Human skin cell, coloured scanning electron micrograph (SEM). The skin is the largest organ in the human body. Its primary function is to form a physical barrier between the internal environment of an organism and the external environment. The epidermis is the outermost layer of the skin and provides a waterproof barrier that protects the other skin layers and other internal structures below it from injury, UV damage, harmful chemicals and infections by pathogens such as bacteria, viruses and fungi. The cell in this image is a flat human keratinised dead cell. Squamous cells like this one are continuously sloughed off and replaced with new cells with humans shedding their entire outer layer of skin every 2-4 weeks. Magnification: x1750 when printed at 10cm wide.