Dictyosomes

Much of the material that is to be secreted by a cell must first be modified by a dictyosome, a stack of thin vesicles held together in a flat or curved array (Fig. 1). ER vesicles accumulate on one side of the dictyosome, then fuse together and form a wide, thin vesicle called a cisterna (pl.: cisternae) that becomes attached to the dictyosome (Fig.2). Soon more ER vesicles gather next to this one and form a new cisterna. The first cisterna becomes embedded more deeply in the dictyosome as more vesicles accumulate on that side, which for obvious reasons is the forming face. At the other side, the maturing face, vesicles are being released; their contents have been processed. After separation, vesicles can move to the plasma membrane and release their contents. The outer edges of dictyosomes form an interconnected network of curving tubes, and these may absorb the contents from the center of the dictyosome cisterna, then detach and move away. It is not known why some dictyosomes concentrate material in the central cisternae whereas others use the peripheral ones.

FIGURE 1: (a) Dictyosomes are stacks of flattened vesicles; the edges form a network of tubules and small vesicles. (b) With rapid dictyosome activity, as in these mucilage-secreting cells, the cytoplasm may almost fill with dictyosome vesicles, and fusion of vesicles with the plasma membrane is so abundant that the plasma membrane appears scalloped (X 12,000). (c) Face view of dictyosome isolated from a plant cell; the network of peripheral tubules is visible (X 60,000). (b and c, Courtesy of H. Mollenhauer, Texas A&M University) 
 

Dictyosomes can form large, complex associations. In animal cells that secrete very large amounts of protein, hundreds of dictyosomes associate side by side and form a cup-shaped structure called a Golgi body or Golgi apparatus. In the Golgi body, the dictyosome's maturing faces are on the inner side of the cup, while the forming faces and associated ER are on the outside. Dictyosomes only rarely aggregate into Golgi bodies in plants, one example being in root hairs: All the dictyosomes are part of a giant Golgi body located at the tip of the hair where growth and cell wall formation occur.

FIGURE 2 (a) Vesicles derived from endoplasmic reticulum migrate a short distance to a dictyosome-forming face. (b) At the forming face, the vesicles fuse into a new dictyosome vesicle, called either a vesicle or a cisterna. (c) The cisterna "moves through" the dictyosome as more vesicles form on one side while other vesicles are released from the maturing face (d). 

Different types of processing may occur within a dictyosome: modification of the vesicle's membrane or modification of its contents. If the vesicle is to fuse with the plasma membrane after release, the vesicle membrane must be made similar to the plasma membrane. If the vesicle is to remain separate from all the other organelles, acting as a storage vesicle, its membrane must be made unique and incapable of fusion.

The alteration of the vesicle's contents involves the addition of sugars onto proteins, forming glycoproteins. Sugar-containing proteins occur in the plasma membrane, the cell wall, and as storage products in seeds. Strong evidence is accumulating that dictyosomes also polymerize sugars to polysaccharides used in cell wall construction.

The movement of vesicles from ER to dictyosomes and then to other sites was hypothesized on the basis of the presence of numerous vesicles located between ER and dictyo- somes and the similarity of the contents of dictyosome cisternae to those of vesicles found fusing with the plasma membrane. This hypothesis predicts that if a secretory cell is given a very brief dose of radioactive sugar or protein precursors, much of the radioactivity is soon found in the ER and later in the dictyosomes. Even later, both are nonradioactive but vesicles at the plasma membrane and external material are radioactive. Experiments have verified both this prediction and the related hypothesis that new membrane is synthesized in the ER and then transported by vesicles to growing organelles. Although organelles appear to be distinct entities when viewed by light or electron microscopy, they are actually highly interrelated by this membrane flow. All membranes of the cell, except the inner membranes of mitochondria and plastids, actually constitute just one extensive system, the endomembrane system.

In a few cases, it has been possible to measure the rate of membrane flow. The insectivorous plant Drosophyllum has leaves covered with glands that produce a sticky secretion. The fluid contains digestive enzymes that are processed by dictyosomes in the gland's cells. Once an insect has been caught, each gland produces a visible drop of digestive fluid at a rate of 1.3 pm3 per minute. Dictyosome vesicles are about 0.15 pm in radius, so each has a volume of 0.014 pm3, and approximately 100 must fuse with the plasma membrane every minute to deliver the 1.3 pm3 of fluid and enzymes. Each vesicle has a surface area of 0.27 pm2, so 27 pm2 of vesicle membrane fuses with the plasma membrane every minute during secretion. From calculations of the average cell volume and surface area of the plasma membrane, these secretory cells could double their plasma membrane every 20 seconds. It is important for these cells to be able to retract membrane material from the plasma membrane and recycle it. The mucilage-secreting cells of root tips are more leisurely: Each cell has approximately 800 dictyosomes that together contribute 14 to 26 pm2 of membrane per minute to the plasma membrane, which has a surface area of about 1000 pm2. Each dictyosome receives a new cisterna every 20 to 40 minutes.