SER plays a crucial role in cellular metabolism. Examples of cells with abundant sER are sebaceous glands, gonadal cells involved in producing steroid hormones (such as Leydig cells in the testis and follicular cells in the ovary), hepatocytes in the liver, and cells of striated muscles. Conversely, some specialized cells, are rich in sER. However, some eukaryotic cells lack sER, such as ova, embryonic cells, and mature RBCs. The smooth endoplasmic reticulum is present in almost all eukaryotic cells. The calcium stored in the sarcoplasmic reticulum is released with the help of ryanodine receptors which initiate muscle contraction and during relaxation, calcium is stored back inside the sarcoplasmic reticulum with the help of SERCA, which is a pump that transports calcium ions from the cytoplasm into the sarcoplasmic reticulum. The entrance doors to the sarcoplasmic reticulum are named SERCA while each cisterna carries sets of closely grouped structures known as ryanodine receptors that function as the exit doors. Both the longitudinal and the junctional sarcoplasmic reticulum (SR) show a specific spatial organization with respect to the myofibrils, being regularly aligned with specific regions of the sarcomeres. The longitudinal tubules join at their ends to form a single dilated sac-like formation which is called terminal cisterna. The longitudinal sarcoplasmic reticulum is a network of many interconnected tubules around each myofibril. The sarcoplasmic reticulum is divided into two domains based on the nature of their functions: the longitudinal sarcoplasmic reticulum and the junctional sarcoplasmic reticulum. It plays an important role in muscle contraction by regulating intracellular Ca 2+ concentrations. The sarcoplasmic reticulum acts as a big chamber inside the cell, that stores most of the calcium required for the contraction of muscles. The name sarcoplasmic reticulum literally means “ net-like structure found inside the muscle fibers” implying that it is a network of tubules that extend throughout muscle cells, wrapping around the myofibrils. It is a network of tubules that extend throughout muscle cells and each segment of the sarcoplasmic reticulum forms a cuff-like structure surrounding a myofibril, the fine contractile fibers that extend the length of the striated muscle cell, wrapping around it but do not come in direct contact with it. The sarcoplasmic reticulum or SR is a highly specialized form of the smooth endoplasmic reticulum which regulates intracellular calcium homeostasis. In muscle cells, a distinct type of smooth endoplasmic reticulum is present which is referred to as sarcoplasmic reticulum. The interior of the smooth endoplasmic reticulum is called the lumen, which is enclosed by a phospholipid membrane. In steroid-secreting cells, sER may also take the form of concentric or spiral arrays of highly fenestrated cisternae. The tubules in sER compared to that of the rER are more variable in diameter and they branch and connect among themselves frequently to form a very compact three-dimensional reticulum. wider, tubular, branched elements mainly present near the cell membrane) and vesicles (i.e., the rounded or oval, vacuole-like elements, scattered in the cytoplasm) formed in a reticular pattern which allows for an increased surface area for the action or storage of key enzymes.
It consists of a membrane-enclosed network of tubules (i.e. Smooth ER is connected to the n uclear envelope and spreads throughout the cytoplasm. The Smooth endoplasmic reticulum, as opposed to the rough endoplasmic reticulum, lacks ribosomes on its surface, hence the distinct name.
Structure of the Smooth Endoplasmic Reticulum In 1971, Günter Blobel (German-born American cellular and molecular biologist) found out that ER plays a central role in the synthesis of proteins. By the early 1960s, the structure, composition, and role of the endoplasmic reticulum in muscle contraction and calcium regulation were delineated. Palade in 1953 observed a net-like (reticulum) structure within (endo) the cytoplasm (plastic) and they named the organelle “e ndoplasmic reticulum”. The world had to wait another 50 years and it was the advent of electron microscopy that opened the door for two scientists to prove the existence of the endoplasmic reticulum. However, their observations were not widely recognized by their contemporaries. French cytologist Charles Garnier and others are said to have observed ER under light microscopy as early as 1902. Discovery of Smooth Endoplasmic ReticulumĮR was one of those organelles to be discovered late despite being one of the largest and functionally important organelles.