What is a 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 and surrounds the root of a hair. It exists within the dermis and epidermis, the two top layers of the skin.

The hair follicle structure is simple and straightforward, but its functions and the growth cycle are quite complex. Any significant alternation 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

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. It is also located at the lower region of the hair follicle. 
  • 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. 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 puberty and during pregnancy.
  • Buldge: The bulge area is in the middle part (called the isthmus) of the hair follicle. It has stem cells that divide and regenerate not only new hair follicles but sebaceous glands and the epidermis too. The buldge also provides the insertion point for the arrector pili—a tiny bundle of muscle fiber connected to each fiber. The contraction of these muscles is what causes 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.

    • Anagen: This is the growth phase. Most hair is constantly growing and can spend several years in this stage, although it varies where the hair is. For instance, the anagen phase in normal scalp hair follicles lasts approximately 2 to 6 years whereas eyebrow hairs last about 70 days in 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 into terminal hair.
    • Catagen: This is the regression phase. Over the course of a few weeks, the hair growth rate slows down and the hair follicle shrinks. This phase lasts 2 to 3 weeks.
    • Telogen: This is 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:

      American Academy of Dermatology. How Hair Grows. 

      Breitkopf T, Leung G, Yu M, Wang E, McElwee KJ. The basic science of hair biology: what are the casual mechanisms for the disordered hair follicle? Dermatol Clin. 2013 Jan;31(1):1-19.

      Ohyama M. Hair follicle buldge: a fascinating reservoir of epithelial stem cells. J Dermatol Sci. 2007 May;46(2):81-9.

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