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Although a discussion of each of the functions of the Golgi apparatus in different kinds of cells and tissues is beyond the scope of this book, two important examples are discussed below.
A list of other well-studied cases is presented in Table 18-1.
1. Formation of the Cell Plate and Cell Wall in Plant Tissues:
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In plants, the cell plate and cell wall form during anaphase and telophase of mitosis and meiosis II. During these final stages of nuclear division, the chromosomes are already separated into two masses in the cell that will become nuclei. Between these two nuclei, pectin and hemicellulose are deposited slowly, forming a plate in the center of the cell, which ultimately grows to the side walls, cutting and separating the protoplasts in two and thereby producing the two daughter cells.
Prior to anaphase, the cell’s Golgi bodies are found outside the spindle. During anaphase, vesicles that appear to be released from the Golgi apparatus invade the center of the spindle (Fig. 18-15) and aggregate about the spindle fibers.
These vesicles are the source of the carbohydrate that forms the cell plate and eventually the cell wall. The nature of the carbohydrate secreted by the vesicles is controversial. Some investigators believe that complete cellulose fibers are secreted and others believe that the final stages of fiber assembly occur after secretion. In either case, the Golgi apparatus is clearly involved in the secretion of the carbohydrate that forms the wall between the two cell halves.
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The plasma membrane of plant cells does not pinch inward or grow inward during cell division as occurs in animal cells. Instead, the membrane forms on both sides of the developing cell plate and grows outward with it. Formation of the membrane results from fusion of the vesicles discharged from the Golgi apparatus.
2. Acrosome Development in Sperm:
The development of the acrosome of sperm cells is an excellent example of the involvement of the Golgi apparatus in the formation of another cellular organelle. The acrosome is a membrane-bound structure at the anterior end of the sperm cells of most animals.
A part of the acrosome membrane appears to be involved in recognition and binding of the sperm cell to the surface of the egg cell during fertilization. The acrosome contains hydrolytic enzymes of which hyaluronidase is the most abundant; it causes the breakdown of the protective surfaces of the egg. As shown in Figure 18-5, the singular large Golgi body of the sperm cell discharges vesicles that migrate to the forming acrosome. At the surface of the acrosome, these vesicles fuse, with the acrosome membrane contributing to the acrosome’s growth.
Because the acrosome is composed of hydrolytic enzymes, it has been suggested that the acrosome is nothing more than a giant lysosome. As the acrosome grows, the Golgi body becomes reduced in size, and in many mature sperm cells disappears entirely.
The outer membrane of the acrosome fuses with the plasma membrane. In mouse sperm cells, it has been shown that the area of the plasma membrane that fuses with the acrosomal membrane contains a large number of concanavalin A binding sites. The increased number of glycoproteins in the membrane is attributed to its origin in the Golgi apparatus.
