Overview – Oral Cavity
Plate 12-1. The Developing Tooth
Plate 12-2. The Tongue, Part 1
Plate 12-3. The Tongue, Part 11
Plate 12-4. The Salivary Gland
Plate 12-5. The Salivary Gland; Acini And Ducts
Our bodies are made of matter. Consequently, the molecular building blocks for cells, tissues, and organs must be gathered from our surroundings. The portal of entry for the matter of which we are made is the subject of this chapter: the oral cavity.
Approximately 500 g (about 1 lb) of solid food and 2.5 L (about 6 lb) of water enter the oral cavity of the average person each day. (These figures are doubtless quite conservative for the average North American and vary considerably among nations, communities, and individuals.) By the time a person reaches age 25 he has taken 54,750 lbs (27.38 tons) of material into the oral cavity. At age 50, 109,500 lbs (54.75 tons) of food and drink have entered and passed through the oral cavity. By age 75, 164,250 lbs (82.125 tons) of material have been taken into the oral cavity, chewed, tasted, swallowed, and passed into the alimentary canal for further digestion and absorption.
The oral cavity performs its tasks efficiently, and usually quite pleasantly. This chapter will deal with three of the most important structures within the oral cavity: the teeth, the tongue, and the salivary glands.
When food is first taken into the mouth, it is reduced to small pieces suitable for swallowing by the cutting and grinding action of the teeth. Each of the 32 teeth is a highly complex organ.
The basic structure of the human tooth is illustrated in Figure 12-1. The exposed part of the tooth, called the Crown, is covered by a cap of Enamel, the hardest substance in the human body, which during tooth formation is laid down upon a core of Dentin. Dentin, a hard, bony substance that constitutes the bulk of the tooth, surrounds a soft central cavity filled with Pulp. Below the gumline lies the Root of the tooth, covered by a modified bone called Cementum, The Cementum, in turn, is tightly attached to a Ligament of dense connective tissue called the periodontal Ligament. As is evident in Figure 12-1, the periodontal Ligamentanchors the tooth to the bony socket in which it sits.
The food that is chewed by the teeth is partially dissolved and put into suspension by the saliva, a complex liquid secreted by the salivary glands. The salivary glands secrete Mucus, salts, water, and a digestive Enzyme called Ptyalin, an Amylasethat splits complex sugars such as starch and Glycogen into simple sugars such as maltose and Glucose. The dissolving action of saliva serves to intensify the taste of food in the mouth; this effect in turn promotes the secretion of digestive juices in the alimentary canal that receives swallowed food. The Mucus secreted by the salivary glands lubricates the oral cavity and pharynx, facilitating swallowing.
The salivary glands are compound tubuloacinar glands; they have a number of secretory units called acini that communicate with the oral cavity through a complex series of ducts. As shown in Figure 12-2, some acini contain Serous Cells, other contain Mucous Cells, and still others contain both serous and Mucous Cells. Each Acinus empties its contents into a small duct called an Intercalated Duct; each Intercalated Duct, in turn, empties its contents into a larger Secretory Duct. The secretory ducts, also called striated ducts, modify the salt and water content of the saliva as demanded by conditions within the oral cavity.
While food is being chewed by the teeth and moistened, lubricated, and partially dissolved by the saliva, it is constantly being moved about by the tongue. The microanatomy of the teeth, tongue, and salivary glands will be illustrated and discussed in the following pages of this chapter.
Figure 12-2, Diagrammatic representation of a salivary gland showing several different kinds of acini.
Adult humans have 32 teeth, or 16 per jaw. We have incisors for cutting, canines for holding and tearing, and molars for grinding food. As omnivores – generalized eaters – we are endowed with the dental equipment necessary to perform a variety of tasks ranging from the grinding of carrots to the tearing of meat. Although canines and molars have different appearances, their general architectural plan is the same (see Figure 12-1). Plate 12-1 shows the structure of a developing tooth that has not yet erupted through the gumline.
Figure A is a light micrograph of a tooth at a relatively early stage of development. The future Crown of the tooth points upward, the Root points downward, and the jawbone (J), or the alveolar bone that will form the socket, is seen to the left. In the middle of the tooth sits the prominent Pulp cavity (PC) that, at this stage of development, is filled with undifferentiated connective tissue cells called Mesenchyme (M). The perimeter of the Pulp cavity is lined with highly specialized cells called odontoblasts (0). Odontoblasts secrete Dentin (D) which will eventually make up the bulk of the mature tooth. In the micrograph at right, the Dentin is darkly stained. Just beneath the Dentin and immediately above the layer of odontoblasts is a clear area made up of material called Predentin (P). Just as bone’s osteoblasts secrete Osteoid, which later becomes mature, mineralized bone Matrix, so do the tooth’s odontoblasts secrete Predentin, material that later becomes mature, mineralized Dentin.
In the mature tooth, the outer surface is made of Enamel. In the micrograph at right, specialized cells called ameloblasts (A) are shown in the act of secreting Enamel (E). Normally, Enamel lies right on top of Dentin. Because of its hardness, however, Enamel often is distorted, displaced, or in some cases dissolved during the rather harsh decalcification procedures used to prepare teeth for microscopy. Here, the Enamel has become separated from the underlying Dentin by an artifactual space (S) that does not exist in life.
The relationship between Dentin and Enamel in the process of tooth development is dynamic. Early in the life of the tooth, when it is but a tooth germ, the odontoblasts at the perimeter of the Pulp cavity are covered by a monolayer of ameloblasts. When the odontoblasts start to secrete Dentin, they deposit it on their outer surface. Consequently, the newly formed Dentin is laid down directly between the layer of odontoblasts and the layer of ameloblasts. When the ameloblasts are confronted by Dentin, they are stimulated to secrete Enamel, which they deposit on top of the layer of Dentin. As a result, the inner core of Dentin is covered by an outer coat of Enamel, a pattern that is evident in the mature tooth.
Outside of the ameloblasts lies another epithelium, the outer Enamel epithelium (OE), which at the base of the developing tooth is separated from the ameloblasts by a jelly-like mass of loose connective tissue called the Stellate Reticulum (SR). As the tooth matures and erupts through the surface of the gum, several events occur. The ameloblasts, being exposed, are rapidly worn away; consequently, Enamel lost through decay or accident does not regenerate. The Stellate Reticulumis transformed from loose connective tissue into the periodontal Ligament, which serves to keep the mature tooth firmly planted in its socket by binding the cementurn at the Root of the tooth to the bone of its socket.
Plate 12-1, Light micrograph of a developing tooth. A, ameloblasts; C, capillary; CT, connective tissue; D, Dentin; E, Enamel; J, jawbone (alveolar bone); M, Mesenchyme that fills Pulp cavity; O, odontoblasts; OE, outer Enamelepithelium; P, Predentin; PC, Pulp cavity; S, artifactual space; SR, Stellate Reticulum. 210 X
In order for the teeth to function properly, food must be placed between them so they can chop and grind it into small pieces suitable for swallowing. This placement is accomplished largely by the tongue, a sensitive, highly innervated, well-coordinated mass of muscle that sits in the middle of the mouth. In addition to moving food about in the mouth, the tongue performs sensory and secretory functions: it is equipped with chemosensitive taste buds that test food quality, mechanoreceptors that monitor texture, and salivary glands that lubricate its epithelial surface. In humans, the tongue is capable of extremely fine movements that permit speech.
Figure A is a light micrograph of a vertical section taken through the posterior portion of the tongue of the squirrel monkey. Here, the surface of the tongue is covered by a stratified Squamous Epithelium (EP). The superficial cells, although filled with Keratin, still contain remnants of nuclei. Consequently, the epithelium is classified as nonkeratinized (some texts refer to it as Parakeratinized). The epithelium is supported by an extensive Lamina Propria (LP). The Lamina Propriabinds the epithelium to the underlying muscle mass (not evident in Figure A) that comprises the bulk of the tongue. The Lamina Propria is highly vascular, containing both blood capillaries (C) and lymphatic capillaries (L). The tongue is extensively innervated, and nerves (N) are clearly visible in the Lamina Propria. In the deeper layers of the Lamina Propria, salivary glands called lingual glands (LG) are evident. The secretory portions of the glands, filled with dark secretory vesicles, empty into ducts (D) that carry the saliva to the surface of the tongue.
Figure B is an electron micrograph of a serial section taken through the stratified Squamous Epithelium depicted in Figure A. The surface of the epithelium bears a series of ridges (arrow) that represent the bases of conical projections called Filiform Papillae. The filiform papillae, which do not contain taste buds, give a rough texture to the surface of the tongue that provides traction against material to be moved in the mouth. The filiform. papillae are supported by projections of the lamina propria called Secondary Papillae (SP).
Figure C is an electron micrograph of a serial section taken through a deeper portion of the Lamina Propria shown in Figure A. Here, a relatively large nerve (N) and an accompanying small branch (N’) are viewed in cross section. Both myelinated and unmyelinated axons are present in these mixed nerves, and a capillary (C) runs through the center of the larger branch. The nerve is surrounded by secretory acini of lingual glands (LG) Several fat cells (A), or adipocytes, are present. Some of these structures are difficult to interpret by light microscopy alone. Fortunately, they are very easy to identify by electron microscopy. Try to find many of the structures shown in the light micrograph in Figure A in the electron micrographs in Figures B and C; this exercise will help you to develop the ability to instantly recognize the components of the tongue when you examine them in the light microscope.
Plate 12-2, Figure A. Light micrograph of vertical section through the tongue of the squirrel monkey. A, fat cell (Adipocyte); C, capillary; D, duct of lingual gland; EP, epithelium; L, lymphatic capillary; LG, lingual gland; LP, Lamina Propria; N, nerve; SP, secondary papilla of Lamina Propria; arrow, base of filiform papilla. 340 X
Figure B. Matched electron micrograph of serial section through epithelium shown in Figure A. C, capillary; EP, epithelium; L, lymphatic capillary; LP, Lamina Propria; SP, secondary papilla of Lamina Propria; arrow, base of filiform papilla. 400 X
Figure C. Matched electron micrograph of serial section through Lamina Propria shown in Figure A. A, fat cell (Adipocyte); C, capillary; LG, lingual gland; M, striated muscle Fiber; N, nerve; N’, small branch of nerve. 775 X
The human tongue has on its surface three types of projections, or papillae: Filiform Papillae, fungiform papillae, and circumvallate papillae. Of these, only the fungiform and circumvallate papillae are equipped with the chemoreceptive structures known as taste buds.
The base of the tongue contains a number of large, manlike projections, the circurnvallate papillae. Arranged in the shape of a V, these papillae, some 12 in number, provide a structural dividing line between the anterior and the posterior regions of the tongue. Each Circumvallate Papilla is a cylindric projection that is surrounded by a valley called the circular furrow. The lateral side of each papilla facing the furrow contains a large number of taste buds. These structures are evident in the micrographs at right.
Figures A and B are a matched pair of light and electron micrographs of serial sections taken through one side of a Circumvallate Papilla on the tongue of the squirrel monkey. Here, the upper surface of the papilla facing the oral cavity (OC) is at the top of the micrograph (Figure A); the circular furrow (F) that surrounds the papilla extends downward. The Circumvallate Papilla, like the rest of the tongue, is lined by a nonkeratinized stratified Squamous Epithelium (EP). The epithelium lies atop a Lamina Propria (LP) that sends a number of small projections, the Secondary Papillae (SP), up into the epithelium. The vast majority of the taste buds (T) of a Circumvallate Papilla are found on the lateral margins that line the circular furrow. The circular furrow provides a space in which saliva, bearing tastant molecules, can flow. Lingual glands, such as those described in Plate 12-2, open into the circular furrow at the base of the circurnvallate papilla.
Each Taste Bud is a small, teardrop-shaped cluster of pale-staining cells that opens to the surface by means of a Taste Pore (arrow). An individual Taste Bud (T) is shown at higher magnification in the electron micrograph in Figure C. This Taste Bud is the same one indicated by the arrow in Figure B. (Its orientation, however, appears different, since the photograph has been rotated 90 ? ? ? so that the Taste Pore faces upward). The Taste Pore (arrow) is evident, as are the large, clear cells of the Taste Bud itself. Most of the cells in the taste bud are long, thin cells with round, basally located nuclei. The apical ends of the cells have Microvilli that extend into a region called the taste pit that lies just beneath the Taste Pore. It is here that tastant molecules are thought to interact with taste cells in the process of chemoreception. The cells within the Taste Bud will be described in more detail in Chapter 20.
Plate12-3, Figures A and B. Matched set of light and electron micrographs of serial vertical sections taken through a Circumvallate Papilla on the tongue of the squirrel monkey. EP, epithelium; F, circular furrow surrounding Circumvallate Papilla; LP, Lamina Propria; OC, oral cavity (Figure A only); SP, secondary papilla of Lamina Propria; T, Taste Bud, arrow, Taste Pore. Figure A, 280 X; Figure B, 500 X Figure C. Electron micrograph of Taste Bud indicated by arrow in Figure B. EP, epithelium; F, circular furrow surrounding Circumvallate Papilla; T, Taste Bud; arrow, Taste Pore. 3,600 X
When food enters the mouth, digestion begins. Digestion, the breakdown of large food particles into smaller ones, is facilitated in the oral cavity by the secretions of many different salivary glands. Taken together, the secretions are called saliva. Although saliva is often thought to be a simple substance, it is really quite complex, containing such chemical constituents as digestive enzymes (Amylase), proteins, carbohydrates, mucoproteins, salts, and ions. Although there are thousands of ingredients in saliva, convention divides salivary secretions into two major categories: serous and mucous. Serous secretions, secreted by Serous Cellswithin salivary glands, are watery and nonviscous and tend to be proteinaceous. Mucous secretions, secreted by Mucous Cells within salivary glands, tend to be mucoid, viscous, and rich in polysaccharides and mucoproteins.
Secretory cells within salivary glands are grouped into acini. Each Acinus contains Serous Cells, Mucous Cells, or both serous and Mucous Cells. All of these types of secretory acini are illustrated in Figures A and B at right, a matched pair of light and electron micrographs of serial sections taken through a salivary gland of the squirrel monkey. Serous and mucous acini look quite different from one another. Serous acini (S), for example, consist of dense, darkly stained, radially arranged pyramidal cells. Their large round nuclei are located at the base of the cell; the apical pole of the cell is packed with electron-dense secretory vesicles. Mucous acini, on the other hand, consist of pale, light-staining cells. The apical pole of the cell is filled with clear mucigen droplets; the Cytoplasm and Nucleus are compressed against the bottom of the cell. Other acini, called mixed acini (MS), contain both mucous and Serous Cells.
Whether mucous, serous, or mixed, all secretory acini within the salivary gland pour their secretions into a system of ducts, illustrated in Figure 12-2. The ducts range from very small to quite large. The smallest ducts are right at the neck of the Acinus itself; the largest duct opens into the oral cavity. The smallest duct, the duct into which each Acinus directly empties its contents, is the Intercalated Duct. As seen in Figures A and B at right, the Intercalated Duct (I) is a tiny tube lined by a simple Cuboidal Epithelium. Its diameter is smaller than that of a typical secretory Acinus. (Were an Acinus a grape, the Intercalated Duct would be its stem). The intercalated ducts, in turn, pour their contents into larger ducts, secretory ducts. A small Secretory Duct (SD), caught near its point of confluence with an intercalated duct, is evident in the micrographs at left. The structure of secretory ducts will be illustrated in more detail in Plate 12-5.
Within a salivary gland, the secretory acini are grouped together into distinct lobules. The individual lobules are separated from one another by strong connective tissue septa. Within a given Lobule, the secretory acini and ducts are held together by a network of loose connective tissue that contains nerves, blood vessels, and many elements of the immune system. A careful look at the electron micrograph in Figure B will reveal small nerve branches (N), capillaries (C), and Antibody-secreting Plasma cells (*). These structures are also present in the light micrograph in Figure A.
Plate 12-4, Figures A and B. Matched pair of light and electron micrographs of serial thick and thin sections taken from a salivary gland of the squirrel monkey. A, fat cell (Adipocyte); C, capillary; I, Intercalated Duct; M, mucous Acinus; MS, Acinus with both mucous an
The three largest salivary glands in the oral cavity are the Parotid Gland, located in front of the ear; the submandibular (or submaxillary) gland, beneath the jaw; and the Sublingual Gland, under the tongue. The parotid is mostly serous; the Submandibular Gland is mixed, though it is more serous than mucous; and the Sublingual Gland, also a mixed gland, is far more mucous than serous.
The Sublingual Gland of the squirrel monkey is illustrated in the electron micrographs at right. Figure A depicts a number of typical secretory acini made of Mucous Cells. Each grape-shaped Acinus appears round in cross section and consists of many Mucous Cells arranged around a small central lumen (L). On the very outside of each Acinus sit one or two highly specialized contractile myoepithelial cells (arrow), frequently called basket cells. Each Myoepithelial Cell, shaped rather like an octopus, has its tentacle-like cellular extensions wrapped around the outside of the Acinus. Under appropriate conditions of neuronal stimulation, the myoepithelial cells contract, expressing the salivary secretions into the small Intercalated Duct (1) that leads from each Acinus. The myoepithelial cells, which are quite similar to Smooth Muscle cells, are spread so thinly that they are hard to see in low-magnification photomicrographs. In Figure A, they look like thin black lines and are most obvious around the perimeter of the Intercalated Duct (arrows, I).
The intercalated ducts lead to secretory ducts (SD), shown in Figure B. The histologic hallmark of the Secretory Duct is its very high density of Mitochondria(Mi), which are for the most part aligned parallel to the long axis of the columnar cells that make up the duct’s wall. When viewed at high magnification with the light microscope, these parallel, vertically oriented Mitochondria appear as stripes, or striations. For this reason, secretory ducts are often called striated ducts. The Mitochondria are intimately associated with the extensive basal infoldings of the Plasma membrane of the cells of the Secretory Duct. Most cells with this configuration are active ion pumps – cells that pump ions against a concentration gradient, a process known as Active Transport. The cells of the Secretory Ductmodify the saliva considerably, adding ions and water to the material produced by the acini. The water and ions are obtained largely from the blood circulating through the capillaries (C) that travel along the length of the duct.
In the image in Figure B, several unusual-looking secretory acini lie next to the Secretory Duct. These mixed acini contain both mucous (M) and serous (S) cells. The Mucous Cells are clustered about the center of the Acinus. The Serous Cellsare arranged in a crescent around the Mucous Cells and are frequently called serous demilunes because of their half-moon shape. The serous cells look quite different from the purely serous acini, such as the Acinus depicted in Figure C. Here, each of the Serous Cells is shaped like a pyramid. Its broad base contains the Nucleus (Nu), many stacked Cisternae of the rough Endoplasmic Reticulum(RER), and a well-developed Golgi Complex (G). The secretory vesicles (V) produced by the biosynthetic apparatus are clustered together in the narrow apical pole of the cell, ready to be released into the tiny lumen (L). Small capillaries (C) and nerves (N) pass close by. When viewed with the electron microscope, the secretory cells of serous acini clearly contain all the organelles necessary to produce large quantities of protein for export. They look quite similar to the secretory cells of the exocrine pancreas.
Plate 12-5, Figure A. Electron micrograph of mucous acini of the Sublingual Gland of the squirrel monkey. C, capillary; I, Intercalated Duct; L, lumen of Acinus; arrows, Myoepithelial Cell. 1,850 X
Figure C. Electron micrograph of serous Acinus from squirrel monkey Sublingual Gland. C, capillary; G, Golgi Complex; L, lumen; N, nerve; Nu, Nucleus; RER, rough Endoplasmic Reticulum; V, secretory vesicles. 5,200 X