An endoplasmic reticulum (ER), a eukaryotic organelle made up of a system of membranous tubes and sacs, that is studded with ribosomes on its surface giving it a rough appearance under the microscope (hence its name).
The ribosomes that give the endoplasmic reticulum a 'rough' appearance are not always bound with the ER. They bind to it when it starts to synthesize membrane-bound proteins destined for sorting.
Synonym: granular endoplasmic reticulum.
Compare: smooth endoplasmic reticulum.
See also: endoplasmic reticulum.