<Li> Strengths: The model explains observations of compartments, polarized distribution of enzymes, and waves of moving vesicles . It also attempts to explain how Golgi - specific enzymes are recycled . </Li> <Li> Weaknesses: Since the amount of COPI vesicles varies drastically among types of cells, this model cannot easily explain high trafficking activity within the Golgi for both small and large cargoes . Additionally, there is no convincing evidence that COPI vesicles move in both the anterograde and retrograde directions . </Li> <Li> This model was widely accepted from the early 1980s until the late 1990s . </Li> <Ul> <Li> In this model, the fusion of COPII vesicles from the ER begins the formation of the first cis - cisterna of the Golgi stack, which progresses later to become mature TGN cisternae . Once matured, the TGN cisternae dissolve to become secretory vesicles . While this progression occurs, COPI vesicles continually recycle Golgi - specific proteins by delivery from older to younger cisternae . Different recycling patterns may account for the differing biochemistry throughout the Golgi stack . Thus, the compartments within the Golgi are seen as discrete kinetic stages of the maturing Golgi apparatus . <Ul> <Li> Strengths: The model addresses the existence of Golgi compartments, as well as differing biochemistry within the cisternae, transport of large proteins, transient formation and disintegration of the cisternae, and retrograde mobility of native Golgi proteins, and it can account for the variability seen in the structures of the Golgi . </Li> <Li> Weaknesses: This model cannot easily explain the observation of fused Golgi networks, tubular connections among cisternae, and differing kinetics of secretory cargo exit . </Li> </Ul> </Li> </Ul>

Where do the microtubules of the spindle orginate during mitosis in both plant and animal cells