Supplementary Figure 4: Maturation of differentiation markers in developing epidermis.

(a) Beginning around E13.5, at the stage when suprabasal cells (except the periderm) are absent, sporadic basal cells coexpress the spinous keratin, K10, along with basal keratins K5 and K14 (not shown). Note also that at this age, the basement membrane marker and hemidesmosome constituent β4-integrin is broadly and diffusely expressed throughout basal cells at E13.5, while it progressively becomes more basally restricted at later ages. (b) Sections of wild-type E14.5 back skin from anterior (less differentiated) to posterior (more differentiated). In anterior regions of single-layered epidermis, K10 and K5 are broadly coexpressed, while β4-integrin remains diffuse. In areas where β4-integrin begins to show some apical enrichment and suprabasal cells are present, K10 becomes more restricted to suprabasal cells, though the basal keratin K5 is diffusely coexpressed there. In posterior regions, the segregation of K10 and K5 becomes more apparent. (c) At E15.5, β4-integrin becomes more restricted to the epidermal-dermal boundary, while K10 and K5 are expressed in opposing domains. An exception is the appearance of sporadic cells positioned in the basal layer which coexpress K10 and K4 (arrows). We suggest that these are cells undergoing differentiation by delamination rather than asymmetric cell division.