Hair Shaft Anatomy
Hair shaft anatomy explained: the cuticle, cortex and medulla, what each layer does, what damages the shaft and how the shaft differs from the hair root.
Reviewed 13 Oct 2026
Hair Library · Category
Hair Biology
How a hair is built, from the bulb deep in the follicle to the shaft you can see, how the growth cycle works and why hair changes colour with age.
Entries
Hair Bulb Anatomy
What the hair bulb is, where it sits in the follicle, what the dermal papilla does for growth, and why some fallen hairs have a small white bulb at the end.
Reviewed 13 Oct 2026
How Fast Hair Grows
How fast does hair grow? About 0.3 to 0.4 mm a day and 1 cm a month. See growth by day, week and month, what changes the rate, and the hair growth cycle.
Reviewed 13 Oct 2026
How Many Hairs Are on the Head
How many hairs are on the human head: 80,000 to 120,000 on a healthy scalp, density by group, men vs women, daily shedding and why it matters for grafts.
Reviewed 13 Oct 2026
Gray Hair: Causes
What causes gray hair: ageing, genes, oxidative stress, stress, smoking and medical conditions. What treatment can and cannot do, and gray hair by age.
Reviewed 13 Oct 2026
Do Eyebrows Grow Back?
Do eyebrows grow back? How long regrowth takes after plucking, shaving, chemo, alopecia areata or thyroid loss, and the signs that eyebrow loss is permanent.
Reviewed 23 Oct 2026
Planned entries
Topics this category still needs. Each is written from peer-reviewed sources and medically reviewed before it is published.
Follicular UnitHair Growth Cycle
Key terms
What is anagen?
Anagen is the growing phase of the hair cycle. For scalp hair it lasts 2 to 6 years according to a Deutsches Ärzteblatt review, and 2 to 7 years according to Cleveland Clinic.
Full entry: How Fast Hair Grows →What is catagen?
Catagen is the short transition phase of the hair cycle, lasting about 1 to 2 weeks. Cell division stops and about two thirds of the lower follicle breaks down, while the dermal papilla stays intact and moves upward.
Full entry: How Fast Hair Grows →What is the hair cortex?
The cortex is the thick middle layer of the hair shaft and makes up most of the hair. It also holds the melanin granules that give hair its natural colour, so dyes and bleach have to reach it to change the shade.
Full entry: Hair Shaft Anatomy →What is the hair cuticle?
The cuticle is the overlapping outer layer of the hair strand, the layer you touch. It decides hair porosity: when its scales lie flat and tight, water struggles to get in.
Full entry: Hair Shaft Anatomy →What is the dermal papilla?
The dermal papilla is a cluster of cells at the centre of the hair bulb. It signals the bulb and helps decide how thick, how long and what colour each hair grows.
Full entry: Hair Bulb Anatomy →What is the hair bulb?
The hair bulb is the rounded base of a hair root, deep inside the follicle, where living cells divide to build new hair.
Full entry: Hair Bulb Anatomy →What is the hair shaft?
The hair shaft is the visible part of a hair above the skin, made of keratin in three layers: the cuticle, the cortex and, in thicker hairs, a central medulla. It forms in the follicle and cannot repair itself after it leaves the scalp.
Full entry: Hair Shaft Anatomy →What are matrix cells?
Matrix cells are fast-dividing cells that surround the dermal papilla in the hair bulb and turn into the hair shaft.
Full entry: Hair Bulb Anatomy →What is the hair medulla?
The medulla is the soft core that runs down the centre of some hairs. It is the least understood of the three layers, and not every hair has one.
Full entry: Hair Shaft Anatomy →What are melanocytes?
Melanocytes are pigment cells that live among the matrix cells of the hair bulb. They make less melanin over time, and age-related graying cannot be stopped.
Full entry: Gray Hair: Causes →What is telogen?
Telogen is the resting phase of the hair cycle. It lasts 2 to 4 months according to a Deutsches Ärzteblatt review, and up to 4 months according to Cleveland Clinic.
Full entry: How Fast Hair Grows →What is terminal hair?
Terminal hair is thick and pigmented, over 60 micrometres wide and longer than 2 centimetres. It grows on the scalp, eyebrows, armpits, beard and pubic region.
Full entry: How Many Hairs Are on the Head →What is trichorrhexis nodosa?
Trichorrhexis nodosa is a common hair shaft problem in which thickened or weak points, called nodes, form along the shaft and make the hair break off easily. It can be inherited or triggered by blow-drying, ironing, over-brushing, perming or heavy chemical use.
Full entry: Hair Shaft Anatomy →What is vellus hair?
Vellus hair is the fine, short, pale hair that covers most of the body. It is under 30 micrometres wide and under 2 millimetres long.
Full entry: How Many Hairs Are on the Head →