Secretion of Organic Substances. Although all the basic mechanisms by which glandular cells function are not known, experimental evidence points to the following principles of secretion, as shown in Figure 1.
1. The nutrient material needed for formation of the secretion must first diffuse or be actively trans ported by the blood in the capillaries into the base of the glandular cell.
2. Many mitochondria located inside the glandular cell near its base use oxidative energy to form adenosine triphosphate (ATP).
3. Energy from the ATP, along with appropriate substrates provided by the nutrients, is then used to synthesize the organic secretory substances; this synthesis occurs almost entirely in the endoplasmic reticulum and Golgi complex of the glandular cell. Ribosomes adherent to the reticulum are specifically responsible for formation of the proteins that are secreted.
4. The secretory materials are transported through the tubules of the endoplasmic reticulum, passing in about 20 minutes all the way to the vesicles of the Golgi complex.
5. In the Golgi complex, the materials are modified, added to, concentrated, and discharged into the cytoplasm in the form of secretory vesicles, which are stored in the apical ends of the secretory cells.
6. These vesicles remain stored until nervous or hormonal control signals cause the cells to extrude the vesicular contents through the cells’ surface. This action probably occurs in the following way: The hormone binds to its receptor and, through one of several possible cell signaling mechanisms, increases the cell membrane permeability to calcium ions. Calcium enters the cell and causes many of the vesicles to fuse with the apical cell membrane. The apical cell membrane then breaks open, thus emp tying the vesicles to the exterior; this process is called exocytosis.
Fig1. Typical function of a glandular cell for formation and secretion of enzymes and other secretory substances.
Water and Electrolyte Secretion. A second necessity for glandular secretion is secretion of sufficient water and electrolytes to go along with the organic substances. Secretion by the salivary glands, discussed in more detail later, provides an example of how nervous stimulation causes water and salts to pass through the glandular cells in great profusion, washing the organic substances through the secretory border of the cells at the same time. It is believed that hormones acting on the cell membrane of some glandular cells also cause secretory effects similar to those caused by nervous stimulation.
Lubricating and Protective Properties of Mucus, and the Importance of Mucus in the Gastrointestinal Tract Mucus is a thick secretion composed mainly of water, electrolytes, and a mixture of several glycoproteins that are composed of large polysaccharides bound with much smaller quantities of protein. Mucus is slightly different in different parts of the gastrointestinal tract, but in all locations it has several important characteristics that make it both an excellent lubricant and a protectant for the wall of the gut. First, mucus has adherent qualities that make it adhere tightly to the food or other particles and to spread as a thin film over the surfaces. Second, it has sufficient body that it coats the wall of the gut and prevents actual contact of most food particles with the mucosa. Third, mucus has a low resistance for slippage, so the particles can slide along the epithelium with great ease. Fourth, mucus causes fecal particles to adhere to one another to form the feces that are expelled during a bowel movement. Fifth, mucus is strongly resistant to digestion by the gastrointestinal enzymes. And sixth, the glycoproteins of mucus have amphoteric properties, which means that they are capable of buffering small amounts of either acids or alkalies; also, mucus often contains moderate quantities of bicarbonate ions, which specifically neutralize acids.
Thus, mucus has the ability to allow easy slippage of food along the gastrointestinal tract and to prevent excoriative or chemical damage to the epithelium. A person becomes acutely aware of the lubricating qualities of mucus when the salivary glands fail to secrete saliva, because then it is difficult to swallow solid food even when it is eaten along with large amounts of water.
