Artifactual vesicles formed from the endoplasmic reticulum when cells are disrupted. They are isolated by differential centrifugation and are composed of three structural features: rough vesicles, smooth vesicles, and ribosomes. Numerous enzyme activities are associated with the microsomal fraction. (Glick, Glossary of biochemistry and molecular biology, 1990; from Rieger et al., Glossary of genetics: Classical and Molecular, 5th ed)