It is likely that the transport through the Golgi apparatus in the forward direction (red arrows) involves elements of both of the views represented here. (A) In the vesicular transport model, Golgi cisternae are static organelles, which contain a characteristic complement of resident enzymes. The passing of molecules through the Golgi is accomplished by forward-moving transport vesicles, which bud from one cisterna and fuse with the next in a cis-to-trans direction. (B) According to the alternative cisternal maturation model, each Golgi cisterna matures as it migrates outwards through a stack. At each stage, the Golgi resident proteins that are carried forward in a cisterna are moved backward to an earlier compartment in COPI-coated vesicles. When a newly formed cisterna moves around to a medial position, for example, “left-over” cis Golgi enzymes would be extracted and transported backward to a new cis cisterna behind. Likewise, the medial enzymes would be received by retrograde transport from the cisternae just ahead. In this way, a cis cisterna would mature to a medial cisterna as it moves.