Endoplasmic Reticulum

Rough Endoplasmic Reticulum (RER):

The RER was first seen in 1945 by Porter, Claude and Fullam, who were able to examine a very thinly spread cultured fibroblast in one of early electron microscope. The presence of this lace like net work (reticulum) in the inner or endoplasmic region of the cytoplasm led to its name.

The membranes of the reticulum show a unit membrane or trilaminar structure usually 6-7 nm thick and encloses a lumen or intracisternal space that may contain some flocculent material of moderate electron density that represents newly synthesized proteins. The membrane of the reticulum encloses intracellular compartment that segregates newly synthesized products. Attached to the outer surface of the membrane are the ribosomes, hence they are termed as rough endoplasmic reticulum.

The amount of rough endoplasmic reticulum in the cell varies from two flattened cisternae to widely distributed, interconnected parallel stacks of cisternae. Rough endoplasmic reticulum is totally absent in mature erythrocytes. It is very extensive in pancreatic acinar cells that produce enzymes. This system of intracytoplasmic membrane may connect with the plasmalemma in a few cell types and with nuclear envelope in others. In some cells, it shows continuity with smooth endoplasmic reticulum. It also communicates with Golgi apparatus by small vesicles.

When the cells are fragmented during homogenization and are subjected to differential centrifugation the endoplasmic reticulum is broken up and forms microsomes. Microsomes derived from rough endoplasmic reticulum are studded with ribosomes. Ribosomes are always found on the outside surface of such microsomes, with the microsomal lumina containing the secretory products of the reticulum.

Functions

•     Synthesis of secreton proteins and their segregation from the remainder of the cytoplasm within the intracistemal space.

•     Concentration of products in some cells e.g. pancreatic acinar cells within the reticulum although concentration usually occurs mainly in the golgi apparatus.

Smooth Endoplasmic Reticulum (SER):

This organelle is primarily in the form of membranous network within the cell; however, it differs from rough endoplasmic reticulum in two important ways. First, it is devoid of ribosomes and thus its membranes appear smooth rather than granular (hence the name smooth or agranular endoplasmic reticulum). Second, it is usually tubular in form rather than cisternal.

SER does not stain different from surrounding cytoplasmic matrix. It was first discovered after the introduction of electron microscopy. Generally the smooth endoplasmic reticulum occurs in the form of tubules which are often interconnected (or merely entangled), tortuous and very closely packed. SER membranes arise from RER and may connect directly with RER and indirectly with Golgi apparatus via small vesicles. The amount of smooth endoplasmic reticulum in the cell also varies with cell type.

Functions:

The smooth endoplasmic reticulum is not associated with any particular or specific function to perform a variety of functions depending on the cell type in which it is present.

It is concerned with the synthesis of lipids (from fatty acids). It also contains enzymes which bring about the various detoxification processes. It is involved in the production of steroid hormones (such as the cells of adrenal luteum and interstitial cells of testis). The smooth endoplasmic reticulum also participates in the synthesis and storage of cholesterol production hormones.