endoplasmic reticulum ER GO:0005783 NIF_Subcellular:sao1036339110 The irregular network of unit membranes, visible only by electron microscopy, that occurs in the cytoplasm of many eukaryotic cells. The membranes form a complex meshwork of tubular channels, which are often expanded into slitlike cavities called cisternae. The ER takes two forms, rough (or granular), with ribosomes adhering to the outer surface, and smooth (with no ribosomes attached). Wikipedia:Endoplasmic_reticulum cellular_component