Hair Follicle

The Structure & Growth Cycle of Hair Follicles

Long Hair
Long Hair. Johannes Simon / Stringer / Getty Images

A hair follicle is a stocking-like structure that contains cells and connective tissue that surrounds the root of a hair. It exists within the dermis and epidermis. The hair follicle structure is simple and straightforward, but its functions and the growth cycle are quite complex. There are more than 2 million hair follicles on the human body.

Hair Follicle Structure

The hair that is visible above the surface of the skin is actually dead.

Living hair exists below the skin's surface. The structure of a hair follicle is relatively simple and straightforward. It's made up many different components, but these are the 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, also referred to as the "matrix," is where cells produce new hairs as hairs die and fall out.
  • Bulb. The bulb is a bulb-shaped structure at the bottom part of the hair follicle "stocking" structure that surrounds the papilla and matrix, and is fed by blood vessels. This is the living part of the hair. 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 puberty and during pregnancy.

    There are several other structures associated with the hair follicle, including arrector pili, which is a tiny bundle of muscle fiber connected to a follicle. The contraction of these muscles is what causes goosebumps. Sebaceous glands are also attached to hair follicles, which produce sebum

    The Hair Growth Cycle

    The rate at which hair grows varies from person to person, but the average is about a half inch per month, or 6 inches per year.

    The hair growth cycle is split up into three phases: anagen, catagen and telogen.

    • Anagen. The growth phase. Most hair is constantly growing and can spend several years in this stage, which lasts approximately 3 to 4 years. Anagen hairs vary in size, from long, thick terminal hairs to short, light-colored vellus hair. Increased hormones during puberty turns vellus hair into terminal hair.
    • Catagen. The transitional phase. Over the course of a few weeks, the hair growth rate slows down and the hair follicle shrinks. The phase lasts 2 to 3 weeks.
    • Telogen. The resting phase, which lasts roughly 3 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 causes a noticeable amount of hair loss.

    Sources:

    http://emedicine.medscape.com/article/835470-overview#a1

    http://www.webmd.com/skin-problems-and-treatments/picture-of-the-hair

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