The cytoplasm in the cells of most of the plants and animals is traversed by a network of anastamosing fibrillar structures called endoplasmic reticulum (ER). Originally known as ergastoplasm, the current name ER was given by Porter and Kaallman (1952).
The ER is connected to the outer nuclear membrane with the ER space opening into the perinuclear space between two membranes. The ER is known to be connected with the plasma membrane.
Morphologically the ER consists of three types of structures viz., Cistemae, – Vesicles and Tublllues.
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The Cisternae are elongated cyllindncal unbranched structures arranged parallel to one another representing a lamellated structure. They are 40 – 50nm in diameter. Cistemae are generally found in cells active in synthesis.
The Vesicles or sacs or oval bodies, attached to the membrane and have a diameter of25-500nm. These are abundant in pancreatic cells.
The Tubules are branched irregularly and are of diverse shapes having a size of 50 – 100|im. The tubules are abundant in cells which are active in the synthesis of steroid compounds. They are also found in the epithelial cells of retina.
Ultra structure:
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All the three components of ER are bounded by a unit membrane 50 – 60A thick. The membrane is trilamenar like the other unit membranes. Palade (1956) has observed some secretory granules in the cavity of endoplasmic reticulum. The membrane has two outer layers of protein enclosing two layers of phospholipids. However the ER membrane is symmetrical unlike in other organelles (mitochondria).
Types of ER:
Two types of ER have been observed in the cells (same or different). These are – smooth ER (agranular) and rough ER (granullar)
The membrane of smooth ER is not granular due to the absence of ribosomes. Smooth ER in mainly seen in tubules. The tubules measure about 500- 1000A in length. AgranularERisseen in cells involved in lipid or steroid synthesis.
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Cells active in protein synthesis do not have agranular ER. Agranular reticulum of muscle cells is called sarcoplasmic reticulum and it forms lace like sleeve around myofibrils. In the pigmented cells of retina, the agranular ER occurs in the form of tightly packed tubules and vesicles called myeloid bodies.
The rough ER has a granular surface due to the presence of a large number of ribosomes attached to it. Since ribosomes are actively involved in protein synthesis rough ER is conspicuous in cells that are actively involved in protein synthesis e.g. meristematic cells, plasma cells, goblet cells etc. The rough ER stains positively by basic dyes.
The cells (Spermatocytes) of certain vertebrates show an unusual type of ER. Usually ER membrane does not have pores or openings.
But in the instance mentioned above pores or annuli have been reported resembling the ones found in the nuclear membrane. There is a diaphragm across the octogonal symmetry (Manul 1968).
The ER performs many vital cellular functions. These are:
(a) Enzymatic activities
(b) Transport of synthetic products
(c) Storage of metabolites
(d) Formation of nuclear envelope
(e) Intracellular impulse conduction etc.
Microsomes:
These are structures that result due to the homogenization of the membranes. The cell membranes break up into fragments, round off and form macrodomes. Macrodomes are actually fragments of ER.
They can be isolated by high speed centrifugation (100,000g) when they settle after nuclear and ribosomal fractions. Observed under electron microscope they appear as membrane bound vesicles of500-1500 at in diameter.
It should be understood however that microscopes are not natural structures found in the intact cell. They are the result of homogenization of cell. The microsome fraction contains fragmented membranes.
Origin of ER:
The origin is uncertain; invagination of the plasma membrane might give rise to a canalicular system. But evidences for this hypothesis are yet to come (Dallmes et al, 1966).