We first examined the expression pattern of K15, K19, and K12 on the human ocular surface. As shown in
Figures 1B and 2A , strong K15 expression was observed in the basal layer of the conjunctiva, while K19 was expressed in both the basal and suprabasal layers
(Fig 2A) . Because K12 was negative
(Fig 1B) , the conjunctival epithelial phenotype was K15
bas/K19
bas-sup/K12
−. Further into the limbus, K12 expression appeared mainly in the suprabasal layers, although weak staining was observed in the basal cells as well
(Figs. 1D 1E) . K19 expression was similar to that in the conjunctiva; however, K15 staining was distinct from the conjunctiva, with positive cells found in the suprabasal layers as well
(Figs. 2A 2C) . The number of K15
+ layers varied in different sections, even in samples from the same donor
(Figs. 1D 2C 2F) . The limbal epithelial phenotype can thus be represented as K15
bas-sup/K19
bas-sup/K12
sup. The central corneal epithelium was uniformly K12
+ and K15
− (Fig. 1C) . In addition, as previously reported by Chen et al.,
31 we found various levels of K19
+ cells in the central corneal epithelium, although the expression level was lower than in the conjunctiva
(Fig. 2B) . The phenotype of the central cornea was therefore K15
−/K19
bas-sup/K12
bas-sup.
We further compared the expression of K15 with that of K14, another basal cell marker. Although K14 expression in the corneal epithelium is considered to be restricted to the basal layer,
11 14 20 28 32 we also found K14 staining in the suprabasal layers of corneal and conjunctival epithelium
(Figs. 2D 2E 2F) . Nevertheless, K14 expression was strongest in the K15
+ basal cells of the limbal and conjunctival epithelium, as well as in K15
− corneal basal cells.