Artefactual spherical particle, not present in the living cell, derived from pieces of the endoplasmic reticulum present in homogenates of tissues or cells: microsomes @S05533@ from such homogenates when centrifuged at \(10^{6}\ \text{g}\) and higher: the microsomal fraction obtained in this way is often used as a source of mono-oxygenase @E02159@.