Understanding Hair Structure and Hair Transplantation


Layer covering the exterior of hair Medulla



Hair stem

Sebaceous glands

Internal root layer

Muscles erecting hair

Hair matrix

Dermal papilla

Hair bulb


Understanding hair structure and hair transplantation, typically, there are approximately 5 million hair follicles in our body. About 100,000-150,000 of these make up our hair. No new hair follicles are formed after birth. In other words, we are born with a fixed number of roots. A loss of 50-150 hairs on average per day is accepted as normal.

Although this number may seem too much for some, it is not very important in the life cycle of hair. In fact, complete loss of roots only occurs in scar tissue or skin loss. In androgenic type hair loss, the roots have been programmed to die. Traces of dead roots are visible when such skin is viewed under a microscope. It is a very tragic bio-story.

Although the number and size of the roots vary, their basic anatomy is the same. A hair strand consisting of alpha-keratin sequences, with thicknesses varying between 0.05-0.09 mm, is divided into 2 basic parts in terms of structure. These are the root and the stem.

The hair body consists of dead cells that have turned into keratin and is the part after the root that is visible on the skin. This part consists of 3 layers: (from the inside out) the Medulla, Cortex and Cuticle. The resistance and color of the hair is determined by the cortex.

The cuticle consists of layers of dead cells and building blocks, acting as a protective shield. The medulla is lost in thin hair, it can only be observed in thick hair.

Hair roots are factories that produce hair. These roots pass the hair they produce through a pattern that determines whether we have straight or wavy hair. In fact, in hair transplantation this detail also determines whether the procedure will be easy or difficult.

Each hair follicle is surrounded by an inner and an outer root sheath. The upper border of our skin begins where the outer root sheath ends. The outer sheath also contains various cells with immunological and neurosensory functions.

The inner sheath determines the shape and thickness of the hair. The most productive center of the root is the hair bulb. It is the most extreme part of the hair follicle in the skin. It consists of living and constantly dividing cells. Hair fibers are produced here.

The melanocytes that determine hair color are also found here. The hair bulb is fed by a capillary extension called the papilla. When this connection is interrupted, it has turned into a graft and needs a new viable bed to survive.

Each hair follicle is in one of the 3 life stages. The problem arises when some of these stages dominate the others. These are: growth (anagen), regression (catagen), rest (telogen). This cycle repeats about 20-25 times in an average person’s life.

Our hair is in the growth phase for 85-90% of its life. These rates and durations differ in various parts of our body. For example, while hair follicles remain in the growth phase for 2-8 years, this period is 2-3 months for our eyebrows.

The resting phase for hair is 2-4 months. The growth rate of hair is 1-1.5 cm per month. Typically, 2-10% of our hair is in the resting phase, and some androgen-containing drugs try to reduce hair loss by forcing the hair in the resting phase to initiate the growth phase.

After this extremely boring medical information, let me tell you a more interesting story. You know, in hair transplantation, we say that we collect hair that is not genetically likely to fall out, that is, from the nape of the hair. Actually, some biological information in this story is missing.

In fact, if hair is going to fall out, once the shedding program is activated all our hair will fall out. The advantage we have is that while human life gives us a taste of shedding in the front parts, 150-200 years is required for the neck area for this to occur in practice. You decide whether we are very lucky or vice versa and please contact us for any questions J.


Normal hair growth cycle

Growth phase


Involution phase Catagen Resting phase


Regrowth phase

Early-middle anagen phase

Regrowth phase Anagen
2-6 years 3 weeks 3 weeks 2-6 years
Average time intervals

Source: Paus R, Cotsarelis G. The biology of hair follicles. N Engl J Med. 1999;341:491. (Brief review of hair follicle biology and pathology.) [PMID: 10441606]