How are endoplasmic reticulum classified?

Two broad types that vary structurally and functionally, with some cooperate functions (lipid synthesis) and are both consisting of membranous system enclosing a lumen, and they divide the cytoplasm into cytosol and lumen, found at same rate in hepatocytic cells. -Smooth ER ( consists of interconnected tubular structures without ribosomes, abundant in cells with active steroid synthesis (Leydig cells in testes) -Rough ER (extensive and consists of interconnected flattened sacks (cisternae) where ribosomes are attached at the cytosolic face, continuous with nuclear envelope which also attaches ribosomes on its cytosolic face, extremely abundant in protein synthesizing cells (salivary glands, pancreatic cells).