5.6 Hair Growth Cycles & Abnormal Hair Growth

5.6 Hair Growth Cycles & Abnormal Hair Growth

1. Hair Classifications: Lanugo, Vellus & Terminal

Human hair is categorized into three distinct biological types based on fiber diameter, pigmentation, structural development, and anatomical location:

                                  HUMAN HAIR TYPES
                                         │
                ┌────────────────────────┼────────────────────────┐
                ▼                        ▼                        ▼
          LANUGO HAIR               VELLUS HAIR             TERMINAL HAIR
   - Fetal downy hair        - Fine "peach fuzz"       - Coarse, long, dark
   - Shed before birth       - Non-pigmented / short   - Pigmented (cortex)
   - Unpigmented / soft      - Face & body surface     - Scalp, brows, lashes
   - Lacks medulla           - Lacks medulla           - Axillae, pubic, beard

1. Lanugo Hair

• Definition: Extremely fine, soft, unpigmented downy hair that forms on the human fetus during the fifth month of gestation.
• Timeline: Lanugo is typically shed in utero during the seventh to eighth month of pregnancy or falls out naturally within the first few weeks after birth, being replaced by vellus hair.

2. Vellus Hair ("Peach Fuzz")

• Definition: Very short, fine, soft, and non-pigmented (or lightly pigmented) hair that covers most areas of the human body, including the face (cheeks, forehead), arms, and torso.
• Structural Note: Vellus hairs have shallow follicles located in the upper dermis and rarely contain a medulla.
• Hormonal Sensitivity: Vellus hair follicles can transition into terminal hair follicles under the influence of circulating androgens during puberty or hormonal shifts.

3. Terminal Hair

• Definition: Long, coarse, thick, heavily pigmented hair that possesses a fully developed cortex and, typically, a distinct medulla.
• Locations: Found on the scalp, eyebrows, and eyelashes throughout life. At puberty, under the influence of androgens (testosterone and DHT), terminal hair replaces vellus hair in the axillary (armpit), pubic, and beard/chest areas in males and females.

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2. The Hair Growth Cycle: Anagen, Catagen, Telogen & Exogen

Hair follicles undergo a continuous, asynchronous biological cycle consisting of three main phases—Anagen, Catagen, and Telogen—followed by the shedding phase (Exogen).

┌─────────────────────────────────────────────────────────────────────────────┐
│                         THE HAIR GROWTH CYCLE TIMELINE                      │
│                                                                             │
│  ANAGEN (Active Growth) ──► CATAGEN (Transition) ──► TELOGEN (Resting Phase)│
│  • Mitosis in matrix        • Mitosis stops          • Complete dormancy    │
│  • Rooted to papilla        • Follicle shrinks       • Club hair sits high  │
│  • Scalp: 2–6+ years        • Forms club hair        • Lasts 3–6 months     │
│  • Optimal for waxing/laser • Lasts 2–3 weeks        │                      │
│          ▲                                           │                      │
│          └──────────────── EXOGEN (Shedding) ◄───────┘                      │
│                            • Club hair falls out as new                     │
│                              anagen hair begins growing                     │
└─────────────────────────────────────────────────────────────────────────────┘

1. Anagen Phase (Active Growth Phase)

• Biological Mechanism: The hair matrix cells above the dermal papilla undergo rapid, continuous mitotic cell division. The growing hair root is firmly attached to the vascular dermal papilla, continuously receiving blood, oxygen, and nutrients.
• Melanogenesis: Melanocytes in the hair matrix actively produce and inject melanin granules into the cortex of the emerging hair shaft.
• Duration:
  • Scalp Hair: Remains in anagen for 2 to 6+ years (growing at an average rate of 0.5 inch / 1.25 cm per month).
  • Eyebrows & Eyelashes: Anagen is very short, lasting only 30 to 45 days (which prevents them from growing long like scalp hair).
• Esthetic Clinical Significance: Anagen is the optimal phase for professional hair removal (waxing, sugaring, electrolysis, and laser). In anagen, epilating the hair pulls the bulb directly from the vascular papilla, maximizing damage to the germinative cells. In laser hair removal, anagen hairs contain the highest concentration of target melanin to conduct thermal energy down to destroy the papilla.

2. Catagen Phase (Transitional / Regression Phase)

• Biological Mechanism: Mitotic cell division in the matrix ceases completely. The follicle canal shrinks and collapses, and the lower portion of the follicle degenerates. The hair bulb detaches from the vascular dermal papilla and keratinizes into a hard, rounded, club-like base called a club hair.
• Duration: A brief transitional window lasting approximately 2 to 3 weeks.
• Clinical Significance: Epilation during catagen removes the detached club hair, but since the papilla has already separated and begun its resting phase, epilation is less destructive to long-term follicle capacity.

3. Telogen Phase (Resting Phase)

• Biological Mechanism: The final resting stage of the hair cycle. The follicle is completely inactive and dormant. The fully formed club hair sits loosely in the shallow, shortened follicle just above the dermal papilla.
• Duration: Lasts approximately 3 to 6 months on the human scalp and body.
• Exogen Subphase (Shedding): During late telogen, the old club hair is naturally shed from the follicle—either falling out on its own or being pushed out by a new anagen hair shaft emerging beneath it as the follicle re-engages with the vascular papilla.

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3. Abnormal Hair Growth Conditions: Hypertrichosis vs. Hirsutism

Distinguishing between generalized abnormal hair growth and endocrine-driven hair disorders is a critical clinical skill tested heavily on state licensing exams.

                               ABNORMAL HAIR CONDITIONS
                                          │
                   ┌──────────────────────┴──────────────────────┐
                   ▼                                             ▼
            HYPERTRICHOSIS                                   HIRSUTISM
   - Non-androgen dependent                       - Androgen-dependent (Male hormones)
   - Excessive growth anywhere on body            - Affects females exclusively
   - Males and females equally                    - Male-pattern terminal hair distribution
   - Causes: Genetics, drugs, trauma              - Causes: PCOS, endocrine tumors, menopause

1. Hypertrichosis

• Definition: An abnormal, excessive growth of terminal or vellus hair on areas of the body where hair is normally sparse or absent, or in densities far exceeding normal limits.
• Hormonal Status: Non-androgen dependent (does not stem from elevated male hormone levels).
• Distribution & Demographics: Occurs in both males and females anywhere on the body (e.g., forehead, cheeks, shoulders, back, arms, or legs).
• Etiology:
  • Congenital / Genetic: Hereditary trait (such as hypertrichosis universalis, historically termed "werewolf syndrome").
  • Acquired / Drug-Induced: Systemic medications such as minoxidil, cyclosporine, or oral corticosteroids.
  • Physical Irritation: Localized chronic friction, repeated mechanical rubbing, or prolonged cast immobilization.

2. Hirsutism

• Definition: The excessive growth of coarse, dark terminal hair in females in an androgen-dependent, male-pattern distribution.
• Hormonal Status: Androgen-dependent (caused by elevated circulating androgens like testosterone/DHT or heightened hair follicle androgen receptor sensitivity).
• Distribution: Manifests specifically on the upper lip, chin, jawline, neck, chest, abdomen, lower back, buttocks, and inner thighs.
• Primary Etiologies:
  • Polycystic Ovary Syndrome (PCOS): The most common endocrine disorder causing hirsutism, irregular menses, and hormonal acne.
  • Adrenal Gland Disorders: Congenital adrenal hyperplasia or Cushing's syndrome (excess cortisol/androgens).
  • Hormonal Transitions: Menopause (due to dropping estrogen levels allowing unopposed androgenic stimulation).
  • Anabolic Steroid Use: Exogenous androgen administration.

Diagnostic Comparison: Hypertrichosis vs. Hirsutism

Exam Anchor — Essential Hair Facts for State Board:

• Pilosebaceous Unit Components: Hair follicle, hair shaft, sebaceous gland, arrector pili muscle
• Hair Shaft Layers (Outside to In): Cuticle $\rightarrow$ Cortex $\rightarrow$ Medulla
• Mass of Hair in Cortex: 75% to 90% (Contains melanin pigment)
• Layer Missing in Fine/Vellus Hair: Medulla (Innermost core)
• Optimal Phase for Epilation / Laser: Anagen Phase (Active growth, rooted to papilla)
• Transitional Phase: Catagen Phase (2–3 weeks; forms club hair)
• Resting Phase: Telogen Phase (3–6 months; quiescent)
• Non-Androgen Excessive Hair: Hypertrichosis (Anywhere on body, males & females)
• Androgen-Driven Male-Pattern Hair in Females: Hirsutism (PCOS, mustache/chin/chest)