cell part endoplasmic reticulum GO:0005783 xenopus_anatomy ER 2016-09-03T19:21:41Z XAO:0005120 eriksegerdell 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).