Golgi Complex. The Golgi apparatus or complex is a membranous system of cisternae and vesicles, usually located in or around the cytocentrum. It is involved in intracellular transport and modification of secretory proteins, membrane proteins, and proteins that remain membrane-bounded within the cell, in distinction to proteins such as hemoglobin and keratin that lie free in the cytoplasm.

The Golgi complex has a characteristic appearance by light microscopy. It may be a small compact structure. It may be a cluster of small structures (termed dictyosomes in earlier literature); or it may be a large netlike structure, the internal reticular apparatus as initially defined by C. Golgi in nerve cells. The Golgi complex is often juxtanuclear and may partially enclose the centrioles. Its size fluctuates with cell type and secretory activity. It is well developed in secretory cells, for example, in the mucus-producing intestinal epithelial cells, in plasma cells, and cells of the pituitary gland. The Golgi complex reduces metal salts, such as salts of osmium and of silver, and may therefore be stained with these compounds. Such staining methods were responsible for the discovery of the Golgi complex.

By electron microscopy, the Golgi complex consists of 3 to 15 large flat sacs or cisternae apposed to one another. They are relatively compressed at their centers and somewhat dilated peripherally. The sacs tend to be bowed, presenting a convex proximal face (toward the nucleus) and concave distal face (away from the nucleus).

These stacked cisternae thus form bowl-shaped structures, and the Golgi complex as a whole looks like a stack of shallow bowls with the concavity directed away from the nucleus. The cisternae may communicate with one another by slender channels at places along their contiguous surfaces. The proximal membranes (those near the nucleus) are thinner than the distal membranes (facing out toward the bulk of the cytoplasm), which are more like those of plasmalemma. At the edge of the lamellated sacs, near their expanded peripheries, vesicles are typically present. Similar vesicles may also be abundant at the distal face. The vesicles vary in size and probably fuse to form larger vesicles. They, like the lateral vesicles, may contain a dense material. The distal face, which is typically engaged in granule formation, has been termed the maturation face (also known as the trans face). The proximal face, relatively free of vesicles, has been termed the forming face (also known as the cis face), (Fig. 1-52).

Proteins destined to be secreted or stored in lysosomes or other membrane-bounded granules are synthesized by polysomes attached to the outer surface of ER. Newly synthesized proteins collect within the lumen of the rough ER and move into smooth connected ER. It is thought that this smooth ER buds off as transport vesicles, which carry quanta of ER content to the Golgi complex. The Golgi complex not only serves as a way station for protein intracellular transport, but proteins are modified as they pass through it. For example, a portion of the carbohydrate moiety of many glycoproteins is added in the Golgi complex (e.g., immunoglobulins and pancreatic enzymes). The high concentration of the glycosyl transferase enzymes on the inner surface of Golgi membranes permits this function.

There are several patterns of secretion. In "nonregulated" secretory cells (eg. plasma cells and fibroblasts), secretion is continuous and is effected by small Golgi-derived secretor vesicles. In "regulated" secretory cells (e.g. pancreatic acinar cells), however, secretion is intermittent and depends on stimulation of hormones or other factors. Here the secretory granules accumulate in the apical cytoplasm and may become rather large, up to 1,500 nm in diameter. In such cells, the ability of the Golgi complex to concentrate secretory protein is especially evident. Distal to the stacked cisternae are "condensing vacuoles" of irregular shape. These organelles further concentrate their content to become zymogen or storage granules. Upon hormonally triggered secretion, the granule membrane fuses with the plasma membrane. At the site of fusion the membranes break down and the contents of the secretory granule are released from the cell.

A major technique for delineating the sequence of protein intracellular transport is pulse-chase electron microscopic autoradiography. With this method, a radioactive metabolite, such as an amino acid or sugar that will be incorporated into the macromolecular product undergoing synthesis, is injected rapidly in a single dose into an experimental animal. As a result, a short, sharply delineated "pulse" of radioactively labeled metabolite enters the synthetic process and is carried through it. After a short time an identical but nonradioactive compound is rapidly injected. This "chases" the radioactive amino acid, diluting it out, so that the radioactive compound can be crisply followed as it passes through the metabolic pool. By sampling tissue at appropriate times for autoradiography, the "pulse labeled" radioactive macromolecules (e.g., proteins) can be visualized at their site of synthesis and followed during transport to the site of discharge.

No portion of the cell has probably been more shrouded in controversy following its discovery than the Golgi apparatus. First described as an apparata reticolare interno (internal reticular apparatus) by the Italian cytologist Camillo Golgi in 1898,... The initial observations were with the light microscope using specimens (especially nerve cells) that had been impregnated with salts or heavy metals that rendered certain parts of the cell, including the Golgi apparatus, dark brown or black against an almost clear background.

With the advent of the electron microscope a portion of the cell containing a mixture of large vacuoles, flattened sacs, and groups of vesicles was shown to reduce osmium tetroxide in much the same manner as the classical Golgi procedure. This structure which constitutes the Golgi apparatus as it is known today undoubtedly helped to form the classic reticulum of Golgi.

There is evidence from labeling studies with radioactive precursors that the Golgi apparatus is a dynamic structure with a complete turnover in many cell types of its membrane constituents every 15 to 30 minutes.

Fig 6. Golgi apparatus...of the epithelial lining of the cat trachea after isolation... and negatively stained. The dictyosomes are partially unstacked to reveal details of indvidual cisternae.

Large secretory vesicles that carry export products from the Golgi apparatus to the cell surface are of two general types. In continuously secreting cells, the mature vesicles detach from the cisternae, often accompanied by an association with clathrin-coated vesicles or clathrin-coated portions of the secretion vesicles and migrate directly to the cell surface where fusion of the vesicle membrane with the plasma membrane ensures delivery of secretory products to the pericellular space and delivery of new membrane to the plasma membrane. In intermittently secreting cells (e.g., acinar cells of the pancreas), secretory vesicles discharged from the mature Golgi apparatus face migrate to the cytoplasm where they function as condensing vacuoles and appear to collect and condense additional secretory products. The fully filled vacuoles, known as secretory granules, are then stored in the cytoplasm to await an appropriate signal to initiate their fusion with each other and with the plasma membrane.

The concept of an endomembrane system (Morré and Mollenhauer, 1974)(Fig.7)

was introduced to indicated the possibility that various membranous compartments of the eukaryotic cells were interrelated and interconnected. Included within the endomembrane system were the nuclear envelope, rough and smooth endoplasmic reticulum, Golgi apparatus, and various cytoplasmic vesicles. Plasma membranes, vacuole membranes, and/or lysosomes were regarded as end products of the system. Organelles such as mitochondria, chloroplasts, and peroxisomes were not included as part of the endomembrane system even though their outer membranes may contact closely or even connect directly with endoplasmic reticulum.

Secretory vesicles that move from the Golgi apparatus to the cell surface provide continuity with the plasma membrane. At the cell surface the membranes of these vesicles fuse with the plasma membrane. The vesicle membrane is incorporated, at least transiently, into the plasma membrane and the vesicle contents are delivered to the cell surface.

The vesicular traffic into and out of the Golgi apparatus involves structures other than the secretory vesicles and the transition vesicles that bleb off the rough endoplasmic reticulum and presumably join to form new Golgi apparatus cisternae. Also involved are clathrin- ("spiny"-) coated vesicles at the mature or trans Golgi apparatus face, condensing vacuoles that give rise to secretory granules, fusiform vesicles, and cisternal remnants as well as various structures apparently derived from the plasma membrane through endocytosis and/or membrane recycling or belonging to the endosome/lysosome/vacuole system.

Biochemically, the Golgi apparatus is a transition cell component that functions as intermediaries between the endoplasmic reticulum and the cell surface. The Golgi apparatus is a primary site of terminal glycosylatin of membrane glycoproteins. Most Golgi apparatus activities can be classified under two broad categories: (1) those of function in the synthesis, assembly, receiving, sorting, and shipping of products destined for secretions and (2) those of function in biogenesis and modification (differentiation) of membranes. Thus, the Golgi apparatus emerges not only as a shipping and receiving center for materials to be secreted but also as a shipping and receiving center for membrane materials for delivery to the cell surface and for imparting to membranes some of the specific characteristics important to the postulated role of the Golgi apparatus in cell surface formation.

Membrane Flow (Vesicular Transport)

Flow kinetics have been determined for mixed membrane proteins (Fig. 17) as well

as for several individual integral membrane proteins. In general, individual membrane proteins move from sites of translation and insertion at the rough endoplasmic reticulum via the Golgi apparatus to the plasma membrane in about 15-20 minutes. Single cisternae are released as large vacuoles from the mature face at a rate of about one every 3 to 4 minutes. These studies, taken together with estimates from short-term labeling and turnover studies with radioactive amino acids from rat liver, suggest that, during active membrane biogenesis, new Golgi apparatus cisternae are formed at one face of the apparatus and discharged at the opposite face at the rate of one very 3 to 4 minutes. With 5 cisternae per stack, the transit time through the Golgi apparatus of 15 to 20 minutes suggests that in situ, a completely new Golgi apparatus is formed every 15 to 20 minutes.

The only well-studied sorting signal is that of the mannose-6-phosphate recognition signal for lysosomal enzymes. Most lysosomal and secretory enzymes are glycoproteins. As such, they are synthesized on rough endoplasmic reticulum, are segregated within the lumens of the endoplasmic reticulum, and pass to the Golgi apparatus. As they pass through the Golgi apparatus, a recognition marker, the mannose-6-phosphate, is added to the lysosomal enzymes. The enzymes so modified are then segregated from normal secretory proteins and are routed to the lysosomes. Lysosomal enzymes lacking the recognition marker fail to recognize the mannose-6-phosphate receptor and are secreted in the normal fashion. In contrast, the enzymes carrying the mannose-6-phosphate signal are segregated into lysosomes and retained within the cells.

There also must be sorting signals to the cell surface for various receptors, adhesion molecules, and antigenic determinants important to growth control, transmembrane signaling, adhesion, attachment, and cell differentiation. Yet, the sorting mechanisms and details through which directionality of sorting and membrane flow are achieved and maintained are largely unknown.

Additionally, the Golgi apparatus requires a source of energy. ATP is involved but the transducing mechanisms and critical control points remain undefined.

While the pattern of Golgi apparatus functioning is similar in all cells that contain the golgi apparatus, important differences also exist in terms of details of the kinds of secretory materials that are processed and segregated by Golgi apparatus vesicles, the manner of discharge of the secretory materials (continuous vs. discontinuous), and in the form and extent of the stacked cisternae. In the sections that follow, the golgi apparatus of representative cell types will be described to illustrate some aspects of this degree of diversity.

1. Pancreatic Exocrine Cells and Zymogen Cells of the Parotid Gland

In these nongrowing cells the Golgi apparatus function is not viewed primarily as one of membrane biogenesis but rather the concentration and segregation of proteins for export into condensing vacuoles (prozymogen granules) and eventually into mature zymogen granules. The mature zymogen granules accumulate near the base of the cell and upon appropriate stimulation coalesce with each other and with the plasma membrane to discharge the stored secretory proteins into the gland lumen. The membrane of the condensing vacuoles appears to originate in continuity with the Golgi apparatus but most of the proteins appear to enter the condensing vacuoles at later stages with the vacuoles physically separated from the stacked cisternae. Another difference between the secretory process of pancreatic exocrine cells and of zymogen cells of the parotid gland is that the discharged membranes do not make a permanent contribution to the plasma membrane. Upon the completion of the discharge process, the excess membrane is largely internalized (recycled).

2. Liver Parenchyma and Intestinal Absorptive Cells

The Golgi apparatus of intestinal absorptive cells shares many common characteristics with that from hepatocytes. The Golgi apparatus of both cell types is engaged in the segregation, modification, and secretion of lipoproteins to the circulation (to the capillary space for liver and to the lymphatics for intestine). The lipoproteins are packaged in the form of discrete particles (low-density and very-low-density lipoprotein particles for liver and very-low-density lipoprotein particles and chylomicra for intestine).   

Liver may be estimated to secrete more than 50 different types of proteins, lipoproteins, and glycoproteins to the circulation. Included among these are the low- and very low-density lipoproteins..., serum albumin, plasma fibronectin, lysosomal enzymes, fibrinogen, and various clotting factors. Most, if not all, of these proteins are packaged and processed via the Golgi apparatus. To what extent sorting takes place, beyond that which segregates lysosomal enzymes from secreted proteins, it not known.

3. Goblet Cells, Enterocytes, and Other Mucin-Secreting Cells

Autoradiography in combination with electron microscopy has established clearly that mucin secretion by colonic goblet cells occurs via the Golgi apparatus. From such studies, it has been calculated that a trans Golgi apparatus cisternae is converted to mucinogen granules every 3 to 4 minutes during active secretion. Discharge to the cell surface is by fusion of the limiting membranes of the mature mucinogen granules with the plasma membrane as for other forms of Golgi apparatus secretions.