Coloured scanning electron micrograph (SEM) of compact, or cortical, bone, which forms a bone's dense outer layer. An osteocyte (centre right), a cell involved in bone remodelling, is seen in one of several small spaces known as lacunae. Cortical bone accounts for 80 per cent of the total bone mass of an adult human skeleton. The picture was created as part of a cooperation with Quintessenz Publishing. Magnification: x1000 when printed at 15cm wide.