what is a hair follicle ?

what is a hair follicle ?



what is a hair follicle ?
That beautiful hair, do you know where you got it from? Yes, indeed, from your parents' genes, but mostly from the million or more hair follicles that cover your scalp and where your hair is born... But the hair follicle, kézako? And how does it regenerate?

Hair follicle: definition

The hair follicle, also known as the pilosebaceous follicle, is the pocket in which both hair and sebum are produced. The hair follicle transforms the keratin present in the hypodermis or dermis into hair, through the process of keratinization.

The sebaceous glands, lateral at the base of the hair follicle, open onto the hair follicle and secrete the sebum needed to lubricate the hair. Hair follicles are present all over our body, except for the palms of the hands and the soles of the feet.

How does a hair follicle develop?

The hair follicle has extraordinary capacity in our organism: it is the only element that can die and resurrect by itself, without external intervention and in a cyclic manner. The first phase, called anagen, is the growth phase, which lasts from 2 to 6 years, and makes the hair grow by 0.3 millimeters per day on average.

 Then comes the catagen phase, the degradation stage, during which the hair follicle "dies". It lasts about 2 to 3 weeks. Finally follows the telogen phase, i.e. rest, which lasts 2 to 4 months and prepares, as it were, for the rebirth of the hair follicle.

 However, as you get older, the number of active hair follicles decreases. In addition, the functioning of the hair follicle can be disturbed, either by hormonal changes or by bacterial or fungal infections, causing superficial or deep folliculitis, alopecia or even alopecia.

Hair growth

The spindle cells located at the junction of the erector group and the follicle are fundamental for hair growth in the anagen stage. Hairs grow in cycles of several stages: anagen (growth), catagen (involution) and telogen (resting). Normally, up to 90% of hair follicles are in the anagen stage, while the remaining 10-14% is catagen and up to 1-2% is telogen. The length of these cycles varies depending on the area of the body. The growth or anagen phase lasts 2-6 years, but varies depending on the body area: in the eyebrows only 2-4 months, in the eyelashes 100-150 days.

THE FUNCTIONING OF THE HAIR FOLLICLE

The hair follicle is the only living organ that can die and resurrect by itself after death, in a perfectly autonomous and cyclical manner. It represents a unique entity in the human body. Since the beginning of this century, immense progress has been made in understanding its cells and the capillary cycles it governs.
Every year, new scientific investigations, which are carried out in molecular biology, provide a better understanding of how it works and how to better combat the development of hair loss.

Cells that prepare the manufacture of hair
Anagen phase: continuous growth
Catagen phase, a key phase
A new start in anagen: the proteins concerned

Cells that prepare the manufacture of hair

The hair follicle houses three different types of cells. All three have well-defined roles in preparing for hair production.

1° THE DERMAL CELLS OF THE PAPILLA (FIBROBLASTS)

They are the ones that attach the follicle to the rest of the body throughout the anagen phase of the hair, a phase of continuous growth. Their proteins are veritable towers of control for this growth, emitting a plethora of signals that represent as many orders to start, maintain or end hair growth.

2° EPITHELIAL STEM CELLS

When gathered in the bulge, they form a reservoir under the sebaceous gland. These cells are indefinitely divisible and pluripotent (multi-purpose, such as embryonic cells). Depending on the orders they receive, they can sometimes be at the origin of hair keratinocytes and sometimes at the origin of sebum sebocytes.

(3) THE PROGENITOR EPITHELIAL CELLS, LOCATED IN THE MATRIX AT THE BASE OF THE FOLLICLE

These cells are larger, more mature. Unlike stem cells, their number of divisions is limited in time (the time of a hair's life cycle) and oriented towards the sole production of the hair. In order to do this, during the anagen phase, they feed the hair shaft, as well as the two epithelial sheaths, one internal and the other external, which surround the shaft and act as its guardian.

In the matrix, the progenitor cells will multiply and divide relentlessly from mother cells to daughter cells (keratinocytes). All of this takes place at an unbridled rate of about 48 hours, which is the fastest rate of reproduction in the body. The new daughter cells are born and chase away the old ones, which die, harden and migrate upwards in a row. This stack of dead cells, called keratinization or keratogenesis, forms the keratin and forms the surface of the hair shaft.

continuous growth


Throughout the anagen phase, hair growth is continuous, with the activity of the different cells of the follicle following a perfectly coordinated sequence:
The dermal cells of the papilla send signals...
to the stem cells in the bulge. These are transformed...
into progenitor cells of the matrix, which divide...
from mother cells to daughter cells, and sire...
the hairline
This lineage will only be extinguished at the end of the life cycle to which it belongs. Its duration can vary between 2 and 6 years depending on the sex and genetic capital of each one.



Catagen phase, a key phase


In order to understand how a new hair is born and develops, scientists have done a lot of analysis of the catagen phase (degradation) of the hair that the unborn regrowth will replace. This is a key phase in explaining the hair renewal cycle.


During this phase, which can last 2 to 3 weeks, the matrix dies, the hair's progenitor cells disappear and the follicle's "heart" stops beating. Nevertheless, the follicle retains its reservoir of stem cells in its remains. It rises towards the epidermis, taking the hair with it. At the end of the catagen phase, its length will have been halved.
The dermal cells of the papilla remain intact but lose all contact with the follicle. The stem cells are then said to be quiescent (dormant) because no longer any stimulus from the papilla animates them or allows them to proliferate. The papilla itself will move upwards and stand close to what is left of the follicle, to "rest" as well.

Then begins the telogen phase (resting phase), it will last between two and four months, during which papilla and follicle will remain inactive and dissociated from each other.

A new start in anagen: the proteins concerned

The rebirth (neo-morphogenesis) of the hair follicle is entirely "homemade": it is managed on-site by a large number of emitting, receiving or relay proteins. These proteins send contradictory signals at a distance, triggering or inhibiting, regulating or accelerating, etc. The whole forms a cascade of orders and counter-orders, a tangle of extremely complex networks, a complexity that we are only just beginning to understand the solfeggio and decipher the orchestration, and which fascinates, not without reason, researchers in every discipline.
 Read our article

But what exactly do we know, at the present time, about the rebirth of the hair follicle, about the regrowth and development of adult hair? To date, researchers have identified several proteins that play a key role in this process:


The proteins β-catenin and Shh (Sonic Hedgehog), at the end of the telogen phase, whistle the start of a new anagen phase by sending signals from the papilla to the follicle. These signals trigger the activity of the stem cells and recruit those that will have to proliferate downwards, forcing the follicle to lengthen and rewrap the papilla.
As soon as the signals are launched, the protein VEGF (vascular endothelial growth factor) activates the number and growth of blood vessels in the papilla, this is the process of angiogenesis. It thus determines the vascular proliferation and blood circulation necessary to stimulate the birth of a bud, then of a regrowth, whose root it will feed during all the anagen phases. Thus can really begin the manufacture of a new hairline, during which three other proteins are involved:

The IGF1 protein (Insulin-like growth factor), so-called because its molecular structure is similar to that of insulin. It guides the development of the follicle and its elongation downwards, leading to the final resurrection of the matrix. The previous hair, dislodged by the new one, is then expelled.

The protein KGF (Keratinocyte growth factor), activated by IGF1. As its name suggests, it is directly involved in the production of keratinocytes by the matrix. It reinforces the cohesion of the keratin and participates in hair growth.

The TERC (Telomerase RNA Component) protein. It prevents the degeneration of dermal cells by delaying the physiological aging of the ends of the chromosomes. By this action, maintains the vitality of the follicle and promotes the metabolism of stem cells into matrix keratinocyte cells. It also actively participates in the keratinization of the cells. PLEASE NOTE: the IGF1, KGF, and TERC proteins will regulate the development of the hair follicle and keratin throughout the anagen phase. They will control the rate of production, which must not be too fast so that the work is well done. They will also prevent the follicle from going into the catagen phase too early.





what is a hair follicle ? what is a hair follicle ? Reviewed by Megan Stone on January 26, 2020 Rating: 5

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