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Follicular units

Hairs are grouped in the form of follicular units.
We have approximately 90 000-150 000 hairs on the scalp. This number varies from person to person depending on density.

  • Blond hair: 150 000
  • Brown hair: 115 000
  • Black hair: 100 000-109 000
  • Red hair: 90 000

Humans have approximately 120 to 260 hairs per cm2 approximately (60-120 follicles per cm2) with a total of 90 000-150 000 on all the head.

 

Are 2400 follicular units the same as 2400 hairs?

No. Since each follicular unit can have 3 or 4 hairs.
For us, 2400 follicular units would correspond to approximately 4800-6000 hairs, depending mainly on the number of follicular units of 3 and 4 hairs in the donor area of the patient.
In this case, 4800 hairs would refer to a patient with low follicular density, while 6000 hairs would refer to a patient with average follicular density.

A patient with 60 follicles/cm2 has approximately = 120-140 hairs/cm2.
Another patient with 90 follicles/cm2 has approximately = 180-200 hairs/cm2.

That is why, when we go down the street, we can see people with a greater hair volume (higher density) and others with less voluminous hair (lesser density).

A follicular unit is made of:

  • 1, 2, 3 or 4 hairs in the same unit
  • 1 hair erector muscle
  • Approximately 9 sebaceous glands
  • 1 peri-follicular vascular plexus
  • 1 neural peri-follicular network
  • The peri-follicle (a circumferential band of collagen with adventitia that defines the unit).

There can be variations according to the ethnic background:

  • European: Follicular units of 2-3 hairs
  • Asian: Follicular units of 1 hair
  • Black: Follicular units of 2 hairs 
Follicular unit

Follicular units of straight hair

Follicular units of curly hair

Hair Growth

Hair does not grow indefinitely, but has a cyclic growth, which is called the hair follicle cycle.
Every follicle has its own cycle, independent of those around.

  • Anagen phase: In this phase the hair, stuck to the papilla, is born and grows. It lasts between 4 and 6 years, although typically an average value of 3 years is considered. The shape of the follicle in this phase is similar to that of an onion, wider at the base than at the stem. The hair grows steadily because the cells in the follicle matrix are constantly dividing themselves by mitosis.
  • Catagen phase: It is a transitional phase. It lasts about 3 weeks, during which growth stops and is separated from the papilla, thus ending the activity of the cells in the matrix, including melanocytes. The bulb takes a cylindrical aspect.
  • Telogen phase: It a phase of rest and hair loss, lasting about 3 months. The hair root takes the appearance of a match and remains inserted in the follicle.
Growth depending on type of hair Duration of the Anagen phase
Hair 3-5 years (1095-1825 days)
Beard 1 year (365 days)
Body hair 13-15 weeks (91-105 days)
Eyebrows 1 month (30 days)
Moustache 4-14 weeks (28-98 days)

Every 2 days and 9½ hours, any hair grows at least one millimetre more.
In men, growth is approximately 1 cm per month.
In women, growth is approximately 1 to 1.5 cm per month.

 

Curiosities of the hair

  • In the animal kingdom, the mammal with the most hairs per cm2 is the otter, with 100 000 hairs/cm2.
  • Hair retains the fundamental function of adorning the face and serves as a personal presentation card for each individual.
  • Using different hairstyles allows us to modify our external appearance.
  • An inappropriate haircut or hairstyle can be an actual catastrophe (there are nowadays in some countries laws by virtue of which the client may report the hairdresser for negligence and professional disability).
  • With a haircut, the common individual is able to affirm their own roots, their gender, to transmit their religious creed, to challenge, to make friends, to cause scandal, to oppose social conventions or likewise to be admitted or dismissed in a job.
  • Throughout the history of civilisations, hair has represented a fundamental element of the human personality, symbol of beauty, fascination, seduction, sometimes power and strength... and nowadays hair can still retain a deep symbolic value.