EDOPLASMIC RETICULUM STRUCTURE AND FUNCTIONS
Discovery:
The ER was discovered by K. R. Porter, Claude and Thompson in 1945, but the name
ER was given by Porter (1953).
Location:
The ER occurs in all eukaryotic cells except ova, embryonic cells and mature RBCs.
It is also absent in prokaryotic cells.
ER is quite extensive in metabolically active cells, simple in storage cells and reduced
in spermatocytes.
Structure:
The endoplasmic reticulum (ER) is an extensive three dimensional, network of
vacuolar system that extends from nuclear membrane up to plasma membrane. ER occupies
10% of total cell volume. Its membrane accounts for 30-60% of all cellular membranes, and
increases the internal surface 30-40 times as compared to the external surface.
ER divides the intracellular space into two distinct compartments i.e. luminal (inside
ER) and extra-luminal (cytoplasm) compartments. The ER exists in three morphological
forms-cisternal, tubules & vesicles. All of them are filled with a fluid called endoplasmic
matrix.
1. Cisternae:
These are flattened, un-branched sac-like structures. They lie parallel to each other.
They are interconnected and may be covered with ribosomes. They occur in cells having
active synthetic roles.
2. Tubules:
These are irregularly branched tube-like structures which form a network along with
other elements. They do not possess ribosomes on their surface. They are more common in
cells engaged in lipid and steroid synthesis.
3. Vesicles:
These are round, spherical or ovoid structures. Diameter is about 25-500/µm. They
are found dispersed in the cytoplasm. They are rich in pancreatic cells. In spermatocytes,
vesicles are the only ER structures found. When cells are disrupted during cell fractionation,
the ER breaks up into small vesicles called microsomes.
Types of Endoplasmic Reticulum:
The endoplasmic reticulum is of two types:
(i) Smooth or Agranular Endoplasmic Reticulum (SER):
The ER in absence of ribosomes appears smooth and is called SER. It composed of
mainly tubules and vesicles.
This agranular reticulum is most common in those cells which are concerned with
steroid or lipid synthesis (such as adrenal and sebaceous glands, gonadial interstitial cells),
carbohydrate metabolism (e.g., liver cells), electrolyte excretion (as in chloride cells of fish
gills), impulse conduction (e.g., in muscle cells) and with pigment production (as in retinal
pigment cells), i.e., nonprotein types of synthesis.
(ii) Rough or Granular Endoplasmic Reticulum (RER):
The ER bearing ribosomes on their surface is called RER. The surface of RER
contains two types of glycoproteins (ribophorin I and II) for attachment of ribosomes. RER
composed of mainly cisternae; tubules are very few. Rough ER predominates in cell which
are actively synthesizing proteins such as enzymes and in glands.
Out of total ER content, about2/3rd is RER and 1 /3 SER. SER and RER are interconvertible. RER is more stable than SER. RER is basophilic due to presence of ribosome.
Functions of ER:
Common Functions:
1. ER provides mechanical support to the cytoplasmic matrix.
2. It provides a large surface area for the synthesis of various materials.
3. ER keeps the various organelles in their position.
4. ER facilitates quick intracellular transport, thus forming the circulatory system of cell.
5. ER extends through plasmodesmata as desmotubules and control the movement of
materials between adjacent cells.
Functions of RER:
1. RER contains ribophorins for holding ribosomes.
2. RER synthesizes proteins destined for secretions, lysosomes or the plasma membrane.
3. RER forms nuclear envelope, plasma membrane and SER.
Functions of SER:
1. SER takes part in synthesis of lipids, steroid hormones ascorbic acid etc.
2. SER helps in detoxification of toxins using cytochrome P-450.
3. In liver cells, SER possesses enzyme bodis called glycosomes for glycogen metabolism
(glycogenesis and glycogenolysis.
4. In retinal cells. SER produce visual pigments from vitamin A.
5. In muscle cells, SER modified into sarcoplasmic reticulum ‘SR) which store and release
Ca2+ for muscle contraction.
6. SER produce organelles like Golgi apparatus, lysosomes, Sphaerosomes and vacuoles.
Modifications of ER:
1. Annulate ER or Annulate lamellae (Mc Culloch 1952):
These are the ER having pores like nuclear pores. They formed from blebing nuclear
envelope. They can be smooth of rough.
2. Sarcoplasmic Reticulum (SR):
A modified form of SER is striated muscle cells (both skeletal and cardiac), supply
Ca2+ to muscle cytoplasm. Help in intra cellular impulse transmission and contraction.
3. Nissl granules (tigroid bodies):
Masses of RER in cyton of a neuron.
4. Ergastoplasm:
This term was introduced by C. Gamier (1900) for highly developed endoplasmic
reticulum with ribosomes in synthetically active cells. Thus, ergastoplasm refers to welldeveloped endoplasmic reticulum along with ribosomes in some parts of metabolically active
cells.
5. Myeloid bodies:
This is also specialization of a granular endoplasmic reticulum. These bodies occur in
pigmented epithelial cells of retina. Each body consists of vesicles and tubules packed
together near the basement membrane of cells. Ribosomes are not associated with these
bodies Each myeloid body is biconvex, 4-5µ long along the axis and composed of stacks of
packed lamellae.