Step by Step
Shaft
Hair shaft structure — from core to surface
The hair shaft is the visible part above the skin, with an internal structure built from three layers: the medulla (central core, containing pigment), the cortex (the bulk of the hair, made of keratin), and the cuticle (the outer, overlapping scale-like layer).
Foll
The follicle, matrix, and papilla
The hair root sits below the skin surface within the follicle — a tube of epithelium extending into the dermis, whose wall cells actually produce the hair. The hair matrix, at the base of the follicle bulb, consists of rapidly dividing cells that generate new hair, nourished by the hair papilla's blood supply.
Cycle
The hair growth cycle — three phases
Anagen is the active growth phase, lasting 2-6 years — at any given time, 85-90% of scalp hairs are in this phase. Catagen is a brief transition phase lasting 2-3 weeks. Telogen is the resting phase, lasting about 3 months, after which the hair is shed.
AP
Arrector pili — the goosebump muscle
The arrector pili is a smooth muscle attached to the hair follicle. When it contracts (triggered by sympathetic stimulation), it produces goosebumps and causes the hair to stand up — a vestigial mechanism that once helped trap heat.
The hair matrix, located at the base of the follicle bulb, consists of rapidly dividing cells — which is exactly why chemotherapy, a treatment that specifically targets rapidly dividing cells throughout the body, causes hair loss as a side effect.
Applied Walkthrough
1
A patient undergoing chemotherapy experiences significant hair loss and asks their care team why this particular treatment affects their hair specifically.
2
Ask: what property of hair growth makes it vulnerable to chemotherapy? The hair matrix, located at the base of the follicle, is made of rapidly dividing cells — and chemotherapy drugs are specifically designed to target and kill rapidly dividing cells throughout the body (since cancer cells also divide rapidly).
3
Because the hair matrix cells divide so quickly to continuously produce new hair, they're caught in the same chemotherapy targeting that's meant for cancer cells — leading to hair loss as a predictable side effect, even though the hair matrix cells themselves aren't cancerous.
4
This connection — chemotherapy's broad targeting of rapidly-dividing cells explaining a seemingly unrelated side effect like hair loss — illustrates why understanding the hair matrix's basic biology has real clinical relevance beyond just knowing hair anatomy for its own sake.
Exam Application
Exams test the hair shaft's three-layer structure (medulla, cortex, cuticle), the follicle structures (matrix: rapidly dividing cells; papilla: blood supply), the three phases of the hair growth cycle (anagen: active growth, 2-6 years; catagen: transition, 2-3 weeks; telogen: resting, 3 months), and the arrector pili's function and mechanism (sympathetic stimulation → contraction → goosebumps).
⚠ Common Trap
The most common trap is assuming hair loss from chemotherapy is unrelated to normal hair biology. It's directly explained by the hair matrix being made of rapidly dividing cells — the same category of cell that chemotherapy is specifically designed to target throughout the body.
✓ Quick Self-Check
1. What are the three layers of the hair shaft, from core to surface?
Medulla (central core, pigment), cortex (bulk of hair, keratin), and cuticle (outer scale-like layer).
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2. What is the hair matrix, and why is it relevant to chemotherapy-induced hair loss?
Rapidly dividing cells at the base of the follicle bulb that produce new hair; chemotherapy targets rapidly dividing cells throughout the body, which is why it affects the hair matrix and causes hair loss.
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3. What are the three phases of the hair growth cycle, and how long does each last?
Anagen (active growth, 2-6 years), catagen (transition, 2-3 weeks), and telogen (resting, about 3 months).
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4. What percentage of scalp hairs are typically in the anagen phase at any given time?
About 85-90%.
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5. What is the arrector pili, and what does its contraction produce?
A smooth muscle attached to the hair follicle; its contraction (triggered by sympathetic stimulation) produces goosebumps and causes the hair to stand up.
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