# Hair Cycle and Structure

> Hair cycle and structure for NEET-PG Dermatology covering anagen, catagen and telogen phases, follicle anatomy, growth rates and how each phase is affected in hair loss.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/hair-cycle-and-structure
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-08
- Last updated: 2026-10-08
- Reference basis: Rook's Textbook of Dermatology; Fitzpatrick's Dermatology; Andrews' Diseases of the Skin
- How to cite: "Hair Cycle and Structure", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/hair-cycle-and-structure

## Direct answer

Hair grows in a repeating cycle of anagen (active growth), catagen (regression) and telogen (resting), followed by shedding in exogen. Scalp hair spends most of its time in anagen, which lasts several years, while about 10 to 15 per cent of scalp follicles are in telogen at any moment. The bulb and dermal papilla of the follicle drive this cycle.

## What you must remember

- **Anagen:** the active growth phase, with about 85 to 90 per cent of scalp hairs in this phase and a scalp duration of roughly 2 to 6 years.
- **Catagen:** a short involution phase of about 2 to 3 weeks in which the follicle shrinks and the bulb moves upward.
- **Telogen:** the resting phase, lasting about 3 months on the scalp, with 10 to 15 per cent of hairs at any time.
- **Exogen:** shedding of the telogen hair as a new anagen hair grows beneath it; normal daily loss is about 50 to 100 hairs.
- **Growth rate:** scalp hair grows about 1 cm per month, or 0.3 to 0.4 mm per day.
- **Structure:** the hair shaft with cuticle, cortex and medulla; inner and outer root sheaths; the matrix at the bulb; the dermal papilla; and the bulge, which holds the stem cells.
- **Clinical link:** anagen effluvium follows chemotherapy, telogen effluvium follows stress or illness, and androgens shorten anagen in androgenetic alopecia.

## Phases compared and linked to hair loss

The four phases differ in duration and proportion, and disruption in each produces a distinct type of hair loss. The table sets out the scalp values and the clinical problem linked to each phase.

| Phase | Approximate duration (scalp) | Share of scalp hairs | Clinical link |
|---|---|---|---|
| Anagen | 2 to 6 years | About 85 to 90 per cent | Anagen effluvium after chemotherapy |
| Catagen | 2 to 3 weeks | About 1 per cent | Brief transition, rarely a problem |
| Telogen | About 3 months | About 10 to 15 per cent | Telogen effluvium after stress or fever |
| Exogen | Shedding phase | Not counted separately | Normal shedding of resting hairs |

Telogen effluvium is diffuse shedding because a large proportion of follicles move into telogen together, usually about 3 months after the trigger. Anagen effluvium is more severe and follows drugs that disrupt rapidly dividing matrix cells.

## Where the hair cycle questions catch candidates

Cycle questions usually test durations and the matching cause of shedding.

- **Trap:** quoting the same anagen duration for body hair and scalp hair. **Correct:** scalp anagen lasts years, while body hair anagen is much shorter, which is why body hair stays short.
- **Trap:** calling telogen effluvium permanent loss. **Correct:** it is usually reversible once the trigger is removed, as the cycle resets within months.
- **Trap:** attributing chemotherapy hair loss to telogen. **Correct:** chemotherapy causes anagen effluvium because it disrupts actively dividing matrix cells.
- **Trap:** assuming any daily hair loss indicates disease. **Correct:** up to about 100 hairs a day is normal; the concern is sustained or visibly increased loss.

## Frequently asked questions

### How long does the anagen phase last on the scalp?

The anagen phase of scalp hair lasts about 2 to 6 years, and this long growth period explains why scalp hair can reach considerable length. Body hair anagen is much shorter, so body hairs remain short.

### What is the normal daily hair loss?

Normal daily hair loss is about 50 to 100 hairs, reflecting the ordinary exogen shedding of telogen hairs. Persistent loss beyond this, especially with diffuse thinning, merits assessment for telogen effluvium or another cause.

### Which phase of the hair cycle is disrupted in chemotherapy-induced hair loss?

Chemotherapy disrupts the anagen phase because it targets rapidly dividing matrix cells, causing anagen effluvium. Hair loss is usually diffuse, begins within weeks of treatment, and regrows after therapy ends.

### Which structure in the hair follicle controls hair growth?

The dermal papilla, a cluster of specialised mesenchymal cells at the base of the bulb, controls follicular growth by signalling to the matrix. The bulge region contains the stem cells that regenerate the follicle in each new anagen phase.

### What is the difference between vellus and terminal hair?

Terminal hair is thick, pigmented and long, whereas vellus hair is fine, pale and short. Androgens convert terminal hair into vellus-like hair in androgenetic alopecia, which is why the scalp thins.