A hair follicle looks like a tiny, onion-shaped pocket nestled in your skin, with a narrow tunnel leading up to a small opening on the surface. Most of what makes up a follicle is hidden below the skin’s surface, extending 3 to 4 millimeters deep into the scalp. To the naked eye, all you can see is the opening (called the ostium) where the hair strand emerges. But beneath that opening sits a surprisingly complex little organ with distinct layers, its own blood supply, and pigment-producing cells.
What You See on the Surface
On the scalp’s surface, a hair follicle appears as a tiny dot or pore where one or more hairs exit the skin. Under magnification with a dermoscope (a handheld tool dermatologists use), healthy follicle openings look like small, round dots with hair shafts emerging from them. When follicles are empty or unhealthy, they take on different appearances: white dots indicate scarred or fibrotic follicles, yellow dots suggest the opening is plugged with oil and skin cell buildup, and black dots mean a hair has broken off just below the surface. These color differences are one way dermatologists assess scalp health without a biopsy.
The Structure Below the Skin
If you could slice a hair follicle in half lengthwise and look at it under a microscope, you’d see a layered tube that widens into a bulb at the bottom, somewhat like a tulip with a long stem. The whole structure has three main zones stacked vertically.
At the very bottom sits the hair bulb, which is the rounded, onion-shaped base. Inside the bulb, a mound of tissue called the dermal papilla pushes upward like a finger poking into a balloon. This papilla is made of specialized cells that control the hair’s growth, thickness, shape, and color. It connects to tiny blood vessels that deliver nutrients. Surrounding the papilla on all sides are rapidly dividing cells (the matrix) that produce the actual hair strand and push it upward.
The middle portion of the follicle is a layered tube. Closest to the hair strand is a thin coating called the shaft cuticle, which is wrapped in three layers that form the inner root sheath. Think of this inner sheath as a mold: it shapes the growing hair as it travels upward. This sheath eventually breaks down and disintegrates about halfway up the follicle. Surrounding the inner sheath is the outer root sheath, a thicker layer that encases the entire follicle from bulb to surface and connects seamlessly with the top layer of your skin. A specific region within this outer sheath, called the bulge, houses stem cells that allow the follicle to regenerate itself cycle after cycle.
The uppermost portion is the narrow channel that opens onto the skin’s surface. A tiny oil gland (sebaceous gland) connects to the follicle near this opening, which is why pores can sometimes look oily or clogged.
Where Hair Color Comes From
Pigment-producing cells called melanocytes cluster in specific spots within the follicle, and their arrangement is part of what you’d see under a microscope. The most active melanocytes sit in large numbers near the base of the dermal papilla, right where new hair cells are being formed. These are the cells doing the heavy lifting of injecting color into the growing hair strand. Other melanocytes are scattered through the outer root sheath and near the bulge area, but these are more like reserves. They carry fewer of the enzymes needed to actually produce pigment and instead serve as a backup population. When these pigment-producing cells decline with age, the result is gray or white hair.
How the Follicle Changes Shape Over Time
A hair follicle doesn’t look the same all the time. It cycles through three phases, and its appearance shifts dramatically with each one.
During the active growth phase (anagen), the follicle is at its longest and fullest. It extends deep into the skin, the bulb is plump and well-defined with its characteristic onion shape, and the dermal papilla is large and well-supplied with blood. This phase lasts two to six years on the scalp, which is why head hair can grow so long.
During the transitional phase (catagen), which lasts only a few weeks, the follicle shrinks. It loses about one-sixth of its diameter as cells in the lower portion die off and the structure collapses upward. The bulb detaches from the dermal papilla and begins to look shriveled and elongated rather than round.
During the resting phase (telogen), the follicle is dormant and much shorter. The dermal papilla sits right at the tip of the follicle, directly in contact with the small cluster of cells that will eventually restart growth. The old hair strand is loosely anchored and will fall out when a new growth cycle begins to push a replacement upward.
What a Healthy Follicle Looks Like vs. a Shrinking One
Healthy terminal hair follicles on the scalp produce strands with an average diameter around 55 to 65 micrometers (roughly the thickness of a fine thread). You can’t see individual follicles with the naked eye, but you can see the difference in the hair they produce. A healthy follicle generates a thick, pigmented strand. A miniaturized follicle, the kind involved in pattern hair loss, produces increasingly thinner, shorter, and lighter hairs over time.
Miniaturization happens when the dermal papilla at the base of the follicle physically shrinks, losing cells and reducing in size. The follicle transitions from producing a thick terminal hair to producing a fine, nearly invisible vellus hair, similar to the peach fuzz on a child’s arm. Under a microscope, a miniaturized follicle looks noticeably smaller than its neighbors, with a shrunken papilla and a thinner bulb. Rather than being a slow, gradual process as once thought, research suggests miniaturization can happen in a single hair cycle as an abrupt, large-step change. This also means it can potentially be reversed in one cycle with effective treatment, supported by both clinical results and tissue samples from patients responding to therapy.
On the scalp surface, miniaturization shows up as a visible difference in hair diameter between strands. In early-stage female pattern hair loss, the average strand diameter in the affected area is around 53 micrometers, while unaffected areas at the back of the scalp measure closer to 62 micrometers. As hair loss progresses, that gap widens, with affected-area diameters dropping to around 42 micrometers while the back of the scalp stays relatively stable near 57 micrometers.
The Hair Strand Itself Under Magnification
If you zoom in on the hair strand that emerges from the follicle, its surface isn’t smooth. Under an electron microscope, a healthy hair shaft has a regular, overlapping pattern of flat cells called cuticle scales, arranged like shingles on a roof. This is what gives healthy hair its shine: those flat, orderly scales reflect light evenly.
When hair is damaged by heat styling, chemical treatments, or coloring, those scales progressively lift, crack, and break away. Mild damage shows as an irregular overlay of the cuticle. Moderate damage reveals cracks, holes, and severe lifting of the scales. In heavily damaged hair, the protective cuticle disappears entirely, exposing the softer inner core of the strand. This visible surface damage is why damaged hair looks dull and feels rough to the touch.