page_title: Endoplasmic reticulum
section_title: Structure
hierarchical_section_title: Endoplasmic reticulum / Structure
caption_attribution_description: English: Endoplasmic Reticulum. See a full animation of this medical topic.
context_page_description: The endoplasmic reticulum is a type of organelle made up of two subunits – rough endoplasmic reticulum, and smooth endoplasmic reticulum. The endoplasmic reticulum is found in most eukaryotic cells and forms an interconnected network of flattened, membrane-enclosed sacs known as cisternae, 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.
  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 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. The smooth endoplasmic reticulum is especially abundant in mammalian liver and gonad cells.
context_section_description: The general structure of the endoplasmic reticulum is a network of membranes called cisternae. These sac-like structures are held together by the cytoskeleton. The phospholipid membrane encloses the cisternal space (or lumen), which is continuous with the perinuclear space but separate from the cytosol. The functions of the endoplasmic reticulum can be summarized as the synthesis and export of proteins and membrane lipids, but varies between ER and cell type and cell function.  The quantity of both rough and smooth endoplasmic reticulum in a cell can slowly interchange from one type to the other, depending on the changing metabolic activities of the cell.  Transformation can include embedding of new proteins in membrane as well as structural changes.  Changes in protein content may occur without noticeable structural changes.
caption: 3D rendering of endoplasmic reticulum
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