The endoplasmic reticulum (ER) is an organelle of cell (biology)|cells in eukaryote|eukaryotic organisms that forms an interconnected network of membrane vesicles.The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F. Fullam in the year 1945. . According to the structure the endoplasmic reticulum is classified into two types, that is, rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). The rough endoplasmic reticulum is studded with ribosomes on the cytosolic face. These are the sites of protein synthesis . The rough endoplasmic reticulum is predominantly found in hepatocytes where protein synthesis occurs actively. The smooth endoplasmic reticulem is a smooth network without the ribosomes. The smooth endoplasmic reticulum is concerned with lipid metabolism , carbohydrate metabolism and detoxification . The smooth endoplasmic reticulum is abundantly found in mammalian testis.