LOWER DIGESTIVE SEGMENT

The peritoneum (see16-3)

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 encloses the abdominal viscera as the visceral peritoneum .

The mesentery is a layer of loose connective tissue (areolar connective tissue) covered with peritoneum. We discuss the histology of the mesentery in Chapter 4 , Connective Tissue.

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 (see 16-3 ).

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 in Chapter 15 , Upper Digestive Segment, 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 ( 16-4 ; see 16-2 and 16-3 )

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 (see 16-2 ):

• 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 evaginations of the mucosa covering the entire surface of the small intestine. 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. About six invaginating crypts, extending deep into the mucosa and ending at the muscularis mucosae, surround an intestinal villus.

The muscularis mucosae is the boundary between the mucosa and submucosa (see 16-3 ).

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 (see 16-4 ).

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.

Microcirculation of the small intestine (see 16-3 )

A difference from the microcirculation of the stomach (compare with 15-8 in Chapter 15 , Upper Digestive Segment) is that the intestinal submucosa is the main distribution site of blood and lymphatic flow .

Branches of the submucosal plexus supply capillaries to the muscularis and intestinal mucosa. Arterioles derived from the submucosal plexus enter the mucosa of the small intestine and give rise to two capillary plexuses:

• 1.

  The villus capillary plexus supplies the intestinal villus and upper portion of the crypts of Lieberkühn.

• 2.

  The pericryptal capillary plexus supplies the lower half of the crypts of Lieberkühn.

A single blind-ending central lymphatic capillary , the lacteal , is present in the core or lamina propria of a villus.

The lacteal is the initiation of a lymphatic vessel that, just above the muscularis mucosae, forms a lymphatic plexus whose branches surround a lymphoid nodule in the mucosa-submucosa. Efferent lymphatic vessels of the lymphoid nodule anastomose with the lacteal and leave the digestive tube through the mesentery, together with the blood vessels.

Innervation and motility of the small intestine ( 16-4 )

Motility of the small intestine is controlled by the autonomic nervous system. The intrinsic autonomic nervous system of the small intestine, consisting of the submucosal plexus of Meissner and myenteric plexus of Auerbach , is similar to that of the stomach (see 15-7 and 15-8 in Chapter 15 , Upper Digestive Segment).

Neurons of the plexuses receive intrinsic input from the mucosa and muscle wall of the small intestine and extrinsic input from the central nervous system through the parasympathetic (vagus nerve) and sympathetic nerve trunks .

Contraction of the muscularis is coordinated to achieve two objectives (see 16-4 ):

• 1.

  To mix and mobilize the contents within an intestinal segment . This is accomplished when muscular contraction activity is not coordinated and the intestine becomes transiently divided into segments. This process is known as segmentation .

• 2.

  To propel the intestinal contents when there is a proximal (orad; Latin os, mouth; ad , to; toward the mouth) contraction coordinated with a distal (aborad; Latin ab , from; os, mouth; away from the mouth) relaxation.

When coordinated contraction-relaxation occurs sequentially, the intestinal contents are propelled in an aborad direction . This process is known as peristalsis (Greek peri , around; stalsis , constriction).

Histology differences between the duodenum, jejunum and ileum ( 16-5 )

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 (leaf-like 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 ( 16-6 )

The intestinal mucosa, including villi surrounded by the crypts of Lieberkühn, is 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.

  Tuft cells .

• 6.

  Intestinal stem cells (ISC) .

Enteroendocrine cells, Paneth cells and ISCs are found in the crypts of Lieberkühn. We discuss below Paneth cells within the context of the protection mechanisms of the small intestine.

Enterocytes: Absorptive cells ( 16-7 )

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 , that contains transverse cytoskeletal filaments.

The brush border of each absorptive cell contains about 3000 closely packed microvilli , which increase by 30-fold the surface luminal area.

The length of a microvillus ranges from 0.5 to 1.0 μm. 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 a nd 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 cyto-keratin-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.

Trafficking of peptides and sugars ( 16-8 )

The microvilli , forming a brush border , contain intramembranous enzymes, including lactase, maltase and sucrase .

Therefore, the brush border not only increases the absorptive surface of enterocytes but is also the site where enzymes are involved in the terminal digestion and absorption of carbohydrates and proteins.

The final breakdown of oligopeptides, initiated by the action of gastric pepsin, is continued by pancreatic trypsin, chymotrypsin, elastase and carboxypeptidases A and B.

Enterokinase and aminopeptidase , localized in the microvilli, degrade oligopeptides into dipeptides, tripeptides and amino acids before entering the enterocyte across symporter channels together with Na ⁺ .

Cytoplasmic peptidases degrade dipeptides and tripeptides into amino acids, which then diffuse or are transported by a carrier-mediated process across the basolateral plasma membrane into the blood.

Concerning the absorption of sugars, oligosaccharides reduce carbohydrates to monosaccharides, which can be transported into the enterocyte by carrier proteins .

Glucose and galactose cross the apical membrane with the help of a s odium glucose/galactose co-trans-porter-1 (SGLT-1) . Na ⁺ -K ⁺ ATPase drives SGLT-1. Fructose, derived from the breakdown of sucrose, enters and leaves the enterocyte by passive diffusion .

A genetic defect in lactase prevents the absorption of lactose-rich milk, causing diarrhea (lactose intolerance) . Lactose is converted by intestinal bacteria to lactic acid, methane and H2 gas, causing an osmotic diarrhea by drawing water into the intestinal lumen. A lactose-H 2 breath test is positive in individuals with lactase deficiency. H2 enters the blood circulation and is expired by the lungs.

In summary , concerning carbohydrates, they can only be absorbed as monosaccharides. A two-step process enables the absorption of glucose and galactose monosaccharides: active transport across the apical membrane of the enterocyte involving SGLT-1, followed by transport across the basolateral membrane by facilitated diffusion.