Small Intestine

The 4- to 7-meter-long small intestine is divided into three sequential segments:

1. Duodenum.

2. Jejunum.

3. Ileum.

The duodenum is about 25 cm in length, is mainly retroperitoneal, and surrounds the head of the pancreas. At its distal end, the duodenum is continuous with the jejunum, a movable intestinal segment suspended by a mesentery. The ileum is the continuation of the jejunum.

The wall of the small intestine consists of four layers:

1. The mucosa.

2. The submucosa.

3. The muscularis.

4. The serosa, or peritoneum.

Histologic differences are seen in the mucosa and submucosa of the three major portions of the small intestine. The muscularis externa and serosa layers are similar.

The Peritoneum

The peritoneum is a serous membrane consisting of a connective tissue stroma (containing elastic fibers, blood and lymphatic vessels, and nerves) lined by mesothelial cells. The parietal peritoneum lines the abdominal wall and reflects to cover the abdominal viscera as the visceral peritoneum.

The mesentery is a layer of loose connective tissue (areolar connective tissue) covered with peritoneum. The mesentery attaches the abdominal viscera to the posterior abdominal wall and it serves as a conduit of blood and lymphatic vessels and nerves to these organs. The blood vessels are components of the subserosal plexus. During digestion, the lymphatic vessels emerging from the walls of the small intestine carry a fluid rich in absorbed fat emulsion, or chyle. Numerous lymph nodes and adipose tissue are seen in the mesentery. The mesentery can be short to anchor certain viscera to the abdominal wall, or longer to enable visceral displacement. As indicated, the esophagus lacks a serosa. The duodenum and ascending and descending colon attach to the abdominal cavity by the adventitia, a loose connective tissue continuous with the surrounding stroma of the abdominal wall.

The omenta and visceral ligaments have a structure similar to the mesentery. The greater omentum has considerable adipose tissue.

Intestinal Wall

The intestinal wall shows an increase in the total surface of the mucosa that reflects the absorptive function of the small intestine.

Four degrees of folding amplify the absorptive surface area of the mucosa:

1. The plicae circulares (circular folds; also known as the valves of Kerkring).

2. The intestinal villi.

3. The intestinal glands.

4. The microvilli on the apical surface of the lining epithelium of the intestinal cells (enterocytes).

A plica circularis is a permanent fold of the mucosa and submucosa encircling the intestinal lumen.

Plicae appear about 5 cm distal to the pyloric outlet of the stomach, become distinct where the duodenum joins the jejunum, and diminish in size progressively to disappear halfway along the ileum.

The intestinal villi are finger-like projections of the mucosa covering the entire surface of the small intestine. Villi extend deep into the mucosa to form crypts ending at the muscularis mucosae. The length of the villi depends on the degree of distention of the intestinal wall and the contraction of smooth muscle fibers in the villus core.

Crypts of Lieberkühn, or intestinal glands, are simple tubular glands that increase the intestinal surface area. The crypts are formed by invaginations of the mucosa between adjacent intestinal villi.

The muscularis mucosae is the boundary between the mucosa and submucosa.

The muscularis consists of inner circular smooth muscle and outer longitudinal smooth muscle. The muscularis is responsible for segmentation and peristaltic movement of the contents of the small intestine.

A thin layer of loose connective tissue is covered by the visceral peritoneum, a serosa layer lined by a simple squamous epithelium, or mesothelium. The parietal peritoneum covers the inner surface of the abdominal wall.

Histologic Differences Between the Duodenum, Jejunum, and Ileum

Each of the three major anatomic portions of the small intestine, the duodenum, jejunum, and ileum, has distinctive features that allow recognition under the light microscope.

The duodenum extends from the pyloric region of the stomach to the junction with the jejunum and has the following characteristics:

1. It has Brunner's glands in the submucosa. Brunner's glands are tubuloacinar mucous glands producing an alkaline secretion (pH 8.8 to 9.3) that neutralizes the acidic chyme coming from the stomach.

2. The villi are broad and short (leaflike shape).

3. The duodenum is surrounded by an incomplete serosa and an extensive adventitia rather than a serosa.

4. The duodenum collects bile and pancreatic secretions transported by the common bile duct and pancreatic duct, respectively. The sphincter of Oddi is present at the terminal ampullary portion of the two converging ducts.

5. The base of the crypts of Lieberkühn may contain Paneth cells.

The jejunum has the following characteristics:

1. It has long finger-like villi and a well-developed lacteal in the core of the villus.

2. The jejunum does not contain Brunner's glands in the submucosa.

3. Peyer's patches in the lamina propria may be present but they are not predominant in the jejunum. Peyer's patches are a characteristic feature of the ileum.

4. Paneth cells are found at the base of the crypts of Lieberkühn.

The ileum has a prominent diagnostic feature: Peyer's patches, lymphoid follicles (also called nodules) found in the mucosa and part of the submucosa. The lack of Brunner's glands and the presence of shorter finger-like villi, when compared with the jejunum, are additional landmarks of the ileum. As in the jejunum, Paneth cells are found at the base of the crypts of Lieberkühn.

Villi and Crypts of Lieberkühn

The intestinal mucosa, including the crypts of Lieberkühn, are lined by a simple columnar epithelium containing five major cell types:

1. Enterocytes or absorptive cells.

2. Goblet cells.

3. Enteroendocrine cells.

4. Paneth cells.

5. Intestinal stem cells.

Enteroendocrine cells, Paneth cells, and intestinal stem cells are found in the crypts of Lieberkühn.

Enterocytes: Absorptive Cells

The absorptive intestinal cell or enterocyte has an apical domain with a prominent brush border (also called a striated border), ending on a zone, called the terminal web, which contains transverse cytoskeletal filaments. The brush border of each absorptive cell contains about 3000 closely packed microvilli, which increase the surface luminal area 30-fold.

The length of a microvillus ranges from 0.5 to 1.0micrometer. The core of a microvillus contains a bundle of 20 to 40 parallel actin filaments cross-linked by fimbrin and villin. The actin bundle core is anchored to the plasma membrane by formin (protein of the cap), myosin I, and the calcium-binding protein calmodulin. Each actin bundle projects into the apical portion of the cell as a rootlet, which is cross-linked by an intestinal isoform of spectrin to an adjacent rootlet. The end portion of the rootlet attaches to cytokeratin-containing intermediate filaments. Spectrin and cytokeratins form part of the terminal web. The terminal web is responsible for maintaining the upright position and shape of the microvillus and anchoring the actin rootlets.

A surface coat or glycocalyx, consisting of glycoproteins as integral components of the plasma membrane, covers each microvillus.

Goblet Cells

Goblet cells are columnar mucus-secreting cells scattered among enterocytes of the intestinal epithelium.

Goblet cells have two domains:

1. A cup- or goblet-shaped apical domain containing large mucus granules that are discharged on the surface of the epithelium.

2. A narrow basal domain, which attaches to the basal lamina. The basal domain houses the rough endoplasmic reticulum and Golgi apparatus, in which the protein portion of mucus is produced and transported, and the nucleus.

The Golgi apparatus, which adds oligosaccharide groups to mucus, is prominent and situated above the basally located nucleus.

The secretory product of goblet cells contains glycoproteins (80% carbohydrate and 20% protein) released by exocytosis.

On the surface of the epithelium, the mucus hydrates to form a protective gel coat to shield the epithelium from mechanical abrasion and bacterial invasion by concentrating specific antimicrobial proteins, including defensins and cathelicidins.

Enteroendocrine Cells

In addition to its digestive function, the gastrointestinal tract is the largest diffuse endocrine gland in the body.

As in the stomach, enteroendocrine cells secrete peptide hormones controlling several functions of the gastrointestinal system.

Intestinal Stem Cells

Intestinal stem cells (ISCs) reside in a niche at the base of the crypts, close to Paneth cells.

Adult ISC, identified by the protein marker Lgr5 (for leucine-rich repeat-containing G protein coupled receptor 5), can differentiate into the secretory goblet cells, Paneth cells, and enteroendocrine cells and the absorptive enterocytes lining the epithelium of the small intestine.

ISCs are multipotent and capable of long-term self-renewal as long as they remain at the crypt niche. Presumably ISCs are subject to positional cues derived from the microenvironment of the niche.

As clusters of enterocytes and goblet cells divide and differentiate, they migrate along the walls of the crypts and villi until they reach the tip of the villus where they are eventually shed.

Following injury, cells committed to the intestinal secretory pathway and expressing Delta-like 1 (DLL1), a ligand of the Notch family of proteins, can return to the stem cell compartment and revert into multipotent ISCs.

Protection of the Small Intestine

The large surface area of the gastrointestinal tract, about 200 m² in humans, is vulnerable to resident microorganisms, called microbiota, and potentially harmful microorganisms and dietary antigens. Microbiota includes bacteria, fungi, parasites and viruses.

In the small and large intestines, goblet cells secrete mucin glycoproteins assembled into a viscous gel-like blanket limiting direct bacterial contact with enterocytes. When the blanket lacks one of its components, mucin glycoprotein 2 (MIC2), spontaneous intestinal inflammation occurs.

Several defensive mechanisms operate in the alimentary tube to limit tissue invasion of pathogens and avoid potentially harmful overreactions that could damage intestinal tissues. The defensive mechanisms include:

1. The intestinal tight junction barrier, formed by apical tight junctions linking enterocytes. The barrier of pathogens is monitored by the immune-competent cells residing in the subjacent lamina propria.

2. Peyer's patches and associated M cells, regarded as the immune sensors of the small intestine.

3. Polymeric immunoglobulin A (IgA), a secretory product of plasma cells located in the lamina propria, reaching the intestinal lumen by the mechanism of transcytosis.

4. Paneth cells, whose bacteriostatic secretions control the resident microbiota of the small intestine.

In addition, it is to be kept in mind the defensive roles of the acidity of the gastric juice, that inactivates ingested microorganism, and the propulsive intestinal motility (peristalsis), that prevents bacterial colonization.

Intestinal Tight Junction Barrier

Intestinal tight junctions link adjacent enterocytes and provide a barrier function impermeable to most hydrophilic solutes in absence of specific transporters.

Tight junctions establish a separation between the intestinal luminal content and the mucosal immune function that occurs within the lamina propria. Plasma cells, lymphocytes, eosinophils, mast cells and macrophages are present in the intestinal lamina propria.

Claudin and occludin are two transmembrane proteins of tight junctions that regulate solute permeability of the transcellular pathway. Flux of dietary proteins and bacterial lipopolysaccharides across leaky tight junctions can increase in the presence of tumor necrosis factor ligand and interferon-a, two proinflammatory cytokines that affect tight junction integrity.

Many diseases associated with intestinal epithelial dysfunction, including inflammatory bowel disease and intestinal ischemia, are associated with increased levels of tumor necrosis factor ligand.

A minor defect of the tight junction barrier can allow bacterial products or dietary antigens to cross the epithelium and enter the lamina propria. Antigens can bind to Toll-like receptor (TLR) on the surface of dendritic cells.

Dendritic cells migrate to a local mesentery lymph node and the antigen is presented to naïve T cells by the major histocompatibility complex to determine their differentiation into T helper 1 (T_H1) and T helper 2 (T_H2) cells that relocate to the lamina propria.

T_H1 cells produce the proinflammatory cytokines tumor necrosis factor ligand and interferon-gamma. T_H2 cells downregulate the proinflammatory activity of T_H1 cells by secreting interleukin-10. If the mucosa immune cell activation response proceeds unchecked, proinflammatory cytokines will continue enhancing further leakage across the tight junction barrier, a condition leading to intestinal chronic inflammatory diseases.

Peyer's Patches

Peyer's patches, the main component of the gut-associated lymphoid tissue (GALT), are specialized lymphoid follicles found predominantly in the intestinal mucosa and part of the submucosa of the ileum. GALTs participate in the uptake of antigens and their exposure to antigen-presenting cells. Therefore, these structures serve important functions that can lead to inflammation or tolerance.

The microbiota is involved in the normal development and maturation of GALTs. In the fetus, lymphoid tissue inducer cells stimulate the development of Peyer's patches in the absence of microbiota.

Peyer's patches consist of cells able to take up and transport luminal antigens and bacteria to antigen-presenting cells leading to immune tolerance or an inflammatory reaction against pathogens.

Peyer's patches are regarded the immune sensors of the small intestine. An equivalent to Peyer's patches in the large intestine are the isolated lymphoid follicles (ILFs), requiring TLRs and nucleotide-binding oligomerization domain 2 (NOD2) to become activated. TLRs are extracellular sensors and NODs are cytoplasmic sensors.

A Peyer's patch displays three main components:

1. The follicle-associated epithelium (FAE), consisting of M cells and enterocytes.

2. The lymphoid follicles, each showing a germinal center and a subepithelial dome area.

3. The interfollicular area, with blood vessels and efferent lymphatic vessels connecting Peyer's patches to the mesenteric lymph nodes.

High endothelial venules, enabling the immigration of lymphocytes, are present in the lymphoid follicles. Activated lymphocytes leave the Peyer's patches through the lymphatic vessels.

The main components of the FAE are M cells and dendritic cells:

1. M cells, forming an enterocyte specialized cell layer that takes up antigens and replaced the brush border by short microfolds (hence the name M cell). M cells differentiate from enterocytes when stimulated by membrane-bound lymphotoxin (LT_1`2) present on local B cells.

M cells form intraepithelial pockets, where a subpopulation of intraepithelial B cells resides and express IgA receptors allowing the capture and phagocytosis of IgA-bound bacteria.

Antigens are transported by M cell and presented to the immunocompetent B cells residing in the intraepithelial pockets.

The population of M cells increases rapidly in the presence of pathogenic bacteria in the intestinal lumen (for example, Salmonella typhimurium). When confronting Salmonella, the microfolds of M cells change into large ruffles and, within 30 to 60 minutes, M cells undergo necrosis and the population of M cells is depleted. Poliovirus, the pathogen of poliomyelitis, uses the Peyer's patches to replicate.

2. Dendritic cell, extending cytoplasmic processes between tight junctions linking enterocytes.

Lymphoid follicles have a germinal center that contains IgA-positive B cells, CD4⁺ T cells, antigen-presenting cells and follicular dendritic cells. A few plasma cells are present in the Peyer's patches. The subepithelial dome contains B cells, T cells, macrophages, and dendritic cells.

Antigens in the intestinal lumen activate TLRs expressed by enterocytes. TLR-antigen interaction stimulates the production of B cell-activating factor (BAF) and cytokines to activate the production of immunoglobulin (Ig) A by plasma cells located in the lamina propria and Peyer's patches.

Intestinal antigens, bound to immunoglobulin receptors on the surface of B cells, interact with antigen-presenting cells at the subepithelial dome region. Antigens are presented to follicular dendritic cells and CD4⁺ T cells to initiate an immune reaction.

In summary, Peyer's patches have the ability to transport luminal antigens and microorganisms and respond to them by inducing immune tolerance or a systemic immune defense response. An example of the functional deficiency of Peyer's patches is Crohn's disease, an inflammatory bowel disease characterized by chronic or relapsing inflammation.

Polymeric IgA

Plasma cells secrete polymeric IgA into the intestinal lumen, the respiratory epithelium, the lactating mammary gland, and salivary glands. Most plasma cells are present in the lamina propria of the intestinal villi, together with lymphocytes, eosinophils, mast cells, and macrophages.

Polymeric IgA molecules secreted by plasma cells are transported from the lamina propria to the intestinal lumen by a transcytosis mechanism consisting of the following steps:

1. Polymeric IgA is secreted as a dimeric molecule joined by a peptide called the J chain.

2. Polymeric IgA binds to a specific receptor, called the polymeric immunoglobulin receptor (pIgR), available on the basal surfaces of the enterocytes. The pIgR has an attached secretory component.

3. The polymeric IgA-pIgR-secretory component complex is internalized and transported across the cell to the apical surface of the epithelial cell.

4. At the apical surface, the complex is cleaved enzymatically and the polymeric IgA-secretory component complex is released into the intestinal lumen as secreted IgA (SIgA). The secretory component protects the dimeric IgA from proteolytic degradation.

5. IgA attaches to bacteria and soluble antigens, preventing a direct damaging effect to intestinal cells and penetration into the lamina propria.

How are plasma cells induced to produce polymeric IgA?

When TLR on enterocytes is activated by microbiota, they secrete B cell-activating factor (BAF) and a proliferation-inducing ligand (APRIL).

In the lamina propria, BAF and APRIL induce the differentiation of B cells into IgA-producing plasma cells.

In addition, the microbiota instructs enterocytes through thymic stromal lymphoprotein (TSLP) to engage dendritic cells in the lamina propria to secrete BAF and APRIL and induce the differentiation of B cells into plasma cells.

One last point: IgA regulates the composition and the function of the intestinal microbiota by affecting bacterial gene expression. By this mechanism, IgA keeps a congenial relationship between the host and the microbiota.

In the discussion of Peyer's patches, it is indicated that M cells express IgA receptors allowing the uptake of IgA-bound bacteria. It is realized that luminal SIgA not only immobilize bacteria but also redirects them to the M cells for internalization and disposal.

Paneth cells

Enterocytes and Paneth cells in particular secrete proteins to limit bacteria pathogenic challenges. We discuss in Chapter 11. Integumentary System, how epithelial antimicrobial proteins (AMPs) protect skin surfaces against microorganisms. We continue the discussion within the context of the antimicrobial defense of the intestinal mucosa involving Paneth cells and enterocytes.

Most AMPs inactivate or kill bacteria directly by enzymatic degradation of the bacterial wall or by disrupting the bacterial inner membrane. A group of AMPs deprive bacteria of essential heavy metal such as iron.

AMPs produced by Paneth cells and enterocytes are retained in the intestinal mucus blanket produced by goblet cells. Therefore, the mucus layer protects the intestinal mucosa by two mechanisms:

1. By creating a barrier that limits direct access of luminal bacteria to the epithelium. 

2. By concentrating AMPs near the enterocyte surface. AMPs are virtually absent from the luminal content.

Paneth cells are present at the base of the crypts of Lieberkühn and have a lifetime of about 20 days. The pyramid-shaped Paneth cells have a basal domain containing the rough endoplasmic reticulum. The apical region shows numerous protein granules representing a diverse array of AMPs, an indication of the microbial diversity and impending threats (Figures 16-16 and 16-17).

Paneth cells produce several AMPs:

1. Defensins (_-defensin 5 [DEFA5] and _- defensin 6 [DEFA6] in humans)

2. C-type lectins, including regenerating isletderived protein 3a (REG3a), also known as hepatointestinal protein/pancreatitis-associated protein (HIP/PAP).

3. Lysozyme and phospholipase A2 (PLA2).

4. Angiogenin 4 (ANG4).

_-Defensins (2–3 kd) target Gram-positive and Gram-negative bacteria, fungi, viruses, and protozoa to produce membrane disruption by the formation of defensin pores. Pores cause swelling and membrane rupture enabling the entrance of water into the pathogen. Defensins can also be chemotactic to CD4+ T cells, CD8+ T cells, monocytes, and macrophages and modulate an inflammatory response. Defensins enhance the recruitment of dendritic cells to the site of infection and facilitate the uptake of antigens by forming defensin-antigen complexes.

Like all C-type lectins, the carbohydrate-recognition domain of REG3a HIP/PAP (15 kd) binds to the glycan chain of peptidoglycan present in the bacterial cell wall of Gram-positive bacteria and causes wall disruption. Peptidoglycan is present in bacteria but not in human cells.

Recall that selectins, a member of the group of Ca2+-dependent cell adhesion molecules, belong to the C-type lectin family that have carbohydrate recognition domains.

Lysozyme is a proteolytic enzyme that cleaves glycosidic linkages that maintain the integrity of cell wall peptidoglycan. PLA2 kills bacteria by hydrolysis of phospholipids in the bacterial membrane.

Paneth cells secrete ANG4, an RNAse with bactericidal properties.

It is important to emphasize that the expression and function of AMPs are highly regulated by the presence or absence of the microbiota (see Figure 16-16).

In the presence of microorganisms:

1. TLR in enterocytes controls the expression of REG3a/HIP/PAP through the TLR-signaling adaptor myeloid-differentiation primary response protein 88 (MYD88).

2. Cytoplasmic NOD2, expressed by Paneth cells, controls the expression of _-defensins when it binds to an internalized peptidoglycan peptide fragment (muramyl dipeptide, MDP) and activates the transcription factor NF-gB.

Note that NOD2 is in a strategic position to contribute to immunogenic tolerance toward the microbiota when confronting MDP: NOD2 can also limit the development of a CD4+ Tcell-initiated immune response. However, _-defensins can be expressed independently of the microbiota by activation of the transcription factor TCF4.

Defensins are produced continuously or in response to microbial products or proinflammatory cytokines (for example, TNF ligand). As mentioned in our discussion on the intestinal tight junction barrier, TNF ligand is a proinflammatory cytokine produced in response to diverse infectious agents and tissue injury.

In summary, enterocytes and Paneth cells produce a diverse group of AMPs that directly kill or inhibit the growth of pathogenic microorganisms that can contribute to inflammatory bowel diseases.

Pathology: Inflammatory bowel diseases

Inflammatory bowel disease includes ulcerative colitis and Crohn’s disease. Both are clinically characterized by diarrhea, pain, and periodic relapses.

Ulcerative colitis affects the mucosa of the large intestine. Crohn’s disease affects any segment of the intestinal tract.

Crohn’s disease is a chronic inflammatory process involving the terminal ileum but is also observed in the large intestine. Inflammatory cells (neutrophils, lymphocytes, and macrophages) produce cytokines that cause damage to the intestinal mucosa (Figure 16-18).

The initial alteration of the intestinal mucosa consists in the infiltration of neutrophils into the crypts of Lieberkühn. This process results in the destruction of the intestinal glands by the formation of crypt abscesses and the progressive atrophy and ulceration of the mucosa.

The chronic inflammatory process infiltrates the submucosa and muscularis. Abundant accumulation of lymphocytes forms aggregates of cells, or granulomas, a typical feature of Crohn’s disease.

Major complications of the disease are occlusion of the intestinal lumen by fibrosis and the formation of fistulas in other segments of the small intestine, and intestinal perforation. Segments affected by Crohn’s disease are separated by normal stretches of intestinal segments.

The cause of Crohn’s disease is unknown. There is increasing evidence suggesting that the disease arises from dysregulated interactions between microorganisms and the intestinal epithelium involving NOD2.

Patients with intestinal bowel disease have an increased number of bacteria associated with the epithelial cell surface, suggesting a failure of mechanisms limiting direct contact between microorganisms and the epithelium.

A contributing factor is the reactive immune response of the intestinal mucosa determined by an abnormal signaling exchange with the resident bacteria (microbiota). In genetically susceptible individuals, inflammatory bowel disease occurs when the mucosal immune machinery regards the microbiota present in normal and healthy individuals as pathogenic and triggers an immune response.

As discussed (see Figure 16-12), cytokines produced by helper T cells within the intestinal mucosa cause a proinflammatory response that characterizes inflammatory bowel disease. In Crohn’s disease, type 1 helper cells (TH1 cells) produce TNF ligand and interferon-a. Because TNF ligand is a proinflammatory cytokine, antibodies to this cytokine are being administered to patients with Crohn’s disease to attenuate proinflammatory activity.

Clinical significance: Malabsorption syndromes

Malabsorption syndromes are characterized by a deficit in the absorption of fats, proteins, carbohydrates, salts, and water by the mucosa of the small intestine.

Malabsorption syndromes can be caused by:

1. Abnormal digestion of fats and proteins by pancreatic diseases (pancreatitis or cystic fibrosis) or lack of solubilization of fats by defective bile secretion (hepatic disease or obstruction of the flow of bile into the duodenum).

2. Enzymatic abnormalities at the brush border, where disaccharidases and peptidases cannot hydrolyze carbohydrates (lactose intolerance) and proteins, respectively.

3. A defect in the transepithelial transport by enterocytes.

Malabsorption syndromes affect many organ systems. Anemia occurs when vitamin B12, iron, and other cofactors cannot be absorbed. Disturbances of the musculoskeletal system are observed when proteins, calcium, and vitamin D fail to be absorbed. A typical clinical feature of malabsorption syndromes is diarrhea.