ABSTRACT
Skin can be divided into two layers: the outer epidermis and the inner dermis. Most epidermal cells are keratinocytes arranged in layers. The basal epidermis also contains melanocytes. Keratinocytes are classified according to their depth in the epidermis and their degree of differentiation. Keratinocytes grow and are replaced via mitosis in the cells of the stratum granulosum as they progress from deep to superficial, losing their nuclei and organelles as they ascend, before forming the stratum corneum. The dermis constitutes 95% of the skin and is structurally divided into a superficial papillary layer, which is composed of delicate collagen and elastin fibres in ground substance, into which a capillary and lymphatic network ramifies, and a deeper reticular layer, which is composed of coarse branching collagen, layered parallel to the skin surface. Adnexal structures such as hair follicles, sebaceous glands and sweat glands span both the epidermal and dermal layers and contain some keratinocytes in their ducts.
