Hair

Aka: Hair, Pilosebaceous Unit, Pilosebaceous Follicle, Hair Follicle, Hair Growth, Hair Bulb, Hair Root, Hair Shaft, Hair Medulla, Hair Cortex, Hair Cuticle, Dermal Papilla, Anagen, Catagen, Telogen, Arrector Pili Muscle, Hair sheath, Internal Root Sheath, External Root Sheath, Vellus Hair, Terminal Hair, hair Anatomy

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II. Background

  1. Normal scalp has more than 100,000 hairs
  2. Hairs normally lost per day: 50 to 100

III. Distribution

  1. Vellus Hair (from vellus follicles)
    1. Non-pigmented fine "peach fuzz" hair covering body
    2. Found in children and adults
    3. Increased in Hypertrichosis
  2. Terminal Hair (from terminal follicles)
    1. Androgen-dependent male-type body hair
    2. Terminal Hairs are long, thick and pigmented
    3. Face (Mustache, beard and sideburns)
    4. Body (Chest, areola, linea alba, inner thighs)
    5. Increased in Hirsutism

IV. Anatomy: Images

  1. See Skin Anatomy
  2. dermHairFollicleGrayBB944.gif Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)
  3. dermHairFollicleXsGrayBB945.gif Lewis (1918) Gray's Anatomy 20th ed (in public domain at Yahoo or BartleBy)

V. Physiology: General

  1. Pilosebaceous Unit (Pilosebaceous Follicle)
    1. Hair Follicle
    2. Sebaceous Gland
    3. Arrector Pili Muscle
  2. Hair Follicle
    1. Hair producing unit based in Hypodermis
    2. Cylinder that ascends through surface epithelium
    3. Hair held within center of follicle's cylinder
    4. Entire follicle encased in connective tissue
  3. Hair Bulb
    1. Located at base of Hair Follicle in Hypodermis
    2. Includes Hair Root which surround Dermal Papilla
  4. Dermal Papilla
    1. Forms center base of Hair Bulb
  5. Hair Root
    1. Epithelial cells actively divide about Dermal Papilla
    2. Hair Root produces 5 cell layers
      1. Hair Shaft forms from inner 3 epithelial layers
      2. Hair sheath forms from outer 2 epithelial layers
  6. Hair Shaft
    1. Inner 3 layers from Hair Root (around Dermal Papilla)
    2. Three layered keratinized structure
      1. Hair Medulla (innermost core)
      2. Hair Cortex (highly keratinized bulk of hair)
      3. Hair Cuticle (hard thin outer later)
  7. Hair sheath
    1. Outer 2 layers from Hair Root (around Hair Shaft)
    2. Two layers
      1. Internal Root Sheath (lightly keratinized)
      2. External Root Sheath
  8. Arrector Pili Muscle
    1. Smooth Muscle attached to Hair Follicle
    2. Contraction erects hair
    3. Innervated by Sympathetic Nervous System
      1. Activated by fear or cold
    4. Minimal function in humans
      1. Results in Goose Bumps
      2. Evolved in furry animals
        1. Increases insulation against heat loss
        2. Animal appears larger

VI. Physiology: Development

  1. Hair Follicles are laid down at 8 weeks gestation
  2. At birth, the scalp is covered by 100,000 terminal follicles
    1. Vellus follicles cover the remainder of the body at birth
  3. With the surge in androgens at Puberty, some vellus follicles transition to terminal follicles
    1. Androgens promote terminal follicle development in the axilla, genitalia and chest
  4. Scalp Hair Follicle diameter changes over time and per gender
    1. Female follicle diameter peaks by age 35 to 45 years, and then decreases with Menopause
    2. Male follicle diameter peaks by mid-20s

VII. Physiology: Phases of cyclic Hair Growth

  1. Follicle cycles are independent of their neighboring hairs (each cycling independently)
  2. Anagen: Active growing phase (85-90% of hair cycle)
    1. Thigh Hair Growth phase lasts 1 to 2 months
    2. Eyebrow and eyelash growth phase lasts 1 to 6 months
    3. Scalp Hair Growth phase lasts 2 to 6 years
      1. Scalp hair grows 0.4 mm/day or 6 inches per year
  3. Catagen: Degeneration, transition or rapid involution phase (2-3%)
  4. Telogen: Resting phase (10-15% of hair cycle)
    1. Hair shedding occurs during this phase
      1. Scalp hairs are shed at 100 hairs/day during Telogen phase
    2. Mustache hair resting phase lasts 1.5 months
    3. Leg hair resting phase lasts 3 to 6 months
    4. As Telogen ends, hair ejected and Anagen starts again

VIII. Pathophysiology

  1. See Folliculitis
  2. See Acne Vulgaris
  3. See Alopecia
  4. See Hirsutism

IX. References

  1. Habif (1996) Clinical Dermatology, Mosby, p. 24
  2. Murphy in Cotran (1989) Robbins Pathology, p. 1277-8
  3. Wheater (1987) Functional Histology, p. 130-2
  4. Dakkak (2024) Am Fam Physician 110(3): 243-50 [PubMed]

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