Endoplasmic reticulum occurs in all the eucaryotic cells except RBC of mammals. It forms an intercommunicating system of channels between nuclear membrane and cell membrane. It was first reported by Porter in 1945. Structurally, endoplasmic reticulum is composed of three different types of structures called cisternae or lamellae, vesicles and tubules. Cisternae are long, flat, unbranched membranous sacs arranged in parallel rows.
Vesicles are normally round or ovoid sacs, often found isolated in the cytoplasm. They range in diameter from 25-500 um. Tubules are irregularly branched, tube-like structures with variable diameters that varies from 50-100um in diameter. Their lumen is filled with a fluid, having secretory granules. It contains many enzymes and other proteins.
Two kinds of endoplasmic reticulum have been recognised. The one with ribosomes attached to its surface is called rough endoplasmic reticulum and that without ribosomes form the smooth endoplasmic reticulum. T
he smooth ones are mostly made of tubules and vesicles, whereas the rough ones are made of cisternae and few tubules. The membrane of rough endoplasmic reticulum is continuous with the membrane of the smooth type and the outer membrane of the nuclear membrane. The membrane of smooth endoplasmic reticulum in turn is continuous with the membrane of the Golgi apparatus and plasmamembrane.
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Besides some common functions like formation of skeletal framework inside the cytoplasm, exchange of ions and other fluids with cytoplasm, and intracellular circulation of various substances, both smooth and rough endoplasmic reticulum are responsible for steroid synthesis, carbohydrate metabolism and lipid synthesis. The rough endoplasmic reticulum becomes the site for protein synthesis.