The Small Intestine
The small intestine is the largest component of the digestive tract and the major site of digestion and absorption. In addition to receiving chyme from the stomach, the initial segment of the small intestine, the duodenum, receives bile from the gall bladder and digestive enzymes from the pancreas. The pancreatic enzymes are produced in an inactive form and only become active in the lumen of the duodenum.
The small intestine is divided into three parts, the duodenum (25 cm), the jejunum (2.5 m) and the ileum (3.5 m).
The mucosa of the small intestine is highly modified. The luminal surface is completely covered by a number of finger- or leaf-like projections called villi, 0.5-1.5 mm in length. The core of a villus is an extension of the lamina propria, and its surface is covered by a simple columnar epithelium. Opening onto the luminal surface at the bases of the villi are simple tubular structures called intestinal glands or crypts of Lieberkuhn. The crypts extend downward toward the muscularis mucosae. The simple columnar epithelium lining them is continuous with that covering the villi.
The predominant cell type of the epithelium is the enterocyte or absorptive cell. Each enterocyte has about 3000 microvilli at its luminal surface, which appear in the light microscope as the fuzzy striated border on the surface of the villi. [Electron microscopy: Microvilli are cylindrical protrusions, about 1 micrometer tall, of the cell membrane enclosing a core of filaments, mostly actin filaments. The actin filaments attach to the plasma membrane at the tip of the microvillus and end in the terminal web near the base of the microvillus. The terminal web consists of actin microfilaments and myosin, and is attached to the zonula adherens of the junctional complex binding epithelial cells to one another near their apical ends.]
The villi and microvilli, together with folds in the submucosa called plicae circulares (below), increase the absorptive surface of the small intestine about 600 times.
The epithelium of the small intestine consists of the following cell types:
- Enterocytesor absorptive cells. These are tall columnar cells with microvilli and a basal nucleus, specialized for the transport of substances. They are bound to one another and other cell types by junctional complexes (zonula occludens or tight junction, zonula adherens, and macula adherens). Amino acids and monosaccharides are absorbed by active transport, monoglycerides and fatty acids cross the microvilli membranes passively. Absorbed substances enter either the fenestrated capillaries in the lamina propria just below the epithelium, or the lymphatic lacteal (most lipids and lipoprotein particles). Enterocytes have a lifespan of about 5-6 days.
- Gobletcells. These mucus-secreting cells are the second most abundant epithelial cell. They are found interspersed among the other cell types. Their mucous is a very large glycoprotein that accumulates at the apical end of the cell, rendering it wide. The slender base of the cell hold the nucleus and organelles. Goblet cells usually appear pale or empty due to the loss of their contents upon preparation. However their glycoprotein content can be revealed with special stains (such as the PAS stain in slide #9). The abundance of goblet cells increases from the duodenum to the terminal ileum. Their lifespan is also 5-6 days.
- Paneth cells. Paneth cells are found only in the bases of the crypts of Lieberkuhn. These cells have an oval basal nucleus and large, refractile acidophilic granules at their apical end. The granules contain the antibacterial enzyme lysozyme, other glycoproteins, an arginine-rich protein and zinc, an essential trace metal for a number of enzymes. Paneth cells also phagocytize some bacteria and protozoa. They may have a role in regulating intestinal flora. They have a lifespan of about four weeks. Paneth cells are easy to identify with the light microscope.
- Enteroendocrine cells. These cells were described in the section on the stomach. In the intestine, they are most often found in the lower part of the crypts but can occur at all levels of the epithelium. Their most abundant products here are cholecystokinin or CCK, which stimulates pancreatic enzyme secretion and gall bladder contraction, secretin, which stimulates pancreatic and biliary bicarbonate secretion, and gastric inhibitory peptide or GIP, which inhibits gastric acid secretion. As in the stomach, these cells are not easily seen without special preparations.
- Mor microfold cells. These cells are epithelial cells that overlie Peyers patches and other large lymphatic aggregations. They are relatively flat and their surface is thrown into folds, rather than microvilli. They endocytose antigens and transport them to the underlying lymphoid cells where immune responses to foreign antigens can be initiated. We do not identify M cells in the lab.
- Undifferentiated cells. These stem cells are found only at the base of the crypts and give rise to all the other cell types. A cell destined to be a goblet cell or enterocyte undergoes about 2 additional divisions after leaving the pool of stem cells, and migrates from the crypt to the villus. It will be shed at the tip of the villus.
Special stains for glycoproteins reveal the glycocalyx at the surface of the intestinal epithelium. The glycocalyx consists of glycoprotein enzymes inserted into the plasma membrane of enterocytes, with their functional groups extending outward. These enzymes are secreted by the enterocytes themselves. They include peptidases and disaccharidases, as well as enterokinase (or enteropeptidase), which converts (inactive) trypsinogen to (active) trypsin. Trypsin, in turn, activates trypsinogen itself as well as other pancratic zymogens.
Lymphocytes are sometimes seen in the intestinal epithelium. They are thought to be sampling antigens in the epithelial intercellular spaces. It is believed that they process the antigens before returning to lymphatic nodules in the lamina propria and undergoing blastic transformation, leading to antibody secretion by the newly differentiated plasma cells.
Within the lamina propria core of each villus is a lymphatic capillary called a lacteal, as well as numerous capillaries. The lacteal is accompanied by smooth muscle fibres arising from the muscularis mucosae. The smooth muscle in the villus allows it to contract intermittantly, expelling the contents of the lacteal into the lymphatic network surrounding the muscularis mucosae. The lamina propria is very cellular, with numerous lymphocytes, plasma cells, macrophages and eosinophils. Lymphatic nodules are quite common and are an important component of GALT. Lymphatic nodules arising in the lamina propria may extend into the submucosa. The muscularis mucosae may be partially or totally disrupted by the nodules. ***The ileum is characterized by having large aggregates of lymph nodules, called Peyers patches, in the submucosa.*** (Unfortunately we have no slides of the ileum, but you should know the term Peyers patches. Look at pictures in an atlas.)
The muscularis mucosae of the small intestine consists of an inner circular and outer longitudinal layer of smooth muscle.
The submucosa consists of dense connective tissue. Adipose cells may be present. Both the duodenum and the jejunum are characterized by modifications of the submucosa. (So is the ileum, although its modification, Peyers patches, arises from the lamina propria.)
***The duodenum is distinguished by the presence of Brunners glands, which occupy most of the submucosa.*** In some areas these glands may penetrate the muscularis mucosae to enter the lamina propria. Brunners glands are branched tubuloalveolar glands that produce a clear, viscous, alkaline (pH 8.1-9.3) fluid, containing neutral and alkaline glycoproteins and bicarbonate ions. (Because of the glycoproteins, Brunners glands also react with the PAS that stains the goblet cells and glycocalyx in slide 9). The secretion of Brunners glands protects the proximal small intestine by neutralizing the acidic chyme from the stomach. It brings the pH of the intestinal contents close to the optimum for the pancreatic digestive enzymes delivered to the stomach.
***The jejunum is characterized by the presence of numerous, large folds in the submucosa, called plicae circulares (aka valves of Kerckring).*** The plicae consist of a core of submucosa and the overlying mucosa. They have a semilunar, circular or spiral form and extend about one-half to two-thirds around the circumference of the lumen. Although they may be present in the duodenum and ileum, they are not as large and are not a significant feature in those regions.
The muscularis externa is as described under General Structure. The two layers (inner circular for mixing and outer longitudinal for peristalsis) are well organized. Features such as Auerbachs plexus tend to be easy to find.
Either a serosa or an adventitia may be present.
Figure 21 shows a low power view of the complete wall of the duodenum. The villi appear packed together and only a few (labelled) can be seen distinctly. The crypts at their bases are not identifiable. The mucosa can be distinguished from the submucosa because of the abundance of Brunners glands in the latter; they have picked up the PAS stain and appear as red circles. The muscularis mucosae is not readily seen at this magnification (and is frequently disrupted by Brunners glands); its approximate course is shown by asterisks. There is a tear at the top of the figure where much of the submucosa has torn away from the muscularis externa, whose circular and longitudinal layers can be distinguished. The boundary is indicated by asterisks. The serosa or adventitia is not identifiable. All pictures of the duodenum shown here are taken from slide 9.
Figure 22 shows a slightly higher power view of the duodenal mucosa and submucosa. The individual villi can be distinguished, and some crypts can be seen at their bases. Goblet cells, staining red, can be seen in the epithelium of both the villi and the crypts. The glycocalyx can be faintly seen as a pink line running along the surface of the epithelium. The lamina propria forming the core of the villi and lying between the crypts contains numerous lymphocytes and other cells. The muscularis mucosae can be distinguished as a pink band.
An abundance of Brunners glands can be seen in the submucosa. They have not broken through to the mucosa in the section shown, and the muscularis mucosae remains undisrupted. Brunners glands open into the intestinal lumen through ducts (not seen here).
Figure 23 shows a high power view of the top part of one villus and part of an adjacent villus. The nuclei of the simple columnar epithelium are lined up in a row at the base of the cells. Goblet cells are interspersed among the enterocytes. At this magnification, the glycocalyx can be readily identified.
Figure 24 shows a crypt opening into the lumen between the bases of two villi. The epithelium lining the inside of the crypt can be seen to be continuous with that lining the outside of the villi. Enterocytes and goblet cells are present in crypts and villi. The lumen can be seen at the top of the crypt, but is obscured at its base. The base of the crypt ends just beyond the field of view at the bottom left. To the right of the crypt, part of another crypt, sectioned obliquely, is seen as the top half of a circular profile.
Many lymphoid cells are seen in the lamina propria. Some blood vessels can also be seen, as can strands of smooth muscle, which arise from the muscularis mucosae and are principally associated with the lacteal (not seen here).
Figure 25 shows the regularly-arranged muscularis externa of the duodenum. The inner circular layer is larger than the outer longitudinal layer. An Auerbachs plexus lies between the two layers (barely identifiable at this magnification). The connective tissue of the adventitia (or serosa) lines the outer surface, and a bit of the submucosa (with Brunners glands) is seen beyond the inner circular layer. The big gap between submucosa and muscularis externa is an artifact (tear).
Figure 26 shows a high power view of an Auerbachs plexus in the duodenum. Two nerve cells bodies can be seen.
Figure 27 is a low power view of a plica circulares in the duodenum. This large fold in the submucosa raises the overlying mucosa with its villi and crypts.
Figure 28 shows a low power view of the jejunum (from slide 39). The mucosa, submucosa and muscularis externa are shown completely, a bit of the serosa or adventitia is shown at the lower right. One of the villi (to the right of the labelled ones) is folded over on itself. Note the absence of Brunner's glands in the submucosa. A few blood vessels can be seen at this magnification. No plicae circulares are included in the field of view. In the muscularis mucosae, the circular and longitudinal layers seem reversed; this is a phenomenon of sectioning.
Figure 29 is at the same magnification as Figure 28, but shows an area of the jejunum in which a plica is present. Only the mucosa, submucosa and a very small part of the inner layer of the muscularis externa are seen. The plica continues to the right beyond the field of view. Compare this plica to the one shown in the duodenum in Figure 27. The plicae in the jejunum tend to be taller and thinner. They are also more frequent. If you scan your own slide 39 you might find branching plicae or plicae arising from other plicae.
Figure 30 shows a medium power view of the mucosa (and a bit of the submucosa) of the jejunum. The muscularis mucosae can be seen as a continuous band. Some of the crypts can be seen to be emptying into the bases of the villi, others, sectioned obliquely, appear as circular profiles. The epithelium appears as a darker red line outlining the villi and continuing into the crypts. The little pale "holes" in the epithelium are goblet cells.
Figure 31 shows a high power view of part of a villus. The epithelium is not very edifying as it is sectioned obliquely and is several layers thick. A few goblet cells can nevertheless be distinguished. However, both the central lacteal (as a cross section) and the smooth muscle strand accompanying it can be seen clearly among the lymphocytes in the lamina propria.
Figure 32 shows a high power view of some crypts in the jejunum. Paneth cells, with refractile granules, are seen at the bases of the crypts (the only place theyre found). The Paneth cells on your slides show up much more clearly than on this computer image. Some goblet cells are also seen in the epithelium. The other cells are enterocytes (absorptive cells). The muscularis mucosae can be readily seen. A bit of the submucosa is present but is bleached out (as a result of trying to get Paneth cells to show up on computer image).
Figure 33 shows an Auerbachs plexus between the two layers of the muscularis externa in the jejunum. This sections happens to have gone through the nucleus and nucleolus of several nerve cell bodies (somata). The nucleus is a paler structure lying in the nerve cell body, the nucleolus appears as a brighter dot. The borders of the nerve cell bodies themselves are a bit hard to see; two of them are outlined with different symbols.
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