<Li> Smooth endoplasmic reticulum </Li> <Li> Cytosol (fluid that contains organelles, comprising the cytoplasm) </Li> <P> The endoplasmic reticulum (ER) is a type of organelle found in eukaryotic cells that forms an interconnected network of flattened, membrane - enclosed sacs or tube - like structures known as cisternae . The membranes of the ER are continuous with the outer nuclear membrane . The endoplasmic reticulum occurs in most types of eukaryotic cells, but is absent from red blood cells and spermatozoa . There are two types of endoplasmic reticulum: rough and smooth . 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 manufacture and metabolism, the production of steroid hormones, and detoxification . The smooth ER is especially abundant in mammalian liver and gonad cells . The lacy membranes of the endoplasmic reticulum were first seen in 1945 using electron microscopy . </P> <P> The ER was observed with light microscopy by Garnier in 1897, who coined the term "ergastoplasm". With electron microscopy, the lacy membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter, Albert Claude, Brody Meskers and Ernest F. Fullam . Later, the word "reticulum", which means "network", was applied by Porter in 1953 to describe this fabric of membranes . </P>

Rough endoplasmic reticulum is the site of synthesis of