Kinds of Endoplasmic Reticulum: Agranular and Granular Endoplasmic Reticulum!
The shape and size of the membrane-bounded ER spaces varies considerably from one cell type to another.
Many cells, in fact, possess two distinct forms of ER which are in continuity with one another.
On the basis of absence or presence of ribosomes, endoplasmic reticulum (ER) is of two kinds:
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[I] Agranular reticulum:
It is characterised by the absence of ribosomes due to which its walls are smooth. It occurs mainly in tubular forms. These tubules measure about 500-1000 A in diameter, forming irregular lattices.
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 (Christensen and Fawcett, 1961).
In interstitial cells of testis of opossum, agranular reticulum fills whole cytosome, having a complex of tubules of 300-400 A diameter. These tubules branch and anastomose to form a complex network.
Agranular reticulum of striated muscle cells is called sarcoplasmic reticulum, which forms lace-like sleeve around the myofibrils. It is called the longitudinal element of reticulum. In vertebrate eye, pigmented layer possesses about 50% of ER in cytoplasm. Reticular tubules have a mean of 750 A diameter.
[II] Granular endoplasmic reticulum:
It is characterized by the presence of ribosomes over the surface of reticulum and hence it is also .called rough type of reticulum. It is plate-like, flattened csternae 400 to 500 A in width. Rough ER predominates in cell which are actively synthesizing proteins such as enzymes and in glands.
Granular and agranular ER are in continuity of the, membranes in regions of contact. Fawcett (1955) observed the reappearance of rough ER liver cells of rat that have been starved and refed begins with the appearance of smooth, tubular ER (together with free cytoplasmic ribosomes).
In exocrine cells of pancreas, granular reticulum consists of reticular sheets and fenestrated cisternae in the basal region of cell. These cisternae measure 5-10µ in length and the, groups are 400-1000 A in diameter.
In apical region of cells, granular reticulum occurs in the form of vesicles. Another peculiar form of granular reticulum occurs in the endoplasmic nebenkern which is formed of a series of concentric reticular membranes organized into spherical or elliptical shape.
Pancreatic cells in guinea pig (vertebrates) functions as a depot and transport system for nascent proteins. The protein (enzyme) molecules are first synthesized on the ribosomes at the outside surface of ER, after which they enter the interior of ER and are formed into intracisterna granules, and finally are released as membrane enclosed zymogen granules. Palade (1961) also suggests that the Golgi complex takes part in this process possibly by ‘packaging’ the intracisternal granules in a surrounding membrance.
1. Ergastoplasm:
This term was introduced by C. Gamier (1900) for highly developed endoplasmic reticulum with ribosomes in synthetically active cells. Thus, ergastoplasm refers to well-developed endoplasmic reticulum along with ribosomes in some parts of metabolically active cells.
2 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 bod.es Each myeloid body is biconvex, 4-5µ long along the axis and composed of stacks of packed lamellae.
A single lamella is a flat disc having fenestrated periphery and solid centre. Usually, myeloid body occurs in groups of 5 or 6 being connected by their convex surfaces. Their function is unknown, but it is supposed that they have function related with photoreception and stimulation of pigment cells to perform the activities necessary in retinal photoreception.