Follicular Units

What are follicular units or follicles?

In the hair grafting sector we know the term "graft", "hair follicle", or follicular unit as a group of follicles (hairs).  The 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 according to density.

• Blondes: 150,000 
• Chestnut tree: 115.000
• Black: 100,000-109,000
• Redhead: 90.000

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

Are 2400 follicular units the same as 2400 hairs?

No. As each follicular unit can have 3 or 4 hairs, therefore the number of follicular units is not equal to the number of hairs.

For us, 2400 follicular units would correspond to approximately 4800 to 6000 hairs, depending above all on the number of 3 and 4 hair follicular units in the patient's donor area.

In this case of 4800 hairs, the patient would have a low follicular density, but in the case of 6000 hairs, the patient would have a good follicular density.

Example: 

A patient with 60 follicles/cm2 corresponds to approximately = 120-140 hairs/cm2.

Another patient with 90 follicles/cm2 corresponds to approximately = 180-200 hairs/cm2.

Therefore, when we walk down the street we see people with a lot of hair (greater hair density) and others with less volume (less hair density).

Composition of a follicular unit.

What is the composition of a follicular unit?

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

What is the survival rate of a follicular unit?

It depends on several factors, among them the quality of the recipient’s capillary bed, the vascularisation and nutritional contribution the bed can provide, as well as several factors that we will list down below:

1. Quality of the Recipient’s Bed?,  Ischemia and Reperfusion

The area receiving the graft must be in optimal conditions, it must not have active infectious processes and its vascularisation must be adequate, so that it can nourish the grafted tissue.

2. Follicular Physical Trauma

Follicular trauma, either due to pulling, stretching, bending or crushing, is enough to diminish follicular growth rate. The deterioration phenomenon can be observed with an electronic microscope. Just a slight touch on the bulb can cause a decrease in growth from 35 to 64%.

3. Conservation Solutions

There are nowadays several types of means for follicular conservation, among them:

• Saline serum 0.9%
• Ringer Lactato
• A-Cell
• Hypothermosol

4. Hydration

Only 3 minutes are enough for the follicles to begin to deteriorate, thus decreasing follicular growth. There is greater deterioration in the follicular units of 1 hair, than in those of 2, 3 and 4 hairs.

5. Density

High density in procedures with more than 50 follicles/cm2 has a low survival rate in relation to others with lower density. This phenomenon occurs due to the effect of ischemia-reperfusion.

6. Temperature

Cold acts as a protective factor against physical damage. A temperature of 4° C (39° F) is ideal to keep the follicles viable with a better survival rate.

7. Time

In 1992, Dr Bob Limmer conducted a study with 200 follicular units with different periods of time outside the body and he found the following:  

What does hair growth consist of?

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.

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.

What does shockloss in hair involve?

A quite often effect, very unpleasant for patients and which happens regardless of the method used, is hair loss in the transplanted area, which may happen 2 to 6 weeks after the operation.

This mainly affects the transplanted hair, but can also affect existing hair. In the course of the second or third week after a hair transplant, most of the transplanted hairs fall out. This effect happens to 98% of patients.

Such a hair loss is completely normal. Patients should know that only hair weakened by the transplant falls off. In contrast, hair roots remain in place, gathering strength and producing new hair after 3-6 months, evidencing the success of the hair transplant.

This also applies to non-transplanted hairs already existing in the area before transplantation. It does not apply though to weak hair follicles, which would no longer be able to produce new hair.

Curiosities of the hair

• In the animal kingdom, the mammal with the most hair per cm2 is the Otter, which has 100,000 hairs/cm2.

• The hair retains the fundamental function of adorning the face and serves as each individual's personal business card.
• The hair can grow an average of 1.25 centimetres per month.
• Our hair grows faster in summer than in winter.
• Through different hairstyles, hair allows us to change our external appearance.
• An inappropriate cut or hairstyle can become a tragedy (today in some countries there are laws so that the client can denounce the hairdresser for negligence and professional incapacity).
• Every day we lose about 100 hairs, which is completely normal.
• A hair can hold up to 100 grams in weight.
• Hair on the face is the fastest growing hair on the whole body.
• With a haircut, it is possible for anyone to affirm their own roots, their sex, transmit their religious creed, challenge, make friends, provoke scandal, oppose social conventions or equally be admitted or dismissed from a job.
• Throughout the history of civilizations, hair has represented a fundamental element of the human personality, a symbol of beauty, fascination, seduction, sometimes power and strength... and in our days hair itself can retain a profound symbolic value.