Making hair grow in vitro
The compartmentalised organisational complexity of the hair follicle is stable and does not change, which makes the production of hair fibres in the laboratory possible when the follicles have been dissected from biopsies and cultured. In vitro hair growth has been developed in the L’Oréal laboratories and today represents a real research tool. After having removed a hair follicle from a scalp biopsy, the intact organ is isolated and cultured in a carefully determined medium. Rather than losing its organisation, the hair follicle maintains its structure and floats in the culture dish while continuing to grow at a speed of 0.3 mm per day (similar to that seen on the scalp).
Its survival time is on average between 15–20 days but quite a number have been maintained in culture for 45 days (a world record), with the hair in vitro reaching a length of 1.5 cm. Through immunohistochemistry and X-ray diffraction studies, L’Oréal laboratories have shown that the structure of the hair remains unaltered for about two weeks. Using in vitro approaches, scientists have also shown that the growth of the human hair follicle is determined by several elements including growth factors, cytokines and hormones. In vitro growth of human hair is a unique way of resolving questions concerning regulation of the hair follicle, production and organisation of the fibre.
Hair pigmentation comes through the continual synthesis of a pigment (melanin) by melanocytes. L’Oréal scientists have located two types of melanocytes occurring in the outer root sheath of the follicle: the active ones (which synthesise melanin) occurring in the bulb, and the inactive ones (which do not synthesise melanin).
During the transitionary phases of the hair cycle (anagen — catagen — telogen), the bulb melanocytes, to a large extent, enter the breakdown cycle of apoptosis while those from the outer root sheath remain in place. At the start of a new anagen phase, some of the melanocytes grouped in the outer root sheath are recruited and migrate to the bottom of the follicle which is forming. Melanin synthesis is reactivated in these melanocytes. The external sheath can thus be considered as a reservoir of melanocytes from which cyclical regeneration of pigmentation occurs.
In the L’Oréal hair biology laboratory, scientists are trying to understand which molecular signals received by the inactive melanocytes in the outer root sheath recruit them into the bulb and reactivate them. This research may open up new ways of facilitating recruitment of melanocytes into the bottom of the follicle, thus avoiding the process of hair turning grey.
Hair stem cells
The succession of regression and renewal phases of the hair follicle implies the existence of reservoirs of stem cells, from which the follicle is regenerated in a cyclical fashion.
By studying the expression of keratin 19 as a hair follicle stem cell marker, L’Oréal scientists have shown that they may be located in two areas: one situated in the proximal part (upper third) of the external sheath, the other in the distal part (lower third). According to the hypothesis put forward, known as the “split/fuse” hypothesis, these two areas which form two reservoirs of epithelial stem cells fuse at the time of transition from the late catagen to the telogen phase, and separate again at the beginning of the next anagen phase.
In addition, analysis of data from recently published literature suggests that the stem cells present in the upper and lower parts of the outer root sheath do not have the same role. Those of the upper part of the follicle are only responsible for the renewal of the outer root sheath, those of the lower part for the renewal of the germinal cells which, at the time of the telogen-anagen transition, renew the matrix, itself responsible for the synthesis of the upper part of the follicle, i.e. the inner root sheath and the hair shaft.
Who would have thought that the life of the hair was such an intricate miniature world? Its delicate analysis involves the deployment of numerous modern physical and biological techniques.