The endoplasmic reticulum is a network of membranes surrounding flattened membranous sacs called cisternae.The membrane of the rough endoplasmic reticulum is in the form of large double-membrane sheets that are located near, and continuous with, the outer layer of the nuclear envelope and has ribosomes attached.The rough ER is the site of protein synthesis. Newly formed proteins are packaged in vesicles and transported to other parts of the cell such as the Golgi apparatus.The RER is found mainly around the nucleus of the cell.Different types of cells contain amounts of RER depending on the activities of the cell.The endoplasmic reticulum is not found in red blood cells, or spermatozoa.