Microscopic morphology and the origins of the membrane maturation model of Golgi apparatus function

The membrane maturation (flow differentiation) model of Golgi apparatus function embodies concepts of saccule formation at one face of the Golgi apparatus from membranes derived from endoplasmic reticulum and utilization of saccules in vesicle formation at the opposite face for delivery to the plasma membrane as existing saccules are displaced from one position within the stack to another. Derivation of the model came almost entirely from light and electron microscopy. Especially important were observations that passage through the Golgi apparatus was accompanied by differentiation of membranes from endoplasmic reticulum-like to plasma membrane-like across the polarity axis of the stacked saccules. The concept of coparticipation of endoplasmic reticulum and/or nuclear envelope, transition, and secretory vesicles and other pre- and post-Golgi apparatus structures through the operation of an integrated endomembrane system was essential to the model. Dynamic aspects confirmed initially by autoradiographing and cell fractionation studies have been corroborated in newer approaches of fluorescent labeling and with living cells.