ABSTRACT
T he skin protects the body o f mammalians from mechanical and chemical damage, is a barrier against infections and prevents dehydration. It can be divided into an upper layer, called epidermis, and a lower layer, called dermis (Fig. 1). The epidermis consists mainly o f keratinocytes, while the dermis is a loose connective tissue containing dermal fibro blasts, macrophages, neurons and other cells. These layers are separated by a basement mem brane (BM), a sheet like structure made from different extracellular matrix molecules includ ing laminin 5, collagen IV and nidogen, that mechanically connects epidermis and dermis and provides signals for cell proliferation and polarisation.4 At the epidermal side, basal keratinocytes are attached to the BM. In healthy conditions only these keratinocytes are proliferating. W hen further differentiating, keratinocytes detach from the BM, move upward and become suprabasal keratinocytes. Suprabasal keratinocytes later terminally differentiate, to form a layer o f dead cells called stratum corneum. It is mainly this layer, which provides the protective barrier func tion to the skin. D uring terminal differentiation keratinocytes fill up with keratin filaments, seal the space between the cell bodies by lipids and degrade the nucleus.1 This process has many similarities to apoptosis, but is much slower and requires the production o f many proteins.20
