As discussed in Chapter 17, salivary glands arise from a cord of epithelium growing into the underlying connective tissue, and the cord later forms a tube. At the end of this tube a cluster of secretory cells forms, and these clusters, which look like bunches of grapes, will have end pieces that are either round or tubelike.
The secretory end pieces are known as acini. There are two kinds of acinar cells, mucous acini and serous acini. Although these cells form a grapelike or tubular endpiece, in cross section they are described as pyramidal cells (Fig. 25-1). The outer edge or base of the cells rests on a basement membrane between the cells and the connective tissue. Within this connective tissue are the nerves and blood vessels necessary for the various aspects of cellular activity. The apex of the cells faces the center of the tube or grapelike structure. The base of the cells is surrounded by connective tissue, and partially surrounding each secretory acinus is a myoepithelial cell (Fig. 25-2). This cell has long cellular projections, resembling a squid. It also has the ability to contract like a muscle. Hence the prefix myo, meaning muscle. These projections surround the acinus, and when the myoepithelial cell contracts, it squeezes the acinus and aids in the secretion of saliva that has accumulated in the hollow center of the acinus and helps move it out the duct system. All types of acini—mucous, serous, and seromucous—secrete their products through the process of merocrine secretion.
A mucous secretion is slightly viscous because of the production of several mucins. Although its product is 99% water, it does have a number of inorganic ions, such as sodium, potassium, and chloride, and very minor amounts of amylase, a carbohydrate-splitting enzyme that begins to break down starches into long-chain sugars. It probably also has some proteins that aid in inhibiting caries and periodontal disease. A mucous acinus is more tubular and has a larger lumen than a serous acinus, and the cell membranes can more easily be seen at adjacent sides of the cells. The nucleus of a mucous cell is usually very flat and lies against the basal end of the cell, and the cell itself is pyramidal in shape. The apical end of these cells appears frothy under the light microscope. With the electron microscope, one can see a great number of mucous droplets that stain very poorly and so give an empty, frothy appearance.
The makeup of serous acinus secretions is similar to that of mucous acini, only without the mucins; a serous secretion is a thinner, more watery secretion. A serous acinus is the primary source of amylase. The secretory granules stain deeply, the lumen is very small and difficult to see, and the adjacent cell membranes are not easily seen. A serous cell is also pyramidal in shape. The nucleus is round and is close to the base of the cell.
In glands that have both mucous and serous components, you can see the separate types of acini, and you can also see them joined together as mixed or seromucous acini. In a seromucous acinus, the mucous cells form a tubelike structure, and on the end of the tube a group of serous cells forms into a half-moon cluster. These are referred to as serous demilunes. The serous demilune cells secrete their product between the cell walls of the underlying mucous cells and their secretion enters the lumen of the gland. As the term seromucous suggests, these acini produce both mucous and serous secretions (see Fig. 25-1).
A salivary gland is surrounded by a connective tissue capsule. The connective tissue not only surrounds the gland but also sends partitions into the gland carrying nerves and blood vessels with it and dividing the gland into lobes and smaller units called lobules (Fig. 25-3).
Salivary glands have varying numbers of lobules, depending on their size, and as mentioned, each lobule is surrounded by connective tissue. There are a series of different kinds of ducts, some of them within the lobule, some between the lobule, and some outside of the gland, in the surrounding connective tissue that leads to the surface of the oral cavity.
Intercalated ducts are very small intralobular ducts that directly drain the acini. The cells in these ducts are not much taller than their nuclei. In some glands the ducts are long and easily seen, whereas in others they are short and rarely seen. These intercalated ducts carry the secretions, unchanged, to the next set of ducts within the lobule.
Intralobular striated ducts are so named because the bases of the cells within these ducts appear to be striped. The basal cell membrane has infoldings in which mitochondria become trapped and aligned between the infoldings. These mitochondria can be stained, causing a striped appearance. These ducts are also called secretory because, as the salivary fluid passes through them, their content is modified. Water and various substances such as sodium, potassium, chloride, and other ions are reabsorbed by being secreted out of the basal end of the cell where they are picked up again by the capillaries and lymphatic vessels. This function is important because it conserves water and electrolytes (see Fig. 25-1).
Interlobular ducts lie within the connective tissue between lobules of the gland. Some of these are striated like the striated intralobular ducts and generally are found only at the beginning of the interlobular ducts and are secretory, but most of them are nonstriated and large. These nonstriated ducts are generally referred to as excretory ducts. These ducts do not modify the salivary secretions but simply carry them out of the gland and to the surface tissues of the oral cavity (Fig. 25-4).
Secretory control comes from the autonomic nervous system, particularly the parasympathetic nervous system (see Chapter 34). The control of secretion is tied to chewing, taste, and smell. Each of these is capable of modifying the amount and consistency of the salivary secretions.
Saliva is formed within the endoplasmic reticulum of individual cells. If you remember from Chapter 17, the control of production within the endoplasmic reticulum comes from the DNA in the nucleus via the varying types of RNA. The future saliva is packaged by a membrane within the Golgi apparatus and moves out into the cytoplasm of the mucous or serous acini. The apical end of the cell accumulates these granules of future saliva, when the gland is stimulated by the parasympathetic nervous system, the granules move to the apical cell membrane, fuse with it, release their substance into the center lumen of the acini. From there the secretion moves out through the intercalated ducts into the striated, intralobular ducts and the striated, interlobular, and extrinsic ducts opening into the oral cavity.
As it passes through the striated ducts, the fluid content actively changes. Water is pulled out of the lumen of the duct, through the striated duct, and into the capillaries lying at the base of the cells of the duct. At the same time, conservation of electrolytes and secretion of potassium and bicarbonate occurs. The fluid within the duct becomes more concentrated. Depending on the type of secretion, the amount of amylase will vary and be available to begin the breakdown of starches before further breakdown in the stomach and duodenum. There are also immunoglobulins in the saliva as well as antibacterial lysozymes that protect the body from bacteria. The saliva also acts as a Ph buffer, which plays a role in resistance to decay, if it is working properly.
Basically, saliva functions to help prepare the bolus of food so that it can be swallowed more easily. It helps to keep the mucous membranes lubricated to protect them from drying out and becoming parakeratinized, or even keratinized. It also may play a role in dehydration. If there are not sufficient body fluids, saliva production will be decreased, and the mouth will become dry, a stimulus to drink more water. As mentioned, saliva also acts as a Ph buffer, and the immunoglobulins and lysozymes within saliva play a role in protection of the body.