E.R., Golgi And Vesicles Thousands of structures with membranes crowd the eukaryotic cytoplasm, the most numerous being called the endoplasmic reticulum (ER), a dense network of membranes running through the cytoplasm of every cell and enclosing a network of interconnected tubes (cisternae), which form a connecting channel throughout the cytoplasm. ‘Endoplasmic’ means ‘inside the cytoplasm’, and ‘reticulum’ means ‘network’. The E.R. can make up a significant percentage of the overall volume of the cell. There are two types of ER: rough ER (RE