Specific distribution of barrier-relevant ceramides in the emerging epidermis and the periderm/subperiderm during chicken embryogenesis

During mammalian embryogenesis the emerging epidermis is temporarily covered by an epithelial monolayer, the periderm. In chicken, a second epithelial layer, the subperiderm, located underneath the periderm develops in later embryogenesis. Together the periderm and the subperiderm are referred to as the PSP unit. The cells of the PSP unit are tightly connected by tight junctions (TJ), thereby providing the embryo with an impermeable bilayered diffusion barrier. The emerging epidermis assumes its barrier function by cornification beginning at embryonic day 17 (E17) before at E18 the PSP unit undergoes desquamation. Lipid analysis of both epithelia after their mechanical separation revealed a dramatic increase to about 100-fold values of barrier-relevant ceramides, i.e. those known to essentially contribute to the diffusion barrier of the cornified envelope, in the emerging epidermis between E17 and E19. In contrast, the content of barrier-relevant ceramides in the PSP unit remained at constantly low levels throughout embryogenesis. These data strongly argue in favour of different mechanisms for the barrier function of the two epithelia. TJ in the PSP unit provide the main diffusion barrier protecting the embryo until beginning of desquamation at E18. At this developmental stage the content of cornified envelope-specific ceramides is substantially elevated, thus enabling the epidermis to fulfil its function as the major diffusion barrier after desquamation of the PSP unit. The observation that barrier-relevant ceramides are formed prior to desquamation of the PSP unit points to a precisely regulated sequence in that desquamation does not occur until the lipid-based barrier of the cornified envelope is completed and suggests in addition that these lipids might be essential regulators of the interaction between the PSP unit and the emerging epidermis.