Microsomes are artificially formed vesicles derived from:
Cell fractionation pioneered by Claude and Palade showed that mechanical homogenization shears endoplasmic reticulum tubular network into small vesicles that reseal spontaneously due to energetically unfavorable exposed hydrophobic edges. These vesicles called microsomes sediment at one hundred thousand times g, one hundred to three hundred nanometers diameter retaining orientation original membrane. Rough microsomes bear ribosomes and contain Sec61 translocon, oligosaccharyltransferase, BiP, permitting in vitro translation translocation assays that defined signal peptide cleavage and core glycosylation. Smooth microsomes derive from smooth ER and contain cytochrome P450, glucose-six-phosphatase and are inducible by drugs. Golgi fragments produce separate Golgi microsomes of different density, mitochondria when broken produce mitoplasts not microsomes, lysosomes release soluble hydrolases rather than resealing same way. Marker enzyme glucose-six-phosphatase and NADPH cytochrome c reductase enrich in microsomes, confirming ER origin. Their artificial nature useful for biochemical reconstitution but not physiological organelles. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.
Ref: Palade Science 1975; microsomes ER fragments 100k g in vitro translation translocation Nobel work.