With this understanding of the basic structural features of the GI tract, we will now take a more focused look at different portions of the tract. Be sure you understand the function of each portion of the tract before you study the structure in detail in the Laboratory Slides. Food enters the digestive tract in the oral cavity, where it is masticated into particles on which digestive enzymes can act more efficiently. In the mouth, food particles are mixed with saliva, which lubricates them and initiates their digestion.
The salivary glands will be discussed in detail in the next laboratory. The tongue is a muscular organ covered by oral mucosa that manipulates the food and contains the sensory organs for taste. The taste buds will be discussed in detail in the Laboratory on Sensory Systems. The esophagus is a muscular tube that transports food from the pharynx to the stomach.
It is lined by a stratified squamous epithelium and has a prominent muscularis mucosa and thick muscularis externa. The muscularis externa of the esophagus is unique in that it transitions from striated to smooth muscle over the length of the tube.
The esophagus ends in the gastro-esophageal junction. The stomach is the site where food is mixed with gastric juice and reduced to a fluid mass called chyme. The layers of the stomach wall follow the basic plan described above.
The gastric glands are the basic structure of the stomach wall and can be thought of as tiny pits, or indentations, lined by epithelial cells. Gastric glands are structured as a gastric pit that opens into the lumen, followed by an isthmus, neck, and base.
There are several types of cells that are important in producing stomach secretions:. The three different regions of the stomach are distinguished on the basis of the histological characteristics of the mucosal glands:. The muscularis externa of the stomach is notable because it contains an additional muscular layer. It is structured with inner oblique, middle circular, and outer longitudinal layers.
This section happened to be cut such that a piece of one of these longitudinal bands may be seen. Look at these slides especially slide at low magnification first to locate the recto-anal junction. Here you will observe a narrow zone of transition from the simple columnar epithelium of the intestine to the keratinized stratified squamous epithelium of skin.
Within the transition zone, you may find stratified columnar or sometimes cuboidal epithelium followed by nonkeratinized stratified squamous epithelium.
While looking, move the image from the colon toward the direction of the recto-anal junction. Observe that the crypts become shorter and shorter, eventually disappearing near the junction.
Also observe that the muscularis mucosae becomes tattered and disappears, allowing the lamina propria merge with the underlying submucosa in this area.
Note the presence of a large number of submucosal veins View Image. When these veins become dilated and varicose, they cause the mucosa to bulge and create the condition commonly known as hemorrhoids. Examine the skin lining the anal region and observe sebaceous and sweat glands, hair follicles, etc.
Also, note the massive amount of smooth and skeletal muscle that form the internal View Image and external anal sphincters View Image , respectively. The primate specimen slide shows these muscles quite well. The micrograph will help you understand the pattern, which arises from the inner circular layer and outer longitudinal layer of smooth muscle cells.
Without the knowledge in which direction the intestinal epithelium is located, it is not possible to discriminate between the two sublayers of the muscularis externa. Note that cells are sloughing off at the tip of the villus. Find some goblet cells, which represent one resident cell type of the intestinal epithelium.
The small lymphocyte is transient and is not a permanent component of the epithelium. Study the composition of the connective tissue core of the villus. It is only one cell layer thick and columnar, as the cells are rather tall. Note the basal lamina at the base of the epithelium.
In some places you can see the apical area where junctions are located. Most of these cells also have short apical microvilli.
Review the reasons for the high rate of cell mitosis in the upper part of the intestinal gland. If you look closely at the labeled goblet cell, you can see that the apex is packed with mucus-containing secretory vesicles. The brush border is PAS positive both because of the glycocalyx that it contains and the pancreatic enzymes that stick to the glycocalyx.
In the GI tract, the presence and appearance of glands can be very telling. Lets begin with the pharynx. The pharynx has no muscularis mucosa or submucosa and its glands can be found imbedded in layers of muscle beneath the epithelium. Observe the crypts of Lieberkuhn. Paneth cells are found at the base of the crypt and are identified by their eosinophilic granules. Examine the villi on this slide with the core of lamina propria containing a lacteal.
Observe the goblet cells and notice the greater number as compared to the duodenum. Study the muscularis externa and serosa. Slide 53 Ileum. The distinguishing feature of the ileum is the presence of large groups of lymphatic nodules in the lamina propria called Peyer's patches. The plicae circulares valves of Kerckring in this slide are not as prominent as those found in the duodenum and jejunum.
Observe the villi and compare to the duodenum and jejunum. Here the villi are much more leaf-like in appearance when compared to the finger-like villi of the duodenum and jejunum. Note the muscularis mucosa and the absence of glands in the submucosa. Examine the muscularis externa and recall the inner circular and outer longitudinal organization. Study the crypts of Lieberkuhn and observe paneth cells. Observe the density of goblet cells covering the villi and lining the crypts of Lieberkuhn.
Goblet cells are most numerous in the terminal ileum. Finally, observe the serosa. These crypts are short glands. The lamina propria which underlies the epithelium has a rich vascular and lymphatic network, which absorbs the digestive products, and there is a muscularis mucosae layer immediately at the base of the crypts. The lymphatic capillaries are called lacteals, and absorb lipids. The vascular capillaries are fenestrated to aid absorption. The muscularis externa layer contains two layers of smooth muscle, an inner circular and outer longitudinal, for continuous peristaltic activity of the small intestine.
There are around or so lymphoid aggregations called Peyer's patches in the mucosa. Look at this photograph of a section through the small intestine, and make sure you can identify the mucosa, submucosa, muscularis mucosae, muscularis externa , and villi. Sometimes the villi are cut in cross-section, and sometimes longitudinally - and you can see this mixture of sections here.
Lymphoid aggregations are commonly found in the sub-mucosa of the small intestine, an you can see one here. The larger aggregations of lymphoid tissue are known as Peyer's Patches. On this magnified image of the mucosa of the small intestine, can you identify: villi, crypts of Lieberkuhn L , muscularis mucosa, lamina propria and lymphoid aggregations?
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