Anagen Phase: Active growing phase lasting 6-8 years.
2. Catagen Phase: Short, degenerative phase lasting 2-3 weeks when the follicle begins to shrink and stop growth.
3. Telogen Phase: Resting phase lasting 1-3 months where hairs are not actively attached and may fall out.

• Note: All phases occur simultaneously across different hair follicles to prevent total hair loss at once.

Factors Affecting Hair Growth

• Chemotherapy: Induces early telogen phase leading to hair loss.
• Pregnancy: Shifts hair into anagen phase, leading to thicker hair; postpartum, hair shifts back leading to potential loss.

Alopecia

• Definition: Thinning or loss of hair, can be influenced by genetic and hormonal factors.
• Pattern Baldness: Genetic condition losing hair from specific regions of the scalp; often dominant in males and correlated with high testosterone levels.

Functions of Hair

• Mostly vestigial, but historically helped with insulation through goosebumps.
• Hair receptors warn of parasites.
• Scalp hair retains heat and provides protection from sunburn.
• Pubic and axillary hair signal sexual maturity and help in scent transmission.
• Guard hairs (e.g., in the nose and ears) help keep out debris.
• Eyelashes and eyebrows serve nonverbal communication purposes.

Nails

• Composition: Clear, hard derivatives of the stratum corneum (the outer layer of the epidermis) packed with hard keratin, including disulfide bridges.

Structure of Nails

1. Nail Body: Main visible part of the nail.
2. Free Edge: The portion hanging over the finger tip.
3. Eponychium (Cuticle): The cuticle at the base of the nail.
4. Lunula: The half-moon visible at the base of the nail, appearing white.
5. Nail Fold: Skin surrounding the nail.
6. Nail Root: The part under the eponychium from which the nail grows, housing the nail matrix sitting on the nail bed.

Glands Associated with Skin

• Types of Glands: Five types including eccrine, apocrine, sebaceous, ceruminous, and mammary glands.

Sweat Glands

1. Eccrine Glands:

   • Function: Typical sweat glands producing a salty secretion to cool the body.

2. Apocrine Glands:

   • Association: Closely linked with hair follicles.
   • Function: Secrete fatty acid-rich sweat in response to stress or sexual arousal, can lead to body odor (bromhidrosis) due to bacterial breakdown.

Sebaceous Glands

• Function: Associated with hair follicles; secrete an oily substance called sebum that keeps hair pliable and prevents dryness.
• Interestingly, lanolin in skin creams is derived from sheep sebum (extracted from wool).

Ceruminous Glands

• Location: Found in the external ear canal.
• Function: Produce earwax, a waterproof substance that keeps the eardrum flexible and repels pests due to its bitterness.

Mammary Glands

• Definition: Present only during lactation and pregnancy in women, classified as modified apocrine glands.
• Function: Produce a thicker secretion released through ducts at the nipple.

Causes of Acne

• Common Myths: Diet rich in fatty foods/chocolate is not a cause of acne.
• Real Causes:
  • Hormonal changes (especially in teenagers)
  • Abnormal keratinization leading to clogged pores
  • Increased sebum production
  • Bacterial infection from Propionibacterium acnes, leading to inflammation.

Detailed Questions for Review

• What distinguishes lanugo from other types of hair?
• Which parts of hair are considered the root, shaft, and bulb?
• How do texture and color variations in hair arise?
• In what phases does hair growth occur, and what influences these cycles?
• What distinguishes the types of sweat glands, and what are their roles?
• What triggers acne and what misconceptions may exist?

End of Notes