Transmission electron micrograph of a thin section of the liver of a 1-day old rat showing a grazing view of rough endoplasmic reticulum (RER, sometimes termed granular endoplasmic reticulum. The 'rough' or 'granular' term refers to the membrane-attached polyribosomes (also known as polysomes) which often occur in a spiral or rosettal conformation. RER-associated polyribosomes are the sites of synthesis of proteins secreted into the RER cisternae and destined for export from the cell.