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 ad ult epidermis, A highly specific polyclonal antibody was made against a C-t erminal peptide of K2e and used to observe K2e expression at different deve lopmental stages. At 12.5 weeks estimated gestational age (EGA) K2e was det ected in trunk skin in scattered cells in the intermediate layer. At 13.5 w eeks EGA, greater numbers of intermediate cells were stained with variable intensity, and staining in this pattern increased with age, Epidermal sheet s from 14 weeks EGA showed that K2e + cells were excluded from developing h air follicles, At 135 days EGA, the following regional patterns were observ ed: in cheek, trunk, dorsal and ventral knee, elbow and dorsal hand there w as moderate to intense staining of upper intermediate keratinocytes excludi ng cells of the hair canals and sweat ducts. The periumbilical region disti nctly lacked K2e staining, while more distal areas showed increasing number s of K2e + cells, The earliest expression of K2e was at 10 weeks EGA in the presumptive nail bed of developing digits. By 135 weeks EGA this pattern h ad shifted to the proximal nail fold, and K2e was absent in the nailbed, K2 e was excluded from developing sweat glands and ducts and from developing h air 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 d etected for K2e with K10, and in palmar skin with K9; however, mostly the f ilamentous staining patterns for these keratins were distinctive. This stud y shows that the complex patterns of temporal and regional expression of K2 e differ from known patterns for other epidermal keratins and suggest diffe rent regulation and function for this epidermal keratin.