Text
Historical Background
Garnier, a French cytologist, in 1899 described
filamentous structures in the cytoplasm of cells in the
pancreas and salivary glands which stained intensely with
basic dyes. He called this material ergastoplasm, which
varied in form and quantity in different phases of the
secretory cycle. Claude (1943), by using staining and
ultraviolet
absorption
methods
identified
ribonucleoprotein particles of 50-300nm, which he called
microsomes and are thought to be involved in protein
synthesis. Porter (1945) observed a system of delicate
branching and anastomosing strands under electron
microscope that formed a lace-like network throughout
the
cytoplasm.
This
endoplasmic
reticulum
was
considered to be a new cell organelle by Porter and
Thompson (1947). Palade (1953) described a small
particulate component consisting of 15 to 20 nm dense
granules that were attached in large numbers to the
outer
surface
of
the
membranes
bounding
the
endoplasmic reticulum. They also occurred singly or in
small aggregates scattered throughout the cytoplasmic
matrix.
INTRODUCTION
Endoplasmic reticulum is a vast network of closed
and open cavities in the form of membrane-bound
tubules, vesicles and flattened sacs. These often remain
concentrated in the endoplasmic portion of the cytoplasm
and are, therefore, known as endoplasmic reticulum, a
name derived from the fact that it looks like a net in the
cytoplasm under electron microscope. The term
endoplasmic means ‘within the cytoplasm’ and the term
reticulum is Latin for a ‘little net’. It is the main
component of the endomembrane system, also called the
cytoplasmic vacuolar system or cytocavity network. This
system comprises the nuclear envelope, the endoplasmic
reticulum and the Golgi apparatus. The entire
endomembrane system represents a barrier separating
cytoplasmic compartments. The endoplasmic reticulum is
composed of a lipid bilayer embedded with proteins, like
the plasma membrane. It weaves in sheets through the
interior of the cell, creating a series of channels between
its folds. This endomembrane system fills the cell,
dividing it into compartments, channelling the passage of
molecules through the interior of the cell and providing
surfaces for the synthesis of lipids and some proteins.
Occurrence
It is absent from the prokaryotes but is present in all
the eukaryotes except germinal cells and mature
mammalian erythrocytes. Development of ER depends
upon metabolic state and stage of differentiation of the
cells, e.g. it is absent from eggs, embryonic cells, mature
erythrocytes; less developed in spermatocytes, and well
developed in fully differentiated and metabolically active
cells, e.g. acinal cells of pancreas and hepatocytes of
liver cells.
Fig.Prokaryotic cell lacking endoplasmic reticulum
Fig. Plant cell showing various cell organelles
Fig. Animal cell showing various cell organelles- 1.
Nucleus;
2. Nuclear pore;
3. Rough endoplasmic
reticulum (RER); 4. Smooth endoplasmic reticulum
(SER); 5. Ribosome on the rough ER; 6. Proteins that are
transported; 7.Transport vesicle; 8. Golgi apparatus; 9.
Cis face of the Golgi apparatus; 10. Trans face of the
Golgi apparatus; 11. Cisternae of the Golgi apparatus.
Ultrastructure
Under electron
microscope,
the
endoplasmic
reticulum shows the presence of three types of
structures- cisternae or lamellae, vesicles and tubules.
1. Cisternae
These are long, flattened, sac-like, unbranched
elements, generally present near the nucleus. Each
cisterna is 40-50µm in thickness and studded with
ribosomes. The ribosomes are attached to ER by a
glycoprotein called ribophorin. The cisternae are
abundant in protein forming cells. Each cisterna of ER has
two surfaces-cytoplasmic or protoplasmic face and
luminal face, also called the endoplasmic or extracellular
face.
2. Vesicles
These are usually rounded or ovoid structures,
ranging in diameter from 25-500µm. These often occur
isolated in the cytoplasmand are also studded with
ribosomes. The vesicles are abundant in protein forming
cells. The vesicles are also called microsomes.
3. Tubules
These are wider, tubular and irregularly branched
elements having diameter of 50-100µm. These are
without ribosomes and so are more in lipid and steroid
forming cells and the cells involved in glycogen
metabolism. In liver cells, fine tubules with glycogen
granules are called glycosomes.
All the three forms of endoplasmic reticulum may
occur either in a single cell or some may be lacking at a
time. These consist of membrane lined channels or
spaces. The channels or spaces contain a fluid called
endoplasmic matrix which is quite different from
cytoplasmic matrix present outside the reticulum. The
membrane of ER remains continuous with the plasma
membrane, nuclear membrane and Golgi apparatus to
form a cytoplasmic vacuolar system. ER forms about 3060%
of
cytoplamic
vacuolar
system
and
10% of total cell volume.
Fig.A three-dimensional view
reticulum
of rough endoplasmic
Types of Endoplasmic Reticulum
On the basis of presence or absence of ribosomes,
ER is of two types:
• Rough Endoplasmic Reticulum
Endoplasmic Reticulum
or
Granular
The RER is characterized by the presence of
ribosomes over the surface of reticulum due to which its
walls are rough and hence called rough reticulum. It is
plate-like flattened cisternae measuring about 400-500Ǻ
in diameter. In rough ER, ribosomes are often present as
polysomes held together by mRNA. These are arranged in
typical rosettes or spirals. The RER contains two
transmembrane glycoproteins called ribophorins I (MW
65,000 daltons) and ribophorin II (MW 64,000 daltons),
to which are attached the ribosomes by their 60S
subunit. This is most common in those cells which are
actively synthesizing proteins, such as enzymes, and in
glands. Rough ER is particularly well-developed in
pancreatic cells, plasma cells, goblet cells and liver cells
where secretory proteins are synthesized on the attached
ribosomes and are translocated through cisternae to
different sites in the cell. The granular type of ER takes
basophilic stain due to its RNA content of ribosomes. The
region of the matrix containing granular type of ER takes
basophilic stain and is named as ergastoplasm, basophilic
bodies, chromophilic bodies or as Nissl bodies by early
cytologists.
Fig.Rough Endoplasmic Reticulum
Fig.Micrograph of
rough endoplasmic reticulum
network around the nucleus
Fig. Rough endoplasmic reticulum in pancreatic
acinal cells of human pancreas
• Smooth Endoplasmic Reticulum or Agaranular
Endoplasmic Reticulum
The SER is characterized by the absence of
ribosomes due to which its walls are smooth. It is tubular
in form measuring about 500-1000Ǻ in diameter and
forming irregular lattices. This is most common in those
cells which are concerned with lipid or steroid synthesis
(such as adrenal and sebaceous glands, gonadial
interstitial cells), carbohydrate metabolism (e.g. liver
cells), electrolytic excretion (as in chloride cells of fish
gills), impulse conduction (e.g. in muscle cells) and with
pigment production (e.g. in retinal pigment cells).
Fig.
Filament and lamellar epithelium of fish.
Longitudinal section of gill filament stained with
Toluidine blue, CC=Chloride cells; PVC=Central
venous sinus.
Fig. Gonadial interstitial cell
Fig. Liver cells
Fig. (a) Fenestrated cisternae of smooth endoplasmic
reticulum in a guinea pig Leydig cell; (b) Smooth
endoplasmic reticulum in an adrenal cortical cell.
Fig. Glycogen in an area of hepatic cell rich in smooth
endoplasmic reticulum
Fig. Nuclear envelope, connected to the rough and
smooth
endoplasmic reticulum.
The SER of striated muscle cells is called
sarcoplasmic reticulum, which forms lace-like sleeve
around the myofibrils and is called the longitudinal
element of reticulum. The sarcoplasmic reticulum (SR),
from the Greek sarx, ("flesh"), is a special type of
smooth ER found in smooth and striated muscle. It is a
highly modified SER and is found as a network of
interconnected
and
branched
tubules
running
longitudinally in the interfibrillar sarcoplasmic spaces
along the length of each sarcomere of muscle fibres. It
was first reported by Veratti in 1902. He described in a
wide variety of species a delicate reticulum throughout
the sarcoplasm, consisting of mutually anastomosing
strands with small enlargements at nodal points. The
reticulum surrounded all of the myofibrils and its
transverse elements appeared to occupy a fixed position
relative to the pattern of striation in the muscle fiber. It
was composed of anastomosing tubules resembling those
of endoplasmic reticulum but without associated
ribonucleoprotein particles.
Fig. Sarcoplasmic reticulum
The only structural difference between this organelle
and the SER is the combination of proteins they have,
both bound to their membranes and drifting within the
confines of their lumens. This fundamental difference is
indicative of their functions: the smooth endoplasmic
reticulum synthesizes molecules while the sarcoplasmic
reticulum stores and pumps calcium ions. The
sarcoplasmic reticulum contains large stores of calcium,
which it sequesters and then releases when the muscle
cell is stimulated. The release of calcium from
sarcoplasmic reticulum upon electrical stimulation of the
cell plays a major role in excitation-contraction coupling.
Fig. Sarcoplasmic retiulum
Enzymes of Endoplasmic Reticulum
A number of enzymes are bound to the membranes of
endoplasmic reticulum. The most important enzymes are:
• Stearases
• NADH-Cytochrome C-reductase.
• NADH diaphorase.
• Glucose-6-phosphatase.
• Mg++ activated ATPase.
• Cytochrome b5 and P-450.
Fig. Glucose-6-phosphatase
Isolation and Chemical Composition
The endoplasmic reticulum can be isolated from
other cell constituents by subjecting homogenized tissues
to differential centrifugation. Electron microscopy reveals
that the membranes of such endoplasmic reticulum
preparations disrupt to form closed vesicles of either a
smooth or a rough form. These membranous entities
were called ‘microsomes’ by Claude (1940). The
membranes of endoplasmic reticulum are lipoproteinous
and trilaminar like the plasma membrane, but
membranes of ER are thinner than plasma membrane
and contain more lipids and less cholesterol. The
endoplasmic reticulum contains many proteins in
common with the plasma membrane but also harbours a
number of enzymes that are predominantly associated
with it. The percentage of proteins ranges from 50-70
and those of lipids from 30-50. Among the lipid contents,
about 70% are phospholipids, most of which comprise
lecithin and cephalin. The marker enzyme for ER is
Glucose-6-phosphatases.
Fig. Plasma membrane
Origin
The most accepted view regarding the origin of ER is
that it is budded off from the nuclear membrane. This
view is supported by the fact that both ER and nuclear
membrane are lipoproteinous and trilaminar. The
intercisternal space of ER is continuous with perinuclear
space and both ER and outer nuclear envelope are
studded with ribosomes. The fluid present in the ER
cisternae and perinuclear space is of similar nature. SER
arises from RER by the loss of ribosomes.
Functions of Endoplasmic Reticulum
• Endoplasmic reticulum acts as a cell circulatory
system and helps in transportation of materials
inside the cell. It is the major component of
cytoplasmic-vacuolar
system
which
acts
as
cytoskeleton and provides mechanical support and a
definite shape to the cell.
• Endoplasmic reticulum acts as segregation apparatus
and divides the cytoplasm into two types of
compartments- one lying within E.R. membranes and
other between the E.R. cisternae. Endoplasmic
reticulum regulates the exchange of materials
between these compartments. It provides enormous
surface area for exchange of materials.
• During cytokinesis in the plant cells, endoplasmic
reticulum provides small sized phragmoplasts of
about 20 µm which arrange themselves at the
equator and later fuse to form the cell plate which
later forms the middle lamella.
• Endoplasmic reticulum of pigmented epithelial cells
of retina acts as photoreceptor.
• Proteins are synthesized on the surface of rough
endoplasmic reticulum. These proteins may be
utilized within the cell or may have to be exported
outside the cell to the site of their utility. The
proteins destined for secretion (i.e. export) from the
cell or proteins to be used in the synthesis of cellular
membranes are synthesized on rough endoplasmbound ribosomes, while cytoplasmic proteins are
translated for the most part on free ribosomes.
Synthesis of tropocollagen, serum proteins and
secretion granules are some examples of secretion
proteins. Infact, the arrangement of rough ER
provides extensive surface area for the association of
metabolically active enzymes, amino acids and
ribosomes. The protein molecules synthsized on
attached ribosomes are discharged and penetrate
into the cavity of ER, where they are either stored or
exported outside.
• Smooth endoplasmic reticulum in collaboration with
Golgi complex is engaged in synthesis of lipids (e.g.
phospholipids, cholesterol, etc.). The smooth ER is
abundant in the hepatocytes and is involved in the
production of lipoprotein particles, which carry the
lipids to other parts of the body. The smooth ER is
also involved in the synthesis of glycogen.
• The ER contains enzymes for the synthesis of
triglycerides, phospholoipids and cholesterol. There
are two electron transport systems that contain two
flavoproteins (NADH-cytochrome-c-reductase and
NADH-cytochrome-b5-reductase)
and
two
haemoproteins (cytochrome b5 and cytochrome P450). Cytochrome P-450 functions as a terminal
oxidase and in the liver is used to detoxify or
inactivate many drugs by oxidation, and to
hydroxylate the steroid hormones. Cytochrome P450 render these drugs water soluble, so that these
may leave the cell and are excreted in the urine.
• The endoplasmic reticulum by virtue of its enzymes
appears to have a role in blood glucose homeostasis
and in the early stages of glycosylation of
glycoproteins.
The enzymes of the endoplasmic reticulum have
important role in the synthesis of cholesterol, steroid
hormones and bile acids.