ABSTRACT
The epidermis is made up of two layers; a basal layer known as the stratum germinativum, which is living, and an outer dead layer called the stratum corneum. The primary cell type in the stratum germinativum is the corneocyte or keratinocyte, which grows from the basal layer outwards to the skin surface. The journey to the surface takes between 12 to 14 days, during which time the cells synthesise the various proteinaceous materials called keratin, they become thin, hard and dehydrated and begin to die. The lifespan of such a cell on the surface is two to three weeks. These cells, together with intercellular lipids synthesized by the keratinocyte, form the outer stratum corneum or horny layer, which is dead. The stratum corneum is the primary protective layer and consists of eight to sixteen layers of flattened, stratified and fully keratinised dead cells. Each cell is about 34 to 44 µm long, 25 to 36 µm wide and 0.15 to 0.20 µm thick, and they are continuously replaced from the basal layer. The water content of the normal stratum corneum is 15 to 20% of its dry weight, but when it becomes hydrated it can contain up to 75% water.
