What is the structure of rough endoplasmic reticulum?

The rough endoplasmic reticulum (rough ER) is a large organelle found in the cytoplasm of some eukaryotic cells. It is comprised of one continuous, bilayer phospholipid membrane that encloses an elaborate network of convoluted, flattened, sac-like structures. These sac-like structures are known as cisternae. The empty spaces between the sacs is called the lumen.

The cytoplasmic surface of the rough ER’s outer membrane is studded with membrane-bound ribosomes that give it a distinctive rough appearance under the microscope. Fine pores at the site where the ribosomes are attached to the membrane allow the synthesized polypeptide to pass into the channel of the ER for transport. 

This outer membrane is continuous with the outer membrane of the nuclear envelope. The direct connection between the rough ER’s lumen and the perinuclear space facilitates the movement of molecules through both membranes.