Ontogeny and regional variability of keratin 2e (K2e) in developing human fetal skin: a unique spatial and temporal pattern of keratin expression in development.

Keratin 2e (K2e) is expressed in the upper spinous and granular cells of adult epidermis. A highly specific polyclonal antibody was made against a C-terminal peptide of K2e and used to observe K2e expression at different developmental stages. At 12.5 weeks estimated gestational age (EGA) K2e was detected in trunk skin in scattered cells in the intermediate layer. At 13.5 weeks EGA, greater numbers of intermediate cells were stained with variable intensity, and staining in this pattern increased with age. Epidermal sheets from 14 weeks EGA showed that K2e + cells were excluded from developing hair follicles. At 135 days EGA, the following regional patterns were observed: in cheek, trunk, dorsal and ventral knee, elbow and dorsal hand there was moderate to intense staining of upper intermediate keratinocytes excluding cells of the hair canals and sweat ducts. The periumbilical region distinctly lacked K2e staining, while more distal areas showed increasing numbers of K2e + cells. The earliest expression of K2e was at 10 weeks EGA in the presumptive nail bed of developing digits. By 13.5 weeks EGA this pattern had shifted to the proximal nail fold, and K2e was absent in the nail bed. K2e was excluded from developing sweat glands and ducts and from developing hair follicles at the hair germ and early peg stages. By 15 weeks EGA in the fetal hair follicle small groups of cells were K2e + and by 19 weeks K2e + cells were seen at the level of the matrix. Some overlap in staining was detected for K2e with K10, and in palmar skin with K9; however, mostly the filamentous staining patterns for these keratins were distinctive. This study shows that the complex patterns of temporal and regional expression of K2e differ from known patterns for other epidermal keratins and suggest different regulation and function for this epidermal keratin.