The study of hair focuses on two main streams of interest: On one hand, the esthetic problem of hair and its management, in other words everything that happens outside the skin; on the other hand, the biological problem of hair, in terms of microscopic, biochemical (hormonal, enzymatic), and molecular changes, in other words the ‘secret life’ of the hair follicle in the depth of the skin. Basic scientists interested in the biology of hair growth and pigmentation have exposed the hair follicle as a highly accessible and unique model that offers unequalled opportunities also to the gerontologist for the study of environmental and age-related effects. Its complex multicell type interaction system involving epithelium, mesenchyme, and neuroectoderm, and its unique cyclical activity of growth, regression, rest, and regrowth provides the investigator with a range of stem, differentiating, mitotic and post mitotic terminally differentiated cells, including cells with variable susceptibility to apoptosis, for study. Finally, a number of intrinsic and extrinsic modulating factors for hair growth and pigmentation have been identified and are being further tested in vitro.
Ageing of hair
Ageing is a complex process involving various genetic, hormonal, and environmental mechanisms. As the rest of the skin, the scalp and hair are subject to intrinsic or chronologic ageing, and extrinsic ageing due to environmental factors. Both occur in conjunction with the other and are superimposed on each other. Intrinsic factors are related to individual genetic and epigenetic mechanisms with inter individual variation. Examples of intrinsic factors are familial premature graying and androgenetic alopecia (AGA). Extrinsic factors include
ultraviolet radiation (UVR), smoking, and nutrition.