Proteins are synthesied on the ribosomes and enter the endoplasmic reticulum cisternae through channels in the membrane. In almost all studies labelled secretory proteins leave the ER and enter the Golgi complex, from where they are secreted outside the cell. In fibroblast, labelled collagen appears to be directly secreted from the RER to the outside of the cell, without involving the Golgi complex.
In many cell types the forming face of the Golgi complex faces the RER cisternae. Parts of the ER adjacent to the Golgi complex are often without ribosomes are called ‘transitional elements,(SER). The ribosomes free ER often shows buds which seperate to form vesicles.