Source: Cell and Tissue Biology: A Textbook of Histology (Leon Weiss, ed). Sixth Edition, Urban & Schwarzenberg 1988 (pp35-45)

Endoplasmic Reticulum.

Two major classes of ER occur. They are rough or granular ER (Fig. l- 40), which has ribosomes on its outside surface. and smooth or agranular ER, whose surface is free of ribosomes. Ribosomes synthesize protein and need not be associated with ER. The association of ER and ribosomes occurs in cells that bound the protein they produce in membranous sacs. For example, erythroblasts synthesize the protein hemoglobin, which remains dispersed through the cytoplasm; their ribosomes are plentiful but little ER is present. In plasma cells, however, which synthesize large volumes of antibody, confine it by membranes and then secrete it, rough ER is abundant. Peptide chains are synthesized in the ribosomes and sent across the ER membrane into the lumen of the ER. The ER thereby isolates synthesized material from the rest of the cytoplasm, permits further assembly of peptides into larger molecules, and conveys the material by means of transport vesicles to the Golgi complex where further synthesis and processing occur. The Golgi then release the secretion enclosed in membranous sacs, the condensing vacuoles, which mature into secretory vacuoles. Rough ER, well developed in secretory cells, is also abundant in cells that synthesize protein and hold it membrane-bounded within their cytoplasm, as in leukocytes and macrophages. These cells contain enzyme-rich membrane-bounded granules, the lysosomes. The formation of these granules parallels the formation of secretory vacuoles, except that the granules are retained rather than released (secreted).

In nerve cells rough ER exists as large, flattened sacs lying on one another in lamellated fashion to form masses, Nissl bodies, identifiable by light microscopy. Hepatic parenchymal cells contain smaller blocks of rough ER. In plasma cells the rough ER is rather uniformly distributed through the cytoplasm except in the region of the cytocentrum. It may be tubular, vesicular, or flattened, depending on the phase of antibody secretion. Rough ER occupies the base of the pancreatic acinar cell. This rough ER, recognizable by light microscopy as basophilic material (because of the affinity of its ribosomes for cationic dye) is termed ergastoplasm.

Smooth ER occurs in a number of cell types and may have diverse functions. It has a role in the production of steroid hormones, and it is abundant in such cells as the Leydig cells of the testis, which produce the steroid testosterone. Smooth ER synthesizes complex lipids from fatty acids. It also detoxifies certain drugs and becomes very prominent in hepatocytes during the inactivation of phenobarbital. In striated muscle, smooth ER is distinctively organized as the sarcoplasmic reticulum whose functions include delivering high concentrations of Ca++ and other ions to critical places in the sarcomere for muscular contraction and relaxation. Smooth ER in megakaryocytes delimits platelet zones in the cytoplasm and, by fusing, frees platelets from the megakaryocyte. Appropriately, this ER is termed demarcation membrane. Carbohydrate synthesis is associated with smooth ER and the Golgi apparatus. The reformation of the nuclear membrane in telophase is accomplished by smooth ER.

The membranes of the ER possess a self-healing capacity after disruption. When fractions rich in ER are recovered from disrupted ultracentrifuged cells, the ER is found as small vesicles (microsomes) (Fig. 1-41). Evidently the tubular system is fragmented, but the membranes reunite or "heal" to form small vesicles.

The eukaryotic ribosome (80S) is composed of two unequal subunits, one large and the other small . Both are highly organized macromolecular assemblies consisting of one or more RNA molecules and numerous different proteins. In bacteria, and prokaryotes in general, ribosomes (70S) and their subunits are somewhat smaller. The sequence of events that occur during the protein synthesis cycle is the same in both eukaryotic and prokaryotic ribosomes and although there are differences in size, these ribosomes greatly resemble each other in the electron microscope.

Polyribosomes may lie free in the cytoplasm and release their peptide chains into the cytoplasm for further combination and complexing. This is how hemoglobin is synthesized. Where the ribosomes attach to the outer surface of ER, the larger unit maintains attachment.

Ribosomes have a short life span. When protein synthesis ceases, they are quickly metabolized and disappear.

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Source: International Review of Cytology, Supplement 17, 1987, pp.255

SANFORD A. GARFIELD AND ROBERT R. CARDELL, JR

Department of Anatomy and Cell Biology, University of Cincinnati, Cincinnati, Ohio 45267

Morphological Studies

The first ultrastructural observations of the ER were made by Porter and colleagues (Porter et al., 1945; Porter and Thompson, 1947) in thin margins of whole cultured embryonic chick cells, and the structure was described as a lace-like "reticulum." Further ultrastructural studies established that the ER was an independent organelle and called it the "endoplasmic reticulum" (Porter and Thompson, 1938). The location of the ER corresponded to the basophilic regions of the cell seen in the light microscope. The earliest observations of this organelle in thin sections described it as a continuous three-dimensional reticulum made up of a network of membrane-enclosed spaces in the form of tubules that were in continuity with layers of flattened cisternae (Palade and Porter, 1954). The tubular elements free of ribosomes were referred to as smooth endoplasmic reticulum (SER) and the ribosome-studded component was called the rough endoplasmic reticulum (RER). Claude (1943, 1946) and Brenner (1947) isolated the endoplasmic reticulum as "microsomes" from disrupted whole cells by means of ultracentrifugation. These microsomes could then be analyzed morphologically and characterized biochemically, and were shown to be rich in phospholipids and ribonucleic acid (Palade and Siekevitz, 1956).

All eukaryotic cells except enucleated red blood cells and sperm have some endoplasmic reticulum. The pancreatic acinar cell ER represents approximately 60% of the total cell membrane compared to 15% in the hepatocyte (Weibel et al., 1969; Bolender, 1974). However, compared to the relative abundance of both RER and SER in the liver cell, greater than 99% of the ER in the pancreatic acinar cell is RER (Bolender, 1973). In contrast, cells predominantly involved with steroid synthesis and lipid metabolism have a higher amount of SER than RER. The SER is greatly expanded in the intestinal epithelial cell during absorption of a fatty meal. The hepatocyte, therefore, lies between these two extremes of differing ER proportions (Fig. 1).

Importantly, the hepatocyte and other cells can respond to changes in blood levels of substrate, hormone(s), and various exogenous and endogenous substances by accumulating predominantly RER or SER as required for a specific activity.

The location and arrangement of ER within the cell also differ according to cell type and function. Some protein secretory cells have extensively developed arrays of RER arranged in parallel stacks, as seen in the pancreatic acinar cells. The RER is distributed throughout the cytoplasm in plasma cells which synthesize and secrete immunoglobulin. Other cells may have RER mainly in an apical or basal orientation as in intestinal epithelial cells or thyroid follicular cells, respectively. While the amount and location of ER within various cell types are an aid to distinguishing one type from another, the biochemical explanation for the occurrence is not always clear.

Biochemical Studies

The ER of the hepatocyte has been studied extensively and has been found to function in protein synthesis and transport, fatty acid synthesis, phospholipid and triglyceride synthesis, glycogen metabolism (including both deposition and depletion), and the metabolism of lipid-soluble xenobiotics as well as many endogenous substances. These same ER functions may be greater or diminished in other cell types, depending upon the cellular/organic activities of that cell and organ: steroid-secreting endocrine cells possess an elaborate ER specialized for steroid hormone synthesis; the ER of intestinal absorptive cells can shift function toward lipid metabolism in response to a fatty meal (Palay and Karlin, 1959; Cardell et al., 1967).

Membranes are involved also in the regulation, sequestering, and/or transfer of ions: Ca2+-ATPases bound to the ER membranes enable the movement of large quantities of Ca2+ from the cytosol to the lumen of the ER (Moore et al., 1974; Alberts et al., 1983; Carafoli and Crompton, 1978). ... Ca2+ sequestration is more highly developed in the muscle cell (MacLennan and Campbell, 1979) and occurs in the specialized ER referred to as sarcoplasmic reticulum.

Functional Adaptation of the Endoplasmic Reticulum

The ER, as exemplified in hepatocytes (Figs. 1, 2, and 3), responds to a wide range of stimuli to enable cellular adaptation to changing conditions in the extracellular milieu. In most cases there is a correlated structural-functional response.

The liver’s central role in the detoxification and metabolism of foreign as well as endogenous substances is underscored by the fact that 29% of the cardiac blood output is received by the liver. Therefore, any substance carried in the circulation can be rapidly acted on by the liver where the metabolism or "elimination" of lipophilic substances such as drugs, anesthetics, pesticides, toxins, and hormones takes place (Welch, 1979).

A role for hepatic ER in carbohydrate metabolism has been suggested since Fawcett (1955) noted SER in close association with glycogen particles. This relationship is clearly demonstrated in Fig. 2. Based largely on morphological observations, that SER is associated with glycogen particles during all stages of feeding-mediated hepatic glycogen deposition and depletion. Apparently the development of SER follows a cyclic pattern. Proliferation of the organelle occurs during early stages of glycogen deposition and remains associated with glycogen particles throughout all stages of carbohydrate deposition until maximum quantities of glycogen are accumulated at which time the quantity of SER is not strikingly great. The initiation of depletion again stimulates SER proliferation and hepatic SER is present throughout all stages of glycogen utilization. At the point where the cell is devoid of glycogen particles SER content decreases but nevertheless is always present (Cardell, 1977).

The role of the SER in the very early events of glycogen synthesis was studied autoradiographically by Cardell and his co-workers (Cardell et al., 1985). The cytosolic regions of glycogen deposition were investigated in livers of fasted ADX rats injected ip with dexamethasone and an iv pulse label of [3H]galactose as glycogen precursor (Devos and Hers, 1979). EM autoradiography revealed that shortly after DEX administration and prior to the appearance of bonafide glycogen, silver grains were overlying restricted regions of the cell apparently specialized for hepatic glycogen metabolism. These cytosomic regions were small ovoid or round areas about 1 m in diameter containing abundant SER and a high concentration of small dense particles (Cardell et al., 1985). Various criteria indicated that these small particles were presumptive glycogen: association of the incorporated labeled precursor with the cytosomic foci, removal of the particles with a-amylase, and positive staining with periodic acid-chromic acid-silver methenamine (Cardell et al., 1985). The cytosomic foci containing presumptive glycogen and SER (Fig. 3A and B) were most abundant during the earliest period of glycogen synthesis. After 4 to 5 hours of DEX treatment these regions began to accumulate typical deposits of cytoplasmic glycogen (Fig. 3C and D). These unique areas of the cytosome of hepatocytes have been defined as ... foci consisting of SER, particles of glycogen at various stages of formation and enzymes and are shown in Fig. 3. It is speculated that the areas are a microenvironment where the SER interacts with a restricted region of the cytosome in which all the necessary "ingredients" for glycogen metabolism are gathered.

Steroid Synthesis and Secretion.  All steroid hormones are synthesize from cholesterol which may be stored in the cells as fatty acyl esters, newly synthesized by the cells, and/or taken up from plasma lipoproteins. In human beings the most important source of this precursor is low-density- lipoprotein (LDL) taken up from blood plasma by receptor-mediated endocytosis. Most of the enzymes involved in steroidogenesis are located in the microsomal fraction, which in steroid-secreting cells is composed largely ... the smooth ER.   However, enzymes for side chain cleavage of cholesterol as well as for production of glucocorticoids, corticosterone and cortisol, and the mineralocorticoid, aldosterone, are located in the inner mitochondrial membrane. Thus, in the adrenal, a precursor may move between these compartments several times before the final product is formed (Fig.35-10). Some of the steroids produced are secreted as sulfates, and the Golgi complex is thought to be the site of sulfation. It has been assumed that this occurs at the end of synthesis just before secretion; however, recent evidence suggests that there may be sulfated precursors as well.

... there is evidence that...  proteins and cytoskeletal elements, particularly actin filaments, may be involved in the transport of cholesterol to the mitochondria (Fig.35-10). Present evidence indicates that steroid hormones are not stored in large quantities in adrenocortical cells and released discontinuously in packets upon stimulation, as is the case in many protein-secreting cells. Instead, they are synthesized and released continuously as individual molecules, with regulation occurring at the level of synthesis.