Upper Gastrointestinal Tract

Salivary glands

The three major salivary glands are the parotid, submaxillary (submandibular) and sublingual. The major salivary glands secrete via a duct system composed of three serially arranged components: intercalated ducts, striated (intralobular) ducts, and interlobular (interlobular) ducts. In addition, there are numerous serous and mucous minor salivary glands located in the oral mucosa and submucosa of the oral cavity (buccal, labial, hard palate, soft palate) and the tongue. You will be referred to these minor salivary glands when you are directed to examine the specimen of the lip, tongue, and hard palate.

Parotid gland
This specimen represents a typical adult parotid gland. The parotid is a purely serous gland. Note the serous acini and their secretory zymogen granules in the apical cytoplasm. Locate the numerous intercalated ducts which lead from the acini and are composed of low cuboidal epithelium. Also find the striated ducts composed of cuboidal to columnar epithelial cells. Finally, observe the interlobular ducts (excretory) that run in the connective tissue septa of the gland and are composed of cuboidal to pseudostratified epithelium. Note the presence of adipose tissue (white fat cells) in the parotid gland.

Submaxillary gland, Verhoeff's Hematoxylin, mucicarmine & methyl green.
This gland is a compound tubuloacinar gland containing both mucous and serous elements (mostly serous). In this specimen, the mucous acini are stained faintly pink and covered with serous demilunes. Locate intercalated ducts, striated ducts, and interlobular ducts (excretory). Elastic fibers are demonstrated in this specimen as black fibers.

Submaxillary gland
Examine the important features of the submaxillary gland: mucous acini and serous acini, intercalated ducts, striated ducts, and interlobular ducts.

Sublingual gland, Verhoeff's Hematoxylin, mucicarmine & methyl green.
The sublingual gland is a mixed serous and mucous gland composed of mostly mucous acini. Serous demilunes are also present on the mucous acini. Here the mucous acini are stained acidophilic and the few serous acini are stained green (mostly present as serous demilunes). The ducts are intensely stained with the methyl green. Note the presence of plasma cells and lymphocytes in the connective tissue interstitium of the gland.

Sublingual gland, Verhoeff's Hematoxylin & Eosin.
Locate the mucous acini and serous acini of the sublingual gland. Also locate intercalated ducts, striated ducts, and interlobular ducts.

Oral Cavity

Lip
Note the skin of the face, the red margin of the lip, and the oral mucosa surfaces of this specimen. The skin of the face is a keratinized stratified squamous epithelium with hair follicles and sebaceous glands, while red margin of the lip lacks hair follicles or glandular tissue. The oral mucosa is nonkeratinized stratified squamous epithelium (sometimes referred to in the oral cavity as parakeratinized epithelium which lacks a stratum granulosum) with the labial minor salivary glands present beneath the epithelium. Observe the acini of these seromucous glands. Note the obvious presence of deep connective tissue papilla into the epithelium of the red margin, compared to the relative lack of dermal papilla in the skin. The papilla account for the red color of the lip (the thinness of the epithelium allows the color of the blood to show through).

Lip
This is another version of the previous specimen. Note the transition between the red margin and oral mucosa. At the point of transition, the stratum granulosum in the red margin disappears and nuclei of living cells appear at the epithelial surface in the oral mucosa.

Tongue
This specimen is representative of the posterior region of the tongue. The epithelium of the dorsal tongue is a stratified squamous keratinized epithelium. Note the cornified layer of cells may have been lost in preparation of this specimen. Most of the dorsal tongue is covered with papillae, of which there are four principal kinds: Filiform, Fungiform, Circumvallate, and Foliate papillae.
The posterior region is where the circumvallate papillae are located and these are present in this specimen. Observe the minor salivary glands of the lamina propria and submucosa (continuous in the tongue). Von Ebner's glands are specialized minor serous salivary glands that drain into the trench around the circumvallate papillae. Taste buds are embedded within the epithelium of fungiform, foliate, and circumvallate papillae. In this specimen, look for taste buds on the lateral walls of the circumvallate papillae. Note the skeletal muscle and its relationship to the glands.

Tongue, monkey
This specimen demonstrates another example of circumvallate papillae. Observe the presence of taste buds in the lateral walls of the papillae. Note that the circumvallate are considerably larger that the neighboring filiform papillae.

Tongue
This specimen demonstrates filiform papillae and skeletal muscle deep to the lamina propria.

Tongue
This specimen demonstrates another example of filiform papillae.

Tongue, mammal
This specimen demonstrates foliate papillae. Identify the taste buds within the epithelium of these papillae.

Tongue, human
This specimen demonstrates fungiform papillae. Fungiform papillae are slightly rounded (mushroom-shaped), elevated structures situated among filiform papillae. The connective tissue core projects into the base of the surface epithelium. Taste buds are present on the dorsal surface of the epithelium of these papillae, but are not observed in this specimen.

Tooth
Identify the major components of the tooth; the enamel, which covers the surface of the tooth, is not present in this specimen, due to extraction of this highly mineralized tissue during tissue preparation, Note the area between the dentin and gingiva where the enamel had existed. Identify the dentin, which is a highly mineralized structure, and is located between the enamel and cementum. Observe the dentinal tubules, which are long, narrow parallel channels in the dentin formed by retreating odontoblasts, which line the pulp cavity and produce the dentin. Identify the pulp chamber (or cavity) and the pulp canal, which are filled with loose connective tissue supply innervation and vasculature to the cells of the tooth.
Identify the cementum, which is located on the outer surface of the root of the tooth. Observe that the cementum is composed of an outer cellular cementum composed of cementoblasts which resemble osteoblasts and produce the avascular noncellular cementum. Identify the periodontal ligament, which anchors the tooth into the alveolar bone of the jaw. The periodontal ligament is comprised of both loose dense connective tissue, and is located adjacent to the cellular cementum. Observe the fine Sharpey’s fibers of collagen that extend from the periodontal ligament into the cementum. Identify the gingiva, which is lined with stratified squamous keratinized epithelium that is tightly attached to alveolar bone and to the enamel at the base of the crown of the tooth.

Upper Segment GI Tract Proper: Esophagus & Stomach

In studying the tubular organs of the GI tract, remember the general organization of the wall of these organs consists of four layers: the mucosa (with epithelium, lamina propria, and muscularis mucosa), the submucosa, the muscularis externa, and the adventitia (or serosa). To help identify these organs, pay attention to these key features on each specimen: (1) the type of epithelium present, (2) the presence of glands, what type and their location, i.e. whether they are present in the mucosa or submucosa, (3) the composition of the connective tissue elements, and (4) the orientation and type of muscle present and its thickness in the muscularis externa. Distinguishing between the different regions of the upper and lower G. I. tract depends on your knowledge and observation of the differences that exist between segments in the four above mentioned features.

Esophagus
Rely on the type of muscle in the muscularis externa to determine what region (upper, middle, lower) of the esophagus is represented on this specimen (i.e.; there is no skeletal muscle present in this specimen). Identify the mucosa (epithelium, lamina propria, and muscularis mucosae), submucosa, esophageal glands, muscularis externa and adventitia. Observe that the smooth muscle of the muscularis mucosa in longitudinally-oriented. Note the presence of small arteries and veins in the adventitia on this specimen. The larger of the veins are known as esophageal veins which may enlarge in the case of cirrhosis of the liver to provide an alternate pathway for blood to be transported from the intestines to the vena cava. Observe the presence of the myenteric (Auerbach's) plexus in between the inner circular layer and outer longitudinal layer of the muscularis externa.

Esophagus, mammal
In this specimen of esophagus, observe that the muscularis externa is composed completely of skeletal muscle rather than smooth muscle in both the inner circular layer and outer longitudinal layer, indicating that it is from the upper third of the esophagus. Identify the mucosa (epithelium, lamina propria, and muscularis mucosae), submucosa, and esophageal glands.

Esophageal-stomach junction
The junction of the esophagus with the stomach is an abrupt transition from nonkeratinized stratified squamous epithelium to simple columnar epithelium. Find the junction and observe the mucosa on either side. The cardiac portion of the stomach on this specimen is a good representation of the cardiac glands of the stomach. Remember the cardiac glands secrete mucous and are irregular tubes seen in cross section below the gastric pits.

Fundic stomach
The most characteristic feature of the fundic region of the stomach is the presence of fundic glands which contain parietal cells and chief cells, as well as mucous neck cells. Examine the fundic glands and observe the surface epithelium, gastric pits, neck region, and base region of the glands. Recall that the same types of glands are also found in the body region of the stomach. Fundic glands are straight long, tubular glands with shallow pits. The greatest density of chief cells is in the basal region of each gland. Thus, the lower base region appears more basophilic than the neck and upper base region. The parietal cells are slightly eosinophilic. Observe the mucosa, muscularis mucosae, submucosa, muscularis externa, and serosa. Also study the submucosal plexus and myenteric plexus of nerves and ganglia.

Fundic Stomach, PAS and Hematoxylin.
This section demonstrates the mucopolysaccharide laden surface lining of epithelial cells of the stomach and the mucous neck cells in the fundic glands.

Pyloric stomach
Pyloric stomach can be identified by its deep gastric pits and coiled pyloric glands which are composed of predominantly mucous acinar cells. The lamina propria is heavily infiltrated with immune cells such as lymphocytes and plasma cells in the diffuse lymphatic tissue. Examine the mucosa, submucosa, muscularis mucosae, and muscularis externa. You can observe in this section that the most dominant layer of the muscularis externa is the circular layer which forms the pyloric sphincter. Observe the presence of the myenteric (Auerbach's) plexus in between the inner circular layer and outer longitudinal layer of the muscularis externa. Locate the submucosal (Meissner’s) plexus of nerves and ganglia.

Pyloric stomach, mammal
This specimen is better preserved that the previous pyloric stomach specimen. Identified the deep gastric pits and coiled mucous pyloric glands. Examine the mucosa, submucosa, and muscularis mucosae. Note the presence of an inner circular layer of smooth muscle and an outer longitudinal layer of smooth muscle in the muscularis mucosae. Identify the submucosal (Meissner’s) plexi in the submucosa. Compare the thick inner circular layer of the muscularis externa to the thinner outer longitudinal layer. Identify myenteric (Auerbach’s) plexi between the layers of the muscularis externa.