Lysosomes are believed to be formed by the joint activity of endoplasmic reticulum, endosomes and Golgi complex (GERL system). The precursors of hydrolytic enzymes are mostly synthesised at the rough endoplasmic reticulum. The latter transfers them to the forming face of Golgi complex either directly or from smooth endoplasmic reticulum through its vesicles. In Golgi complex, the precursors are changed to enzymes. The enzymes are then packed in larger vesicles which are pinched off from the maturing face. Golgian vesicles are joined by endosomes to produce lysosomes.