Keratin 19 as a biochemical marker of skin stem cells in vivo and in vitro: keratin 19 expressing cells are differentially localized in function of anatomic sites, and their number varies with donor age and culture stage

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 analyzed keratins and filaggrin in over 50 samples of human embryonic and fetal skin, identifying four periods of epidermal development: embryonic, stratification, follicular keratinization, and interfollicular keratinization. Keratins of 50 and 58 kD were consistently present, while 56.5 and 67 kD keratins appeared during stratification and increased with development. Filaggrin was first detected at 15 weeks, associated with keratinizing cells around hair follicles, and later in the granular and cornified layers. The findings indicated a relationship between protein expression and epidermal morphologic changes, with keratin expression marking tissue commitment to stratification and keratinization, and no unique "fetal" keratins were identified.