The endoplasmic reticulum is composed of sac-like structures called cisternae that are folded and bound together by a cytoskeleton. There are two types of endoplasmic reticulum: smooth endoplasmic reticulum and rough endoplasmic reticulum.Smooth endoplasmic reticulum gets its name from its smooth folded surface.he endoplasmic reticulum is composed of sac-like structures called cisternae that are folded and bound together by a cytoskeleton. There are two types of endoplasmic reticulum: smooth endoplasmic reticulum and rough endoplasmic reticulum.