Osteoporotic bone, coloured scanning electron micrograph (SEM). Here part of the surface of the bone (grey) has been degraded to show the underlying tissue (brown). This is due to an imbalance in the activity of osteoclasts (cells that reabsorb bone tissue) and osteoblasts (cells that form new bone tissue). Osteoporosis (brittle bone disease) causes a loss of bone mass and an increase in the bone's porosity, making it more brittle and likely to fracture. It commonly affects post-menopausal women, who experience a decrease in levels of the hormone oestrogen. It may also develop after injury or infection. Treatment is with hormone replacement therapy (HRT) and drugs that slow the rate of bone loss. Magnification: x380 at 10 centimetres wide.