AN APPLE A DAY

摘要

The skin and its appendages,including hair follicles,sebaceous glands and sweat glands provide our body with overall protection.Two main layers form the skin: the outer epidermis and the dermis below.The dermis is a connective tissue important for skin firmness and elasticity.It contains relatively few fibroblast cells embedded in a matrix of collagen and elastin proteins.The epidermis is a stratified epithelium composed of different layers of densely arranged keratinocyte cells(figure 1).The epidermis builds a barrier which prevents water loss and the entry of microbes.The basal keratinocytes in the deepest zone of the epidermis constantly undergo a differentiation process leading ultimately to cornified dead cells in the stratum corneum.Because the outermost cells are always shed,new basal keratinocytes have to be produced.This is the role of keratinocyte stem cells which are dispersed in the basal layer of the epidermis.Only these cells retain the capacity to proliferate and thus to form new keratinocytes to replace those that get lost by desquamation.