Fig. 4. Loss of epidermal structure resulting from reduced cell-cell contacts, significant apoptosis and vacuolization of keratinocytes during early guard hair follicle development in c^IBN (IBN) and downless (dl) compared with wild-type mice. High-resolution light microscopy and electron microscopy images of embryos at E14.5 (A) and E15.5 (B). (A, a) Hair follicle placode stage 0/1 in wild-type embryo. Focal reorganization of basal keratinocytes with clustering of mesenchymal cells. (b,c) Cell-cell contact maintained by intercellular junctions, e.g. desmosomes. No vacuoles. (d) Hair follicle placode stage 0/1 in c^IBN embryo. Focal reorganization of basal keratinocytes, especially cell parallelization, and an attempt at clustering of mesenchymal cells with mitosis (arrow). (e) Apoptotic cells in epidermis and mesenchyme (black arrows). (e-g) Lack of cohesion of basal cells (white arrows) with associated vacuoles containing lipid-like material. (h,i) Hair follicle placode stage 0/1 in downless embryo. (h) Focal reorganization of basal keratinocytes via cell parallelization and clustering of mesodermal cells. (i) Increased apoptosis in epidermis (black arrows). (j) Lack of cohesion of basal cells (white arrows). (k) Vacuoles containing lipid-like material in epidermis (white arrows). (B, a) Hair follicle placode stage 2 in wild-type embryo. Development of hair peg and significant clustering of mesenchymal cells forming the dermal condensate. (b,c) Epidermal cell-cell contacts maintained by intercellular junctions, including desmosomes. No vacuoles. (d) Hair follicle placode stage 0 in c^IBN embryo. No placode development detectable. (d,e,h) Significant apoptosis (black arrows). Cell-cell contacts are maintained by intercellular junctions, including desmosomes, and intercellular spaces are less than at E14.5. Vacuoles containing lipid-like material present. (h,j) Cellular disorganization with poor stratification. (k) Hair follicle placode stage 0 in downless embryo. (k,i,l) No placode development detectable. Cell-cell contacts are maintained as in c^IBN and there are fewer intercellular spaces than at E14.5. (m) Vacuoles containing lipid-like material present. P, basal keratinocytes, clustering in hair placodes; M, mesenchymal cells, clustering to form dermal condensate; V, vacuoles; D, dermis; E, epidermis.