The endoplasmic reticulum (ER) is a type of organelle made up of two subunits – ROUGH endoplasmic reticulum (RER), and smooth endoplasmic reticulum (SER). The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae (in the RER), and tubular structures in the SER. The membranes of the ER are continuous with the outer nuclear membrane. The endoplasmic reticulum is not found in RED blood cells, or spermatozoa.Cell biologyThe animal cellAnimal Cell.svgComponents of a typical animal cell:NucleolusNucleusRibosome (little dots)VesicleRough endoplasmic reticulumGolgi apparatus (or, Golgi body)CytoskeletonSmooth endoplasmic reticulumMitochondrionVacuoleCytosol (fluid that contains organelles, comprising the cytoplasm)LysosomeCentrosomeCell membraneMicrograph of rough endoplasmic reticulum network around the nucleus (shown in lower right-hand side of the picture). Dark small circles in the network are mitochondria.The two TYPES of ER share many of the same proteins and engage in CERTAIN common activities such as the synthesis of certain lipids and cholesterol. Different types of cells contain different ratios of the two types of ER depending on the activities of the cell.The outer (cytosolic) face of the rough endoplasmic reticulum is studded with ribosomes that are the sites of protein synthesis. The rough endoplasmic reticulum is especially prominent in cells such as hepatocytes. The smooth endoplasmic reticulum lacks ribosomes and functions in lipid synthesis but not metabolism, the production of steroid hormones, and detoxification.[1] The smooth endoplasmic reticulum is especially abundant in mammalian liver and gonad cells.