The epithelial lining of the oral cavity is of the stratified squamous type. In contrast to the skin it is nonkeratinized. The major salivary glands arise as invaginations of the oral epithelium during the second month of embryonic development, and they are involved with the secretion of the watery, mucus, and enzymatic content of saliva. A discussion of these glands will be postponed for now.
TONGUE
The tongue is easily recognized because of its interlacing bundles of skeletal muscle that are disposed in three planes, and by its covering of stratified squamous epithelium that is elevated on the dorsal surface of the tongue, into papillae. There are three types of papillae in man: filiform, fungiform and cicumvallate.
#116 Tongue, human, circumvallate papillae, (H & E)
The lingual papillae are vertical elevations or projections of the epithelial surface, and they contain a core of connective tissue derived from the lamina propria. In this section there are a number of filiform papillae and a single circumvallate papilla. Note the numerous taste buds on the lateral walls of the circumvallate papilla. Some of these
have been sectioned through the taste pore. There are three types of cell within the taste bud: sensory, supporting and basal, but you should not attempt to distinguish them. Find a taste bud in which the taste pore is the plane of the section. There are massive serous glands (glands of von Ebner, a.k.a. lingual salivary glands). In some sections the ducts of these glands may be seen to drain into the furrow of the circumvallate papilla. Note the bands skeletal muscle (artifactually separated) and identify blood vessels and nerves. There is considerable lymphatic invasion, particularly around the secretory portions of the serous glands.
#87 Tongue, Circumvallate Papillae. Mallory-Azan
Be sure that you examine a slide that has been stained with the trichrome stain, Mallory- Azan, so that you can compare the differences in the staining properties with H&E. Both slender filiform papillae and large circumvallate papillae, encircled by a deep furrow or trench, occur on your slide. Serous glands (of von Ebner) open into the base of the furrow or the circumvallate papillae. Taste buds are scattered along the lateral walls of the circumvallate papillae; these appear as pale-staining oval bodies within the darker-staining epithelium. Identify bands of skeletal muscle.
ESOPHAGUS
As in both the oral cavity and in the pharynx, the mucosal surface of the esophagus is lined by stratified squamous epithelium that is nonkeratinized in man. In herbivores, the esophagus has a keratinized epithelium. The well-developed muscularis externa and the stratified squamous epithelial lining are well adapted for the rapid transport of food from the pharynx to the stomach. Diagnostic features of the esophagus are the combination of stratified squamous surface epithelium, the considerable thickness of the muscularis mucosae (up to 0.2 - 0.4 mm thick). In the upper third the muscularis externa contains largely skeletal muscle, in the middle third, a mixture of skeletal and smooth, and in the lower third, only smooth muscle.
#32 Esophagus, middle third, Human, (H&E)
The esophagus and trachea adhere to each other in this slide. Examine the wall of the esophagus starting with the stratified squamous nonkeratinized epithelium. Underlying the epithelium is a layer of loose connective tissue and the muscularis mucosae. Note the intermingling of both skeletal and smooth muscle in the muscularis externa. This is a good time to review the diagnostic features of smooth and skeletal muscle, and to compare them with the adjacent connective tissue of both the lamina propria and the submucosa. Also, identify the various types of vessels within the submucosa, as well as other layers.
#33 Junction of Esophagus and Stomach, Human (H&E)
Locate the abrupt transition between the stratified squamous epithelium of the esophagus and the simple columnar epithelium of the stomach. Compare the muscle of the muscularis externa of the esophagus with that of the previous esophageal slides. How can you diagnose whether you are looking at the upper or lower portion of the esophagus? Note both the diffuse infiltration of lymphocytes and the scattered lymph nodules within the lamina propria of the stomach. A more detailed study of the various cell types of the gastric epithelium is considered on the next slide.
The stomach extends from the esophagus to the duodenum; it is divisible into the cardiac, fundus, (body) and pyloric regions. The distensible stomach is involved in both the mechanical and chemical breakdown of food, and also serves as a temporary reservoir. Its epithelium is specialized for secretion and is of the simple columnar type. The gastric mucosa contains gastric pits (foveolae); these are surface invaginations that also serve as the ducts of the underlying intrinsic gastric glands. Three basic cell types contribute to the secretion of gastric juice, and each has a characteristic appearance under the light and electron microscope. All of these cell types can be seen in the fundus and body of the stomach.
1. Mucus-secreting cells. These cells form the surface epithelium and extend inward to line the gastric pits. Nuclei are basal, and the supranuclear cytoplasm containing mucinogen granules appears clear or vacuolated with H & E stain. Mucous neck cells occur in the junctional region of the gastric pits and glands, and it is in this region that cell proliferation for the renewal of the epithelium occurs.
2. Parietal cells. These pyramidal or spherical cells appear wedged in between other cells of the gastric glands. They are characterized by their finely granular acidophilic cytoplasm due to an abundance of mitochondria, and by their central, spherical nucleus. They contain an extensive intracellular canalicular system that communicates with the lumen of the gland (visible at the EM level). These cells are involved in the elaboration of hydrochloric acid and intrinsic factor.
3. Chief (zymogen) cells. As their name implies, these cells are involved in the secretion of enzymes, particularly the proteolytic enzyme pepsinogen (pepsin in the active state). As is characteristic of cells involved in protein synthesis and secretion, these cells contain basophilic cytoplasm, particularly at their base due to the extensive development of rough endoplasmic reticulum. Although at the LM level the supranuclear cytoplasm generally appears clear or vacuolated, electron micrographs reveal an accumulation of membrane- bound secretory granules in this region.
#34 Stomach, Body, (H&E)
Identify the mucosa, submucosa, and muscularis externa. Locate the following elements of the mucosa: the luminal surface mucous secreting cells, the gastric pits and the cells lining them. Parietal cells are particularly prominent and chief cells and mucous neck cells are present. Note the loose, cellular areolar connective tissue surrounding the gastric pits, the muscularis mucosae, which forms a boundary between the mucosa and submucosa, and the blood vessels in the submucosa. You may be able to find nerve bundles and the ganglion cells of the submucosal plexus (Meissner's plexus) in some slides, but they are not obvious in all of them. Examine the muscularis externa and notice that the smooth muscle is oriented in several different planes. A serosa covers the external surface of the gland in this section. You may be able to find the myenteric plexus (Auerbach's plexus) between the external and adjacent inner layers of smooth muscle.