Blackhead. Coloured scanning electron micrograph (SEM) of hairs emerging from a blockage in the skin. The material which caused the obstruction, the dead cells and debris associated with a pimple or blackhead, is attached to the emerging hair. Blackheads and pimples form when a sebaceous gland becomes blocked with a plug of dried sebum (an oily excretion). Magnification: x180 when printed 10 centimetres wide.