Smooth endoplasmic reticulum. Transmission electron micrograph (TEM) showing smooth endoplasmic reticulum (ER, thin lines) inside a cell that is synthesising steroid hormones. Here, the smooth ER is typically prominent, and seen as concentric whorls of membranes. Nearby lipid droplets (dark circles) supply the cholesterol needed for the biosynthesis of steroids. In tissues that are not steroidogenic such as the liver, smooth ER is also abundant and used for detoxification and metabolic reactions of carbohydrates. Magnification: x42,000, when printed 10 centimetres wide.