Microsomes are fragments of the rough ER. (Left) Electronmicrograph showing the cytoplasm in an acinar cell of the guinea pig pancreas. rs, rough ER; ss, smooth ER; m, mitochondria. (Right) An EM section of a microsome pellet isolated from pancreatic cells. Note the presence of various kinds of membrane-bound vesicles (mc₁,mc₂, and mc₃), most of which are studded with dense particles that were later identified to be ribosomes. dm, damaged mitochondria; g, dense granules. From Palade and Siekevitz (1956b).