The endoplasmic reticulum (ER) consists of a membranous network that extends through the cytoplasm and is continuous with the outer nuclear membrane. Thus, the ER lumen is continuous with the perinuclear space. The ER is divided into the smooth ER (sER) and the rough ER (rER), with the latter having ribosomes attached to the cytoplasmic surface. The membranous network forms flattened cisternae in the rER and tubular structures in the sER, the relative amount of which varies between different cell types and metabolic states. The rER is responsible for the synthesis of membrane proteins and proteins that are secreted to the extracellular space. The sER is involved in synthesis of lipids and steroids. In addition, the ER is one of the major storage sites for intracellular ions; maintaining homeostasis in the cell by regulating the release and uptake of ions.
The ER is recognized by a network-like staining in the cytosol, which is usually stronger close to the nucleus and weaker close to the edges of the cell. As the ER is continuous with the outer nuclear membrane, staining is often seen around the nucleus. In the Subcellular Section, the ER is counterstained with an antibody marker targeting calreticulin (shown in yellow).
Read more about the proteome of the endoplasmic reticulum.