The hair follicle is an appendage of the skin of the skull. It evolves into either down or hair depending on a number of parameters including age and hormonal status.
An average adult human head of hair comprises nearly 120,000 hairs, which are in constant renewal alternating phases of growth, involution, rest and then fall out. Every day, on average, 50 to 100 hairs fall out physiologically.
Hair grows irregularly and unsynchronised, in the form of cycles (15 to 20 in a lifetime) punctuated by phases of varying duration.
Three phases follow one another:
The hair has died and will be replaced by a new hair, which will in due course cause its predecessor to fall out. Around 10% of follicles are in this phase at any one time. This phase lasts approximately 2 to 6 months.
On the scalp, random groupings of 1 to 4 hairs commonly called follicular units (FUs) grow around a common base. On average, the most numerous UFs are those of 2 or 3 hairs. They are very important in hair implantology because it is the unit of measurement used to define the density of hair in the areas of collection and in the bald areas, but also the number of grafts necessary to obtain an optimal and natural result during an intervention. Number and density Depending on each patient, their age and ethnic origin, we observe a variable number of hairs and therefore UFs. Schematically and on average, patients:
Density is a fundamental parameter in hair implantology . It allows us to evaluate the donor area of grafts (at the back and sides of the head) which could be a limiting factor in the case of an area too poor in UF, as well as the recipient area.
These three criteria are important whenever hair loss occurs.
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