During the subsequent weeks, luminal vacuolization and degeneration of some of the proliferating cells

result in recanalization of the duodenal lumen. From the 4th to the 10th week of development, a large

portion of the midgut is herniated through the umbilicus. This may be due to the rapid growth of the

intestinal tract at this time in relation to the abdominal cavity. At approximately 8 weeks of gestation,

the midgut begins to rotate in a counterclockwise manner 90 degrees around the axis of the mesenteric

vasculature. The extra-abdominal portion of the gut returns to the abdominal cavity approximately 2

weeks later. Around this time point an additional 180-degree rotation occurs. These two rotations

provide 270 degrees of total rotation and result in the typical anatomy that is found in humans. A

failure of these precise steps produces a spectrum of anatomical variants which are grouped together as

malrotation of the intestinal tract.1

DUODENUM

The duodenum comprises the first portion of the small intestine and plays an important role in

connecting the foregut organs to the midgut. It anatomically begins at the duodenal bulb which is

immediately distal to the pylorus and terminates at the ligament of Treitz, where it joins the jejunum.

The duodenum is approximately 20 to 30 cm in length and is divided into four distinct areas.

The first portion of the duodenum is approximately 5 cm in length and is referred to as the bulb or

cap. This area is directly attached to the pylorus and extends laterally and cephalad. It serves as an

attachment for the hepatoduodenal ligament and traverses over the common bile duct, portal vein,

pancreatic head, and the gastroduodenal artery. The mucosal surface of the duodenal bulb is smooth

until its junction with the second portion of the duodenum, where the concentric Kerckring folds begin.

This portion of the duodenum is prone to ulceration, with approximately 90% of duodenal ulcers

occurring here. Unfortunately, due to its anatomic positioning, these ulcers may erode into the

gastroduodenal artery which lies directly posterior, causing potentially life-threatening bleeding.

The second portion of the duodenum (descending duodenum) extends from the origin of the

Kerckring folds to the beginning of the transverse duodenum and travels over the right renal

vasculature, the medial aspect of Gerota fascia, the inferior vena cava, and to the right of the L1 and L2

vertebra. It is approximately 10 cm in length and 3 to 5 cm in diameter. The Kerckring folds (plicae

circulares) are concentric mucosal folds which are 1 to 2 mm thick and 2 to 4 mm high and are

separated by 2 to 4 mm of smooth, flat mucosa. This portion of the duodenum serves as an entry point

for pancreatic and biliary secretions into the gastrointestinal tract. This is typically through the major

papilla (ampulla of Vater), which is a valvular structure arising in the midportion of the descending

duodenum, approximately 7 to 10 cm from the pylorus. Through this point, the confluence of the

common bile duct and the main pancreatic duct (duct of Wirsung) join the duodenum. The valvular

function of the papilla is regulated through the muscular sphincter of Oddi. The minor pancreatic duct

(duct of Santorini) enters through the minor papilla proximal to the ampulla of Vater in 50% to 60% of

patients and endoscopically appears as a 1- to 3-mm polypoid structure.

The second portion of the duodenum is important surgically as it represents the entry of the

duodenum into the retroperitoneum. Surgical evaluation of this part of the duodenum requires

mobilization from its posterior and lateral attachments, described as a Kocher maneuver. This allows for

further evaluation of the duodenum, pancreatic head, and bile duct.

The third (transverse) and fourth (ascending) portions of the duodenum complete the duodenal

sweep. The third portion of the duodenum is about 10 cm in length and courses transversely from right

to left, crossing the midline anterior to the spine, aorta, and inferior vena cava. This portion is closely

attached to the uncinate process of the pancreas. The superior mesenteric artery (SMA) and vein (SMV)

course anterior to the third portion of the duodenum to provide blood supply to the gut. The transition

between the third and fourth portions of the duodenum is marked by the passage of the SMA in front of

the duodenum. The SMA forms an acute angle as it originates from the aorta. An abnormally narrow

angle can result in obstruction of the duodenum at this location (SMA syndrome). The fourth portion of

the duodenum is approximately 5-cm long and courses upward and obliquely to reach the ligament of

Treitz, marking the end of the duodenum and the return of the small bowel to the peritoneal cavity.

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Figure 48-1. Arterial blood supply to the duodenum.

Following its embryologic origins, the vascular supply to the duodenum arises from branches of the

celiac trunk for the foregut portion, whereas the distal (midgut origin) duodenum is supplied by

branches of the SMA (Fig. 48-1). Venous drainage includes a series of pancreaticoduodenal veins which

drain into the SMV–portal vein system. Lymphatic drainage follows the vascular supply with drainage to

the pancreaticoduodenal nodes. From here, lymph drains superiorly to the hepatic nodes or inferiorly to

the superior mesenteric nodes.

JEJUNUM AND ILEUM

2 Distal to the ligament of Treitz, the jejunum and ileum form the remainder of the small intestine. The

boundary between the two is arbitrarily determined such that 40% of the intraperitoneal small intestine

comprises jejunum and 60% comprises ileum. This portion of the bowel is suspended within the

peritoneal cavity by a thin, broad-based mesentery that is attached to the posterior abdominal wall. The

jejunum and ileum are freely mobile within the peritoneal cavity. The jejunum is the widest portion of

the small intestine, whose caliber progressively decreases as it approaches the ileocecal valve. The

mucosa of the jejunum has a thick lining and is characterized by prominent plicae circulares that

become shorter and less frequent in the ileum. The total length of jejunum and ileum varies, but is

usually between 5 and 7 m in length. The small intestine terminates in the right lower quadrant at the

ileocecal valve. The ileocecal valve exhibits motor characteristics separate from the terminal ileum and

colon, postulated to prevent reflux of fecal material from the colon into the small intestine.2

The arterial blood supply of the jejunum and the ileum arises from the SMA. The main vascular

branches form arcades within the mesentery. The vasa recta, intestinal arterial branches, enter into the

intestinal wall without anastamosing. The vasa recta of the jejunum are straight and long, in contrast to

the vasa recta of the ileum, which are shorter with greater arborization (Fig. 48-2). The venous and

lymphatic drainage follow the arterial supply. The main venous outflow is through the SMV which,

along with the splenic vein, becomes the portal vein.

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Figure 48-2. The superior mesenteric artery supplies bloodflow into the small intestinal arteries (vasa recta) which branch within

the mesentery. In the jejunum (A), the vasa recta are straight and long, this is in contrast to the vasa recta within the ileum (B),

which are shorter with greater arborization.

MICROSCOPIC ANATOMY

General Considerations

3 The wall of the small intestine is composed of four distinct layers—mucosa, submucosa, muscle, and

serosa. The role of the mucosa is absorption and secretion. The luminal mucosal surface forms circular

folds known as plica circularis or valvulae conniventes in all segments of the small intestine distal to the

first portion of the duodenum. The submucosa contains an elaborate network of blood vessels,

lymphatics, and nerves. The muscular portion of the wall includes outer longitudinal and inner circular

muscle layers. Between the muscle layers lies the myenteric (Auerbach) plexus. The muscular layers are

responsible for coordinating peristaltic movements. The outermost layer, the serosa, is composed of a

thin layer of mesothelial cells overlying loose connective tissue. The serosa covers only the anterior

surface of the retroperitoneal segments of small bowel, but it completely covers the portions of small

bowel that are invested with mesentery.

Mucosa

The mucosa lines the luminal surface of the small intestine. It consists of three layers: epithelial cells,

lamina propria, and a narrow layer of smooth muscle, the muscularis mucosae. The basic structural unit

of the mucosa is the crypt and villus. Villi are finger-like projections of mucosa 0.5 to 1 mm high

extending into the intestinal lumen that have a columnar epithelial surface and a cellular connective

tissue core of lamina propria. Each villus contains a central lacteal (lymphatic), and a vascular network

consisting of a small artery, vein, and capillaries. Ninety percent of the cells of the villi are columnar

epithelial cells responsible for absorption and secretion. These cells are 22 to 25 μm high with basally

located nuclei. The apices of these cells have microvilli, produced by numerous folds in the apical

membrane, which account for the brush border appearance. Microvilli are approximately 1 μm long and

0.08 μm wide. Their surface is coated by glycoproteins rooted in the cell membrane. These glycoprotein

filaments are referred to as the glycocalyx and are essential for digestion and absorption.3 The lateral

membranes of neighboring enterocytes are connected by tight junctions, an apparent fusion of adjoining

plasma membranes just below the level of the brush border. Movement of ions and water can occur by

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either a transmembrane or a paracellular route through tight junctions, which behave as selective pores.

Between the villi lie the crypts of Lieberkühn. Stem cells within the crypts of Lieberkühn are the

source of the four major types of differentiated cells: the absorptive enterocyte, goblet cells,

enteroendocrine cells, and Paneth cells. Absorptive enterocytes differentiate as they migrate from the

crypt compartment up toward the tip of the intestinal villus. Cells then undergo apoptosis and are shed

into the intestinal lumen. Most of the intestinal lining is renewed over a period of approximately 5 days.

Despite the rapid rate of cellular turnover, intestinal epithelial cells exhibit complex patterns of gene

expression that vary according to their location on the two main spatial axes of the gut, the vertical

(crypt-villus) and horizontal (proximal to distal) axes. For example, cells destined to become

enterocytes do not begin to express a variety of genes important in digestion and absorption until the

cells have migrated out of the crypt and up the villus. In addition, many epithelial cell genes are

selectively expressed in the proximal small intestine, whereas other genes are specifically expressed

only in the ileum.4

Several other cell types are present in the mucosa. Mucus-secreting goblet cells are present in both

the crypts and villi. Goblet cells have a narrow base containing the nucleus and a wide apical membrane

with a large number of granules containing mucin. Mucin is secreted in a merocrine fashion by the

goblet cell and functions as a lubricant and has a cytoprotective function.5

Paneth cells are pyramidal cells that reside in the crypt base. They contain large eosinophilic

secretory granules located at their apical surface. It is thought that Paneth cells play a role in host

defense based on their abundant expression of lysozyme and defensins, a family of small peptides that

are also found in human neutrophils and exhibit microbicidal activity toward many different bacterial

organisms in vitro. However, examination of the role of Paneth cells in the small intestine by lineage

ablation in transgenic mice revealed no alteration in host defense mechanisms, thus the actual function

of Paneth cells are yet to be delineated.6

Enteroendocrine (also referred to as amine precursor uptake and decarboxylation, or APUD) cells may

reside in either the crypts or the villi, depending on the particular neuroendocrine substance they

produce. Specific areas of the small intestine have higher concentrations of specific neuroendocrine

substances than other areas. These cells do not contact the intestinal lumen, unlike exocrine cells, and

their secretory granules are located below the nucleus near the basement membrane. This suggests that

these cells secrete their contents into the circulation rather than into the intestinal lumen.

Submucosa

The submucosa is a dense connective tissue layer with a rich network of blood vessels, nerves, and

lymphatics. The submucosa contains Meissner plexus and is the strongest layer of the intestinal wall.

Brunner glands are found in the submucosa of the duodenum and secrete mucus and bicarbonate into

the intestinal lumen. These secretions aid in the neutralization of the gastric acid load which enters the

duodenum. Peyer patches are localized collections of lymphoid follicles that are most prominent in the

submucosa of the ileum. These are typically 8 to 10 mm in diameter and are most abundant early in life,

gradually disappearing with age.

Physiology

4 The small intestine plays important physiologic roles in motility, blood flow, growth, digestion,

absorption, immune function, and endocrine secretion. In fact, the small intestine is the largest

endocrine organ in the human body.7 The secretion of numerous hormones and neurotransmitters are

specific to distinct anatomic zones within the small intestine (Fig. 48-3). There is no specific cell mass

which produces these hormones, but rather individual cells scattered along the gastrointestinal tract.

Gastrin is a peptide produced in the gastric antrum and in the duodenum by the G cells and secreted

into the circulation in response to gastric distension, vagal stimulation, amino acids, and hypercalcemia.

Gastrin exists in three functional forms (G-34, G-17, and G-14). Its release is inhibited by low

intraluminal pH, somatostatin, secretin, gastric inhibitory peptide (GIP), vasoactive intestinal peptide

(VIP), glucagon, and calcitonin. Gastrin binds to CCK2/gastrin receptors on ECL cells causing a release

of histamine which in turn stimulates the parietal cells in a paracrine fashion. Gastrin also causes an

increase in the gastric blood flow and the release of pepsinogen by the chief cells and pancreatic

enzymes from the pancreatic centroacinar cells.

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