Hair grows in groups of often two, rarely one, three or four adult hairs and one or two miniaturized hairs. These natural groups are called “follicles”.
Hair grows per cycles which last between 3 and 5 years. Each hair is programmed for about thirty to fifty cycles in one life. Their diameter ranges from 60 to 100 microns, that is to say 0.1 mm. Follicles are separated from each other by 1 mm.
A hair is characterized by several variables: its colour, its texture, its length and its diameter. All these characteristics have an effect on the apparent density.
Head hair develops from hairs through a differentiation. During embryonic development, the body is covered with hairs, a very thin layer of down called ‘lanugo’.
As the foetus develops into an embryo and then a future child, hairs differentiate and progressively evolve into body hairs or head hair, according to the area they are in. It is pretty amazing to notice the broad range of body hairs a human being has. They are characterized by their shape, their texture and their length.
In 99% of cases, hair loss is the result of a genetic programming. How to be convinced of this reality? If hair loss was due to a toxicity, a lack of oxygen, an allergy or even to a deficiency of minerals or vitamins – that is to say due to a ‘general’ cause –, it would affect all the hair on the head and even the body hairs. Yet, what do we almost systematically observe? A progressive and regular evolution following a ‘pattern’ – a well-defined geography we find in many patients. We are confronted to a phenomenon which seems programmed and it is indeed so. From birth, we bear in us, hidden in the heart of our chromosomes, the genetic information that will decide the evolution of our hair.
It is often said that hair loss breaks out at different ages. 30% of men lose their hair at 30 years old, 50% at 50, 60% at 60, and so on. A hereditary factor frequently comes into play: there is often a similarity between the severity and the evolution of hair loss observed by a patient, and those of a relative who also suffers from hair loss (a father, a grandfather, a brother, an uncle or a cousin). Unfortunately, it is impossible to predict precisely how a particular person will go bald and over what period of time. Looking for miniaturized hair enables the surgeon to determine which areas are likely to get bald.
It is really hard to follow objectively the evolution of hair loss. We must be very rigorous and use reliable and proven methods.
A first way consists in classifying hair losses according to their severity and their topography. The first person who defined baldness stages is Norwood. His classification system is still widely used and enables to classify oneself in an appropriate ‘stage’ in order to follow one’s evolution towards the next stage.
Unfortunately, this classification is not representative of all the types of evolution as they are far more numerous than the few major specific cases listed by Norwood.
In women, Ludwig’s classification prevails over Norwood’s one.
In 2004, two American colleagues, James Arnold and Bernard Cohen, imagined a system of personalised follow-up consisting in measuring precisely the ‘hair mass’ on a given surface. The system is simple: we must gather hair coming from a surface of 1 cm² in a tiny lock of hair and then measure its diameter thanks to an electronic equipment. This diameter represents thus the hair mass index at the given place.
Cohen, B.H. Hair loss profile, index, and severity scale. In: R. Haber R and D. Stough, editors. Hair Transplantation. Phifadelphia: Elsevier. 2006; 12.
These measures are long to implement but represent undeniably the best way to follow the spontaneous evolution of hair loss, as well as the result of a medication or a surgical treatment.
Have you ever been in front of a forest of firs whose trunks are aligned with each other? Have you noticed that, from a distance, the wood seems dense, but when you get closer and even go inside, the feeling of density decreases? The explanation is easy: from a distance, the trunks – which are at different depths – seem to be all on the same line, they block the view and give the impression of filling all the space.
The phenomenon is perfectly similar with hair. Indeed, if we measure the ‘floor surface’ covered by hair, we notice it represents only 8 out of the 300 cm² of the scalp, barely 2.5% of the total surface.
We just stated that the actual density is actually extraordinarily low in comparison with the impression of density it gives. Even more impressive: it is possible to reduce the original actual density by 40%, without seeing any difference. This is called the apparent density.