Endoplasmic reticulum

The endoplasmic reticulum (Latin reticulum = "litter net") is a three-dimensional cavity system consisting of vesicles, tubules and plate-shaped cisternae on the outside of which ribosomes are anchored or lined up at relatively regular intervals. The membranes, which are always arranged in pairs, have a distance of 20 - 60 nm from each other, which can increase significantly (dilate) at corresponding points during high metabolic activity and protein storage. The endoplasmic reticulum is mainly responsible for protein synthesis, protein remodeling, detoxification, storage of carbohydrates and calcium, and hormone production.

The rough endoplasmic reticulum: The membranes of the rough endoplasmic reticulum may be in continuous communication with the nuclear membrane, causing the interior of the RER to merge into the perinuclear space (space between the inner and outer nuclear membranes). From the nuclear pores, mRNA enters the rough ER as a protein blueprint. There, at the ribosomes, mRNA is converted into proteins (translation), which are folded from a long strand into an interwoven "bundle" while still in the ER. Depending on the task, these proteins either remain as enzymes in the extensive complex of the smooth ER to perform various functions there. A large proportion of the proteins are not produced for the cell's own use, but are released as secretions. The RER forms small transport vesicles that migrate toward the Golgi apparatus or cell membrane to fuse with membranes, releasing their protein-rich contents.

The RER is particularly expressed in cells with a high protein synthesis capacity, e.g. plasma cells, exocrine gland cells, neurons, osteo-, chondro- and fibroblasts. Rough ER can also continue directly into smooth (agranular) ER, which is occasionally observed, especially in liver cells. Ergastoplasm or Nissl clod (or tigroid substance; substantia chromatophilica; English chromatophilic substance) is the term used to describe the RER consisting of largely parallel membranes in association with adjacent free ribosomes in neurons.

Thesmooth endoplasmic reticulum: The smooth or agranular endoplasmic reticulum (sER) has no ribosomes on its inner surface. Usually it is the smaller part of the ER, only in cells that are responsible for lipid metabolism or that are specialized in the production of steroid hormones, the smooth endoplasmic reticulum forms the larger part. The tubular structure of the smooth endoplasmic reticulum, which extends through the entire cell interior, has a large surface area that is required for various functions, such as calcium storage.

Sarcoplasmic reticulum: The so-called sarcoplasmic reticulum (SR) is a special form of smooth ER, which is mainly found in muscle cells. There it spans the individual muscle fibrils (contractile structures in striated muscles). There the main protein of the membrane of the SR is the Ca2+-ATPase (adenosine triphosphatases). After each muscle contraction, calcium is pumped from the cytoplasm into the ER. This prevents the muscle from contracting again, whereas the release of calcium into the cytoplasm is what causes them to contract in the first place.