The epidermis is the outermost layer of skin – the layer we can see from the outside. It itself can be subdivided into 5 different layers, the uppermost of which is called the stratum corneum. This outer layer is composed of dead cells called corneocytes, which are filled with the protein keratin. These form protein-based ‘bricks’. In between these are a mixture of compounds, primarily lipids, that act as the ‘mortar’, holding the ‘bricks’ together.