<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 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 lacy membranes of the endoplasmic reticulum were first seen in 1945 by Keith R. Porter, Albert Claude, Brody Meskers and Ernest F. Fullam, using electron microscopy . The word reticulum, which means "network", was applied to describe this fabric of membranes . </P>

What is the definition of endoplasmic reticulum in biology
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