The Secret Life of the Hair Follicle

The study examined the expression of keratin K14 in human skin, particularly in the epidermis and hair follicles, using monospecific antisera and cRNA probes. It was found that K14 expression and filament organization differed significantly between the hair follicle and epidermis. In the outer root sheath (ORS) cells, which are mitotically active, K14 expression was low, and keratin filaments were fewer and more loosely organized compared to basal epidermal cells. As ORS cells differentiated, they increased K14 expression and formed denser keratin bundles. In contrast, matrix cells, which can differentiate into the inner root sheath, cuticle, and hair shaft, did not express K14 or form keratin filaments. Instead, they produced hair-specific keratins and dense filament bundles without inducing K14 expression. The study suggested that the patterns of K14 expression and filament organization in skin epithelial cells were linked to their pluripotency and highlighted the distinct differentiation programs of hair follicles compared to other stratified squamous epithelia.

The study investigated keratin and keratin mRNA expression during the differentiation of stem cells into epidermis and hair follicles, as well as follicle morphogenesis. It found that type I keratin K14 was expressed early in embryonal basal cells and elevated in the basal layer of developing epidermis, but suppressed in developing matrix cells, indicating a biochemical distinction between diverging cell types. This expression pattern suggested a narrow developmental window for irreversible divergence in basal and matrix cells. In contrast, a hair-specific type I keratin was expressed late in hair matrix development, indicating that keratin expression might be a consequence of cell organization and differentiation rather than a cause.