Abstracts
Résumé
La chevelure est un des traits caractéristiques de l’espèce humaine. Bien qu’elle subisse une perte quotidienne d’une centaine de cheveux, elle est néanmoins permanente. En fait, les 100 000 à 150 000 follicules pileux d’une chevelure normale sont les seules annexes chez l’homme à se renouveler de façon cyclique, asynchrone et stochastique, à partir d’un double réservoir de cellules souches. Plus précisément, les interactions épithélio-mésenchymateuses, à l’origine de la morphogenèse du follicule, sont réactivées au cours du cycle pilaire, pour le régénérer à travers une néo-morphogenèse qui rend cette structure unique chez l’homme. Ce renouvellement touche aussi le compartiment pigmentaire à l’origine de la couleur des cheveux, qui se régénère à partir d’un réservoir de mélanocytes progéniteurs. Enfin, la forme même du cheveu est programmée par le bulbe. Le follicule apparaît donc comme une annexe cutanée autonome avec son propre contrôle hormonal, son propre cycle, une structure incroyablement complexe et stable, résumant à lui seul les grandes lois de l’homéostasie tissulaire. C’est cette vie que nous révélons dans cet article.
Summary
The human hair follicle is a unique appendage which results from epithelio-mesenchymal interactions initiated around the 3rd month of development. This appendage has a very complex structure, with a dermal compartment and an epithelial compartment. The dermal compartment comprises the connective tissue sheath and the dermal papilla, both of which are irrigated by microvessels. The epithelial compartment is made of highly replicating matrix cells giving rise to three concentrical domains, namely the outer root sheath, the inner root sheath and the hair shaft. The pigmentation unit, responsible for hair color, is made of fully active melanocytes located on top of the dermal papilla. Altogether a hair follicle contains more than 20 different cell types, engaged in different differentiation pathways and/or interacting with each other. This complex appendage has a unique behavior in mammals since, after a hair production phase, it involutes in place before entering a resting phase after which it renews itself under a cyclical but stochastic way, out of a double reservoir of pluripotent stem cells able to also regenerate epidermis. For yet unknown reasons, this well ordered process can be disturbed, provoking alopecia. The pigmentation unit also renews itself under a cyclical way, out of a melanocyte progenitor reservoir which progressively declines with time, provoking the hair whitening process. Finally, the shape of the hair shaft is programmed from the bulb. What makes this appendage unique and fascinating is its high degree of autonomy, its incredibly complex though stable structure, the number of different cell types interacting under an equilibrated way and its potential of regeneration. It represents a true paradigm of tissue homeostasis, exemplifying in a small living cylinder all the fundamental laws of cell-cell and tissue interactions. This life is revealed in this short synthesis.
Appendices
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