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Section I • Histology and Cell Biology

Small Intestine

The small intestine is tubular in shape and has a total length of about 21 feet. The effective internal surface area of the small intestine is greatly increased by the plicae circulares, villi and microvilli.

Plicae circulares (circular folds or valves of Kerckring) are foldings of the inner surface that involve both mucosa and sub-mucosa. Plicae circulares increase the surface area by a factor of 3.

Villi arise above the muscularis mucosae and they include the lamina propria and epithelium of the mucosa. Villi increase the surface area by a factor of 10.

Microvilli of the absorptive epithelial cells increase the surface area by a factor of20-30. 'Ihe surface area of microvilli is increased even further by the presence surface membrane glycoproteins, constituting the glycocalyx to which enzymes are bound.

The luminal surface ofthe small intestine is perforated by the openings of numer­ ous tubular invaginations (the crypts of Lieberkiihn) analogous to the glands of the stomach. The crypts penetrate through the lamina propria and reach the muscularis mucosae.

The small intestine completes digestion, absorbs the digested food constituents (amino acids, monosaccharides, fatty acids) and transports it into blood and lym­ phatic vessels. Three subdivisions-duodenum, jejunum, ileum-perform dif­ ferent digestive functions.

For carbohydrates, pancreatic amylase completes the hydrolysis of poly­ saccharides to disaccharides that is initiated by salivary amylase in the mouth.

For proteins, pancreatic enzymes, trypsin, chymotrypsin, elastase and carboxypeptidase mediate further digestion to small peptide fragments.

•For dietaryfat, pancreatic lipase hydrolyzes fat to free fatty acids and monoglycerides. These combine with bile salts to form micelles. The micelles diffuse across the surface membrane of microvilli and then to the smooth endoplasmic reticulw11 (SER) where triglycerides are resynthesized and processed into chylomicrons in the Golgi apparatus.

Chylomicrons are transported out the basal surface into the lamina pro­ pria and diffuse into the lacteals in the center of the villus.

Sugars and amino acids are absorbed through the enterocytes and then transported to the capillary network immediately below the epithelium.

The small intestine participates in the digestion and absorption of nutrients. It has specialized villi on the epithelial surface to aid in this function.

The duodenum is the proximal pyloric end of the small intestine. Distal to the duodenum is the jejunum, and then the ileum.

In the small intestine, the chyme from the stomach is mixed with mucosa! cell secretions, exocrine pancreatic juice, and bile.

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Section I • Histology and Cell Biology

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Figure 1-8-6. Duodenum with villi (curved arrow) and submucosal

Brunner glands (arrow)

Patches of lymphatic tissue are in the lamina propria (arrowheads).

In the jejunum, the digestion process continues via enterocyte-produced enzymes and absorbs food products. The plicae circulares are best developed here. In the ileum, a major site of immune reactivity, the mucosa is more heavily infiltrated with lymphocytes and the accompanying antigen-presenting cells than the duode­ num and jejunum. Numerous primary and secondary lymphatic nodules (Peyer's patches) are always present in the ileum's mucosa, though their location is not

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Chapter 8 • Gastrointestinal System

fixed in time. In the infant, maternal IgGs that are ingested are recognized by the Fe receptors in microvilli and endocytosed to provide passive immunity. In the adult, only trace amounts ofintact proteins are transferred from lumen to lamina propria, but IgAs produced in GALT in the ileum are transported in the opposite direction into the lumen.

Histologically, the duodenum contains submucosal Brunner's glands, and the ileum contains Peyer's patches in the lamina propria. The jejunum can be eas­ ily recognized because it has neither Brunner's glands nor Peyer's patches.

Throughout the small intestine, the simple columnar intestinal epithelium has 5 types of differentiated cells, allderived from a common pool of stem cells that line the villi and crypts.

1 . Goblet cells secrete mucous that protects the surface of the intestine with a viscous fluid consisting of glycoproteins (20% peptides, 80% carbohydrates). The peptides are synthesized in the rough endoplasmic reticulum (RER) and the oligosaccharides added in the Golgi. Granules containing the condensed mucous collect in theapicalregion ofthe cell (hence the goblet shape) and are exocytosed together. Mucus protection must be balanced with permeability since digested foods and fluids must reach the epithelial surface.

2.Enterocytes have 2 major functions. Enterocytes participate in the final di­ gestion steps and they absorb the digested food (in the form ofamino acids, monosaccharides, and emulsified fats) by transporting it from the lumen of the intestine to the lamina propria, where it is carried away by blood vessels and lymphatics. Enterocyte-produced enzymes are bound to the glycocalyx ofmicrovilli and include oligo peptidases which hydrolyze peptides to amino acids; and disaccharidases and oligosaccharidases which convertthe sugars to monosaccharides (mainly glucose), galactose, and fructose.

3.Paneth cells are cells located at the base of the crypts, especially in the jeju­ num and ileum; theycontain visible acidophilic secretory granules located in the apical region ofthe cells. These cells protect the body against pathogenic microorganisms by secreting lysozyme and defensins (or cryptins) that de­ stroybacteria. Their life span is about 20 days.

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Figure 1-8-7. Cells at the base of crypts of LieberkOhn include Paneth cells (A) with large apical granules of lysozyme, and an adjacent stem cell (circle) undergoing mitosis.

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Section I • Histology and Cell Biology

4.Enteroendocrine cells ofthe small and large intestine, like those ofthe stom­ ach, secrete hormones that control the function of the GI tracts and associ­ ated organs. They are located in the lower halfofthe crypts and are detectable by silver-based stains.

5.Stem cells are located in the crypts, about one-third of the way up from the

bottom. Their progeny differentiate into all the other cell types. The epithelial lining ofthe small intestine, particularlythat covering the villi,completely renews itself every 5 days (or longer, during starvation). The newly creat­ ed cells (goblet, enterocytes, and enteroendocrine cells) migrate up from the crypts, while the cells at the tips of microvilli undergo apoptosis and slough

off. There is also a group of fibroblasts that accompany these epithelial cells as they move toward the tips ofthe villiOther. cells move to the base ofthe crypts, replenishing the population ofPaneth and enteroendocrine cells.

The lamina propria underlying the villi contains an extensive network of small blood vessels and capillaries. The capillary bed is immediately below the intestinal epithelium. The fenestrated capillaries carry out the absorbed sugars and amino acids, which get delivered directly to the liver via the portal circulation. In the center of each villus is a dead-end lymphatic with a large lumen. These lymphatic vessels are called lacteals because they transport emulsified fat; thus, the lacteals appear white after a heavy fat meal. The lymphatic vessels also carry out activated lymphocytes.

The serosais the connective tissue and mesothelial covering ofthe small intestine. This is the peritoneum ofgross anatomy. The duodenum is largely retroperitoneal and is only partially covered by a serosa. The jejunum and ileum are peritoneal structures and are almost completely covered by a serosa and are suspended by a double layer of a mesothelium emanating from the body wall. This double layer is the mesentery and is traversed by the neurovascular structures supplying the small intestine.

Gut-associated lymphatic tissue (GALT): Throughout the intestine, the lamina propria is heavily infiltrated with macrophages and lymphocytes. Peyer's patches are patches of GALT that are prominent in the ileum. M cells in the epithelium transport luminal antigens to their base, where they are detected by B lympho­ cytes and taken up by antigen-presenting macrophages. Lymphocytes that have interacted with antigens in a Peyer's patch migrate to the lymph nodes, mature, and home back to the intestine where they differentiate into plasma cells and produce IgA. IgA is transcytosed to the intestinal lumen and adsorbed onto the glycocalyx, where it is strategically situated to neutralize viruses and toxins and inhibit bacteria from adhering.

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Chapter 8 • Gastrointestinal System

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Figure 1-8-8. Ileum with Payer's patches (arrow) and central lacteals (arrowheads) in the lamina propria of the villi

A second function of GALT is immune suppression. This is a mechanism that enables the animal to ingest and digest foods without launching an attack against these foreign invaders, by suppressing the immune response to antigens in the food_ The same antigens injected into the blood would mount an immune re­ sponse. Mast cells are also present in the lamina propria. They secrete histamines, which are chemotactic agents for neutrophils and eosinophils. Mast cells are in­ volved in local defense against enteric parasites.

Clinical Correlate

The malfunction of GALT with autoimmune basis results in chronic inflammatory diseases ofthe gut. Crohn's disease (which affects mostly the ileum) and ulcerative colitis (which affects the large intestine) are examples

•The immune system mistakes microbes normally found in the intestines for foreign or invading substances and launches an attack.

•In the process, the body sends white blood cells into the lining ofthe intestines, where they produce chronic inflammation and generate harmful products that ultimately lead to ulcerations and bowel injury.

Large Intestine

The large intestine includes the cecum (with appendix), ascending, transverse, and descending colon, sigmoid colon, rectum, and anus. The large intestine has a wide lumen, strong musculature, and longitudinal muscle that is separated into 3 strands, the teniae coli. The inner surface has no plicae and no villi but consists of short crypts of Lieberkiihn.

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Section I • Histology and Cell Biology

Generalfeatures

•The colon is larger in diameter and shorter in length than is the small intestine. Fecal material moves from the cecum, through the colon

(ascending, transverse, descending, and sigmoidcolons), rectum, and anal canal.

•Three longitudinal bands of muscle, the teniae coli, constitute the outer layer. Because the colon is longer than these bands, pouching occurs, creating haustrabetween the teniae and giving the colon its characteristic "caterpillar" appearance.

•The mucosa has novilli,and mucus is secreted by short, inward-projecting colonic glands.

•Abundant lymphoid follicles are found in the cecum and appendix, and more sparsely elsewhere.

•The major functions of the colon are reabsorption offluid and electro­ lytes and temporary storage offeces.

Colonicmotility

•Peristalticwaves briefly open the normally closed ileocecal valve, passing a small amount of chyme into the cecum. Peristalsis also advances the chyme in the colon. Slow waves, approximately 2/min, are initiated at the ileocecal valve and increase to approximately 6/min at the sigmoid colon.

•Segmentation contractions mix the contents of the colon back and forth.

•Mass movement contractions are found only in the colon. Constriction of long lengths of colon propels large amounts of chyme distally toward the anus. Mass movements propel feces into the rectum. Distention of the rectum with feces initiates the defecation reflex.

Absorption

The mucosa of the colon has great absorptive capability. Na+ is actively trans­ ported withwaterfollowing, and K+ and HC03-are secreted into the colon.

Defecation

Feces: Contains undigested plant fibers, bacteria, inorganic matter, and water. Nondietary material (e.g., sloughed-off mucosa) constitutes a large portion of the feces. In normal feces, 30% of the solids may be bacteria. Bacteria synthesize vitamin K, B-complex vitamins, and folic acid; split urea to NH3; and produce small organic acids from unabsorbed fat and carbohydrate.

Defecation: Rectal distention with feces activates intrinsic and cord reflexes that cause relaxation of the internal anal sphincter (smooth muscle) and produce the urge to defecate. If the external anal sphincter (skeletal muscle innervated by the pudenda! nerve) is then voluntarily relaxed, and intra-abdominal pressure is in­ creased via the Valsalva maneuver, defecation occurs. Ifthe external sphincter is held contracted, the urge to defecate temporarily diminishes.

Gastrocolic reflex: Distention of the stomach by food increases the frequency ofmass movements and produces the urge to defecate. This reflex is mediated by parasympathetic nerves.

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Section I • Histology and Cell Biology

Table 1-8-4. Gastrointestinal Physiology

Appetite

Appetite is primarily regulated by 2 regions ofthe hypothalamus: a feeding center and a satiety center. Normally, the feeding center is active but is transiently inhibited bythe satiety center.

Mechanical Distention

•Distention of the alimentary tract inhibits appetite, whereas the contractions of an empty stomach stimulate it.

•Some satiety is derived from mastication and swallowing alone.

Miscellaneous

Other factors that help to determine appetite and body weight include body levels of fat and genetic factors.

(Continued)

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The·ducts that drain the glands increase in size and are lined by an epithelium that transitions from cuboidal to columnar to pseudostratified to stratified co­ lumnar cells. The smallest ducts, intercalated ducts, have myoepithelial cells; the next larger ducts, striated ducts, have columnar cells with basal striations, caused by basal infolding of cell membranes between prominent mitochondria. These columnar cells make the saliva hypotonic by transporting Na and Cl ions out of saliva back into the blood.