A cisterna (plural cisternae) is a flattened membrane disk that makes up the endoplasmic reticulum and Golgi apparatus. A Golgi stack may contain anywhere from three to twenty cisternae but most contain about six. Golgi cisternae can be separated into four classes; cis, medial, trans, and TGN (trans-Golgi network). Each cisternae class contain different enzymes working within it. This helps to prevent any redundant enzymatic activity. Almost all cisternae in the Golgi apparatus present multiple stacking. There are only eight instances in which the stacked characteristic has been lost.
Cisternae pack and modify proteins and polysaccharides. Biosynthetic cargo proteins travel through cisternae and undergo glycan remodeling and other modifications. Cisternae package the proteins and then send them to transport carriers. They also package polysaccharides that are synthesized in the Golgi apparatus. The compounds enter at the cis face of the Golgi stack and exit out the trans side where most of the packaging occurs.
The functions of cisternae change as it undergoes micromaturation. Immature cisternae receive COPII vesicles from the endoplasmic reticulum. During this stage, new cisternae can be produced. The next stage begins when cisternae at carbohydrate synthesis swap material via COPI vesicles. This is when glycosylation and polysaccharide synthesis occur. Mature cisternae reach the final stage where cargo proteins are sent to transport carriers and finally the cisternae disassemble.