The Structure and Growth Cycle of Hair Follicles

A hair follicle is a stocking-like structure that contains cells and connective tissue and surrounds the root of a hair. It exists within the dermis and the epidermis, the two top layers of the skin. For a helpful visual, think of the hair follicle as a vase and the hair as the stem of a flower. 

The structure of hair follicles is simple and straightforward, but its functions and its growth cycle are quite complex. Any significant alteration to the normal growth cycle of a hair follicle may lead to a hair condition like alopecia areata or telogen effluvium. 

The Hair Follicle Structure

Hair follicles are made up of many different components, but these are the four key structures.

Papilla

The papilla is made up of connective tissue and blood vessels that nourish growing hair. It exists at the very base of a hair follicle.

Germinal Matrix

The germinal matrix, which is also referred to as the "matrix," is where cells produce new hairs as hairs die and fall out. It is also located in the lower region of the hair follicle. 

Bulb

The bulb is a bulb-shaped, rounded structure at the bottom part of the hair follicle "stocking" that surrounds the papilla and the germinal matrix and is fed by blood vessels. This is the living part of the hair. In fact, hair that is visible above the surface of the skin is actually dead.

The bulb holds several types of stem cells that divide every 23 to 72 hours, faster than any other cells in the body. The bulb also contains hormones that affect hair growth and structure during different stages of life, such as during puberty and during pregnancy.

Bulge

The bulge area is located in the middle part (also known as the isthmus) of the hair follicle. It contains stem cells that divide and regenerate not only new hair follicles but the sebaceous glands and the epidermis, too.

The bulge also provides the insertion point for the arrector pili—a tiny band of muscle tissue. The contraction of these muscles is what causes hairs to stand on end when you get goosebumps.

The Hair Growth Cycle

The rate at which hair grows varies from person to person, but the average growth rate is about a half-inch per month or six inches per year. The hair growth cycle is split up into three phases: anagen, catagen, and telogen. Here are some more details about each phase. 

Anagen

This is the growth phase. The anagen phase in normal scalp hair follicles lasts approximately two to six years, whereas eyebrow hairs last about 70 days in the anagen phase.

Anagen hairs also vary in size, from long, thick terminal hairs to short, light-colored vellus hairs. Increased hormones during puberty turn vellus hair (almost colorless) into terminal hair (darker and larger).

Catagen

This is the regression phase. Over the course of a few weeks, the hair growth rate slows down and the hair follicle shrinks. The catagen phase lasts two to three weeks.

Telogen

This is the resting phase, which lasts roughly three months. After a few months, hair stops growing and detaches from the hair follicle. New hair starts to grow and pushes the old, dead hair out.

During periods of stress, more hair enters the telogen phase and begins to fall out. Humans shed anywhere from 50 to 100 scalp hairs a day, but stress can cause a noticeable amount of hair loss.