Structure of Small Intestine
|MUCOSA AND MUSCULATURE OF DUODENUM|
The freely mobile portion of the small intestine, which is attached to the mesentery, extends from the duodenojejunal flexure to the ileocolic orifice, where the small intestine joins the large intestine. This portion of the small intestine consists of the jejunum and the ileum. They run imperceptibly into each other, the transition being marked by a gradual change in the diameter of the lumen and by several structural alterations.
The walls of the jejunum and ileum are virtually identical in structure but have slight modifications that make them histologically distinct. Like the rest of the gastrointestinal tract, the jejunum and the ileum each has four layers: the mucosa, submucosa, muscularis externa, and serosa. The innermost layer, the mucosa, is thickly plicated by macroscopically visible circular folds (plicae circulares, Kerckring folds). These folds vary in height, projecting into the lumen by 3 to 10 mm. Some of these plicae extend all the way around the internal circumference, some of them extend only halfway or two thirds of the way around the circle, and still others spiral around the circle twice or even more times. The circular folds projecting into the lumen slow down the progression of the luminal contents to a slight degree, but their most important function is to increase the absorptive surface area of the intestinal lumen. The visible increase in surface area created by the circular folds is mirrored on the microscopic level by tiny fingerlike projections, intestinal villi.
In fact, the entire mucosal surface of the small intestine, over and between the circular folds, is covered with intestinal villi, projections that are 0.5 to 1.5 mm long (just barely visible to the naked eye). The mass of these villi (estimated at 4 million altogether in the jejunum and ileum) accounts for the velvety appearance of the mucosa. They are somewhat longer and broader in the jejunum than they are in the ileum. The valleys or indentations between the villi form nonramified pits, each of which harbors tubular structures, the intestinal glands (crypts of Lieberkühn). Along the villi, the entire inner surface of the small intestine is covered by a single layer of epithelial cells, the majority of which are enterocytes, highly prismatic columnar cells with a surface covered by microvilli, microscopic projections from these cells’ apical surfaces. Between these columnar cells are interspersed three other types of cells: goblet cells, Paneth cells, and enteroendocrine cells. The goblet cells secrete an alkaline, mucous fluid that coats the whole mucosa. Most goblet cells are found lining the crypts or along the lower parts of the villi, but a considerable number of them are located near the apex of the villi, where they seem to be squeezed between neighboring enterocytes. The Paneth cells are found almost exclusively near the base of the glands. They are easily identified histologically due to the eosinophilic granules they contain. Paneth cells are primarily involved in moderating the bacterial normal flora of the small intestine. They do so by secreting the antimicrobial enzyme lysozyme, as well as α-defensins. They are able to phagocytose bacteria and other invaders within the intestinal lumen. Lastly, the enteroendocrine cells (argentaffin cells, yellow cells, cells of Schmidt or of Kultschitzky) contain basal-staining granules with a high affinity for silver and chromium. These cells are typically found at the bottom of intestinal glands but can migrate upward. They regulate the activity of the digestive system by releasing hormones such as cholecystokinin (stimulates secretion of the gallbladder and pancreas and inhibits gastric emptying), secretin (stimulates pancreatic secretion and inhibits gastric secretion), motilin (stimulates peristalsis), and gastric inhibitory peptide (stimulates insulin secretion and inhibits gastric secretion) into the bloodstream. Enteroendocrine cells in the small intestine may also secrete somatostatin (inhibits release of gastrin and gastric secretion) and histamine (stimulates gastric secretion from parietal cells) in a paracrine fashion, affecting nearby tissues. Lymphocytes, eosinophils, neutrophils, macrophages, mast cells, and plasma cells are also sometimes seen in the epithelial lining of the small intestine, but these have generally migrated from the underlying layer of the mucosa, the lamina propria.
The lamina propria lies deep to the epithelial surface of the mucosa, but it extends into both the circular folds and intestinal villi, forming the core of each villus. This diffuse reticular connective tissue allows for the easy diffusion of nutrients and gasses to and from the epithelial lining of the intestine. The lamina propria frequently assumes a lymphatic character owing to the large number of lymphocytes that migrate through it. The lamina propria also contains thin fibers of smooth muscle that radiate from the muscularis mucosae and extend upward to the tips of the villi. When these fibers are relaxed, the villi have a smooth surface, whereas they become jagged or indented when the fibers contract. These muscular fibrils are assumed to act as motors that maintain the pumping function of the villi. At the core of each villus is a lymphatic vessel, the central lacteal, which transports fat-soluble substances and lymph to the cisterna chyli and from there to the venous circulation. The muscularis mucosae, which separates the lamina propria from the submucosa, is com- posed of two thin layers of smooth muscle, which keep the movable mucosal layer in place. The outer longitudinal layer is thinner than the inner circular layer, from which the muscular fibers in the core of the villi, mentioned above, emanate.
|SMALL INTESTINE MICROSCOPIC STRUCTURE|
The submucosa is a relatively stout layer that is located deep to the mucosa. It is made up of type I collagen bundles forming dense, irregular connective tissue. By altering the angles of its meshes, this submucosal network is able to adapt itself to changes in the diameter and length of the intestinal lumen. The submucosa contains a rich network of arteries, veins, and lymphatics that supply the submucosa and overlying mucosal structures. It also has a substantial network of viscerosensory and visceromotor axons; preganglionic parasympathetic axons terminating in the submucosa synapse on the submucosal plexus (Meissner plexus), a collection of ganglia (nerve cells) scattered throughout the small and large intestines that contribute to the enteric nervous system. The muscularis externa is a large and powerful two-layered coat of smooth muscle that covers the submucosa. The thick inner circular layer and the thinner outer longitudinal layer are connected by convoluted transitional fascicles in the area where they border on each other. Between the two layers is a network of viscerosensory and visceromotor axons. As in the sub-mucosa, preganglionic parasympathetic axons synapse with the myenteric plexus (Auerbach plexus), which are the ganglia located between the two layers of smooth muscle. The myenteric plexus and submucosal plexuses are major components of the enteric nervous system. The muscularis externa is responsible for creating the powerful movements of peristalsis that move intestinal contents progressively down the lumen, or in retrograde motion during vomiting.
Lining the outside surface of the small intestines is the final layer, the serosa (visceral peritoneum). This layer is composed of mesothelial cells on the surface that are connected to the muscularis externa by a thin layer of loose connective tissue. The mesothelial cells release fluid that lubricates the external surface of the small intestine and helps to prevent irritation and adhesions between the intestines and other peritoneal structures. The serosa covers the entire circumference of the small intestines, except for a narrow strip where the mesentery anchors the intestines to the posterior body wall.
Though very similar in many ways, the jejunum and ileum differ in several respects. The lumen of the ileum is narrower and the diameter of the total wall is thinner than that of the jejunum. The average diameter of the jejunum measures 3 to 3.5 cm and that of the ileum 2.5 cm or less. Due to this difference, the contents of the intestine are more visible through the wall of the ileum than the jejunum. Because of this, in the operative view, the jejunum typically has a whitish-red hue, whereas the ileum takes on a darker appearance. The circular folds within the lumen vary in frequency and height, as do the villi. They decrease in height and number as the small intestine approaches the cecum, and in the distal ileum the folds appear only sporadically.
In the jejunum, lymphatic tissue is encountered only in the form of solitary lymphoid nodules that appear as pinhead-sized elevations on the surface of the mucosa.
They become more numerous and more pronounced as they near the large intestine. However, within the ileum, such nodules are very pronounced, forming aggregated lymphoid nodules (Peyer patches). They are invariably situated opposite the attachment of the mesentery and are generally of an elongated oval or ellipsoid shape, their longest diameter always coinciding with the longitudinal axis of the intestinal lumen. Their average width is 1 to 1.5 cm and they vary in length from 2 cm up to 10 or 12 cm or, occasionally, even more. They differ in number from one individual to another, the average total fluctuating from 20 to 30. Another difference between the jejunum and ileum concerns the fat content of their mesenteries. In the adult, the mesentery of the ileum contains more fatty tissue and appears to be thicker than that of the jejunum. The blood vessels that supply each region also have a different appearance, which is described in Plates 1-1 and 1-2.
The principal task of the gastrointestinal tract is to supply the body with its caloric requirements and metabolic material. The structures of the entire gastrointestinal tract, from the mouth to the large intestine, are optimized to accomplish this task. It is within the small intestines, especially the jejunum and ileum, where the majority of absorption occurs through the long row of epithelial cells, enterocytes, which coat the inner surface of the small intestine. These epithelial cells, together with the villi they cover, should properly be considered the organ of absorption. The surface area available for absorption is maximized in several ways. The circular folds of the small intestine (including the epithelial cells, lamina propria, muscularis mucosae, and submucosal layers) increase the surface area grossly. The villi that project from the lumen and circular folds further increase the surface area available for absorption. Finally, the apical surfaces of the enterocytes themselves have a striated border, which is actually composed of microvilli extending from each enterocyte into the lumen. It has been calculated that each epithelial cell is provided with about 1000 microvilli, which increase the cellular surface approximately 24 times. The microvilli seem to vary only a little in size (average length, 1 micron; width, 0.07 micron) and have a core of actin microfilaments extending down their length to attach to a network of fibrils, the terminal web, at the apical edges of the cells. Contraction or relaxation of this web can widen or narrow the space between adjacent villi.
At some time after the ingestion of a meal containing fat, fine lipid droplets can be observed in the intermicrovillous spaces; slightly later, the droplets appear in the area of the terminal web, where they accumulate in minute vesicles, which owe their existence to a pinocytotic activity, probably of the intermicrovillous plasma membrane. The droplets then can be found in the main body of the epithelial cell, where they coalesce to form larger units in vesicles or cisternae, which are connected with each other by intracellular tubules. The fat drop- lets pass toward the lateral cell surfaces. From the inter- cellular spaces, the droplets traverse the basement membrane and the interstitial spaces of the lamina propria to enter the central lacteals of the villi. The lacteals carry fats and fluid proximally via lymphatic channels that ultimately drain to the cisterna chyli, thoracic duct, and, finally, left subclavian vein. For this reason, fat-soluble substances can bypass the liver, which receives the substances from the lumen that are transported to it via the hepatic portal vein.
|EPITHELIUM OF SMALL INTESTINE|
The nucleus of the enterocytes is typically located in its basal region, near the Golgi apparatus. Mitochondria and other organelles of the cell body show no particular or specific features. To maintain a separation between the lumen of the intestines (which is technically outside the body) and the extracellular space within the body, the apical region of enterocytes and other cells of the intestinal epithelium are bound to each other by junctional complexes in the vicinity of the terminal web. The enterocytes are anchored to the underlying connective tissue of the lamina propria by tight junctions. This allows the enterocytes to be selective about the substances that are released into the lamina propria and, thereafter, the bloodstream.