Accessory Organs of Digestion

Accessory Organs of Digestion

Human dietary needs are broad. We require a wide variety of food types, many of which challenge the digestive system. Logically, the structures that help you to digest will be located at the top end rather than at the bottom end of the system. The accessory organs of digestion include the liver, the gallbladder, and the pancreas, each of which derives from the foregut. Moreover, they each bud off of the foregut within the other unique aspect of foregut anatomy the ventral mesentery. The fact that the foregut is the only region containing both accessory organs of digestion and a ventral mesentery is no coincidence, of course.

FIGURE 3.10 The duodenum is whipped around by the liver and stomach.
Like the tail of a dog or the end of a whip, the duodenum bends into a C-shape, cocks upward with ascension of the liver, and swivels back to the body wall.

Liver and Gallbladder

The liver begins to bud off of the foregut tube during the fourth week of embryonic growth. At this point, it is simply called the hepatic diverticulum (Fig. 3.11). The top, or cranial, part of the diverticulum goes on to become the liver, which quickly becomes the largest organ in the fetus. Blood cell production is an early function of the liver. The smaller, bottom part of the diverticulum becomes the gallbladder. Together, the liver and gallbladder lie within the ventral mesentery in the upper right quadrant, where the rapidly expanding liver has migrated as a result, in part, of the stomach expansion. This creates a dynamic in which the upper half of the abdominal cavity is dominated by a stomach on the left and a liver on the right, with a stretched ventral mesentery lying in between them (see Figs. 3.6 and 3.7).

FIGURE 3.11 Origin of the accessory organs of digestion.
The accessory organs of digestion (liver, gallbladder, and pancreas) first emerge as buds of the foregut tube in the space provided by the ventral mesentery. (A “B) As the organs enlarge and move, the foregut mesentery goes with them and persists in the same way that the dorsal mesentery does. (C  ,E)

The ventral mesentery continues beyond the liver to the anterior abdominal wall; in the adult, this film of tissue is called the falciform ligament. Remember that the mesenteric space (between the two layers of mesoderm that form it) is available as a route for nerves and blood vessels to travel through the abdominal cavity without puncturing or being inside the peritoneal sac. The falciform ligament provides just such an opportunity.

The liver grows so large that it impacts the diaphragm above it, much like a helium balloon that rises to the ceiling. This compression of the liver, coated by mesentery, against the diaphragm, likewise coated on its underside by the somatic layer of mesoderm, erodes the coatings and leaves the liver tissue in contact with the fascia of the diaphragm. This is called the bare area of the liver (Fig. 3.12). At the margins of the bare area of the liver, the mesoderm coating reflects onto the adjacent diaphragm. The peritoneal sac is still sealed shut along these reflections, but a number of blind pouches are left where fluid within the sac can accumulate.

FIGURE 3.12 The liver ascends during growth (A) and impacts the diaphragm (B).
This impact pushes back (reflects) the peritoneal coating of the liver and of the diaphragm, making a kind of bare area on the top of the liver. The liver essentially fuses to the diaphragm, which seals off the reflected arcs of peritoneum and keeps the peritoneal sac a closed space

The gallbladder forms because the liver produces more bile than the body needs, and this bile must be stored somewhere. As the liver is expanding from the original bud off of the foregut tube, the connection that it maintains to the tube winnows into a narrow bile duct. A part of this duct pouches out to form the passive gallbladder and the cystic duct that connects it back to the bile duct (see Fig. 3.11D,E). Because gallbladder problems are common clinical presentations, the specific position and name of all the ducts and blood vessels near it are important to learn.
The liver and gallbladder bud off of the gut tube at virtually the lower, or distal, limit of the ventral mesentery. With the gut tube in its original, linear state, this lower limit of the ventral mesentery is shaped like the bottom of a sling, and it forms a sort of trough. After the stomach and liver have rotated, sagged, and risen, this lower limit of mesentery is oriented straight up and down, and it faces to the right (see Fig. 3.7A). It forms the perfect sling for transmission of the ducts that connect the liver and gallbladder back to the gut tube. Their ducts form an elegant, branching design before joining with the duct from the pancreas right before entering the proximal part of the duodenum (see Fig. 3.11E). These hepatic and bile ducts run within the sling at the distal end of the ventral mesentery, which in the adult is termed the hepatoduodenal ligament.
This ligament ensheaths three important, large structures related to the physiology and circulation of the gut tube: the hepatic artery, which serves the foregut organs; the portal vein, which delivers all gut tube venous blood to the liver; and the bile duct, which is the site of common clinical disorders (e.g., gallstones). These three structures are collectively called the portal triad, and recognizing their gross anatomy during mobilization or surgery of the bowel is a critical skill.

Pancreas

The pancreas is the final accessory organ of digestion that forms from the foregut tube. It actually begins as a separate dorsal bud and ventral bud, each with its own connecting duct to the foregut. The dorsal pancreatic bud generally is larger, and the ventral bud eventually rotates toward it (Fig. 3.13). As with many tissue structures that are similar to one another, once the ventral and dorsal buds come into contact, they functionally fuse. The fused pancreas stays connected to the duodenum through the main pacreatic duct, which also incorporates the bile duct. Thus, just before they enter the wall of the duodenum, the main pancreatic duct and the bile duct merge to form a hepatopancreatic ampulla. The ampulla invades the wall of the duodenum at a location called the major duodenal papilla. Thus, all the efforts devoted to developing accessory organs of digestion converge into one small input line.

FIGURE 3.13 The pancreas forms from two buds.
The ventral bud, which is connected to the base of the bud that grows the liver, rotates in concert with the duodenum (A). When it merges with the dorsal bud (B), the main and accessory ducts usually merge as well.

Of the accessory organs of digestion, the liver and gallbladder remain intraperitoneal, whereas the pancreas migrates to a retroperitoneal position. The liver and gallbladder receive all their arterial blood supply from branches of the celiac trunk, but the pancreas receives blood from both the artery of the foregut (celiac trunk) and the artery of the midgut (superior mesenteric) (see Fig. 3.8).

Midgut

The midgut includes the long run of bowel between the proximal duodenum and the transverse colon (Fig. 3.14). The great length of the midgut is achieved by an unusual growth process in which the gut tube is excused from the fetus through the umbilical hiatus and then, substantially elongated, is returned with significant rotation.
The adult parts of the gut tube that develop from the midgut are the rest of the duodenum, the jejunum, the ileum, the cecum, the ascending colon, and part of the transverse colon. You can think of the midgut as the length of the small intestine and part of the large intestine. Basic midgut function is simple to squeeze the digested food matter against the inner, absorbing surface of a very long tube to extract nutrients that have been released by the digestion initiated in the foregut. The midgut both elongates greatly and rotates during its development. It also remains tethered to the posterior abdominal wall by the dorsal mesentery all the while, which explains why this mesentery comes to look like an opened Asian, or Oriental, fan. From the very beginning, it is supplied by the artery of the midgut, the superior mesenteric artery branch of the aorta (Fig. 3.15).

FIGURE 3.14 Midgut derivatives.
The midgut matures into the final third of the duodenum, the entire small bowel (jejunum and ileum), and the ascending and transverse portions of the colon.

The first event during midgut development is elongation, which causes the tube to project ventrally, or toward the umbilicus of the embryo (Fig. 3.16). And it just keeps going. The midgut elongates so much that it actually herniates into the umbilical cord. This is considered to be a normal herniation, or a physiological herniation. The migration is so patterned, in fact, that a cranial limb and a caudal limb of the tube can be identified on opposite sides of an axis formed by the superior mesenteric artery that feeds the midgut (see Fig. 3.16A). The cranial limb super elongates and takes on the squiggly packing of the small intestine while it is herniated into the umbilical cord. The caudal limb, which will become the cecum, appendix, and ascending colon, expands less dramatically before it returns to the fetus. While still in the umbilical cord, the midgut loop rotates 90 counterclockwise around the axis created by the superior mesenteric artery (as viewed from the front). This is the first of three such rotations before the tube finally settles into place during the tenth week of embryonic development. This may help to explain why the beginning of the large intestine is located on the lower right side of the abdominal cavity; it started out as the caudal limb of the midgut herniation. After 270 of counterclockwise rotation, it is banked against the right side of the body wall.
As the midgut tube is growing and rotating, a piece of the caudal limb does not grow at the same rate as the rest of the surrounding tissue much like the tip of a long, skinny balloon as you inflate it. This pouch of slow growth is a diverticulum of the cecum portion of the midgut tube that becomes the adult appendix (see Fig. 3.15B,C). For many people, removal of the appendix is a first exposure to the world of doctors and hospitals. Finding the appendix beneath the skin of the abdomen during a physical examination is based on knowing how the gut tube rotates before birth; because the cecum ends up in the well of the right hip bone (see Fig. 3.15A), you can feel for the appendix between the hip bone and the umbilicus.
After the midgut has elongated, it returns to the abdominal cavity of the fetus in much the same way as you might suck in a strand of spaghetti through pursed lips. The cranial end returns first and collects in a giant squiggle in roughly the middle part of the lower abdomen. Technically, the mesentery that slings it begins at the duodenojejunal junction, or flexure, just to the left of the midline. The mesentery ends at the first part of the elongation to be retroperitoneal, which is the area of the cecum in the lower right quadrant. Thus, the dorsal mesentery of the intestines is rooted to the back of the abdominal wall along an oblique line that runs from upper left to lower right. It slings well over 10 feet of intestinal tube, despite a root that is closer to 10 inches in length. This is what gives the dorsal mesentery at this location a fan-like shape (much longer at its periphery than at its base).
The caudal end of the midgut loop returns to the fetal abdomen along the periphery, which is the only space left available because of the position of the small intestine. Follow the gut tube from the cecum up the right side of the abdomen (ascending colon) to the liver, where it turns medially and runs across the abdomen as the transverse colon. The midgut portion of the tube transitions to the hindgut portion where the major source of blood supply to the transverse colon transitions from the superior mesenteric artery to the inferior mesenteric artery. This is a subtle transition in the sense that the morphology of the transverse colon shows no abrupt change; it merely turns inferiorly along the left side of the abdominal cavity (descending colon) in parallel to the cecum and ascending colon on the right.

FIGURE 3.15 The small intestine suspends like a fan.
The long tube of the small intestine includes a subtle transition from a jejunum to an ileum before emptying into the large cecum. The jejunal part is coiled into the upper quadrants of the abdomen (A) and typically transitions to the ileum in the left lower quadrant. The ileocecal junction (B) and the appendix are key features of the right lower quadrant. The arterial supply from the superior mesenteric arcades through the dorsal mesentery (C). 

FIGURE 3.16 Normal herniation of the midgut.
As the midgut herniates into the umbilical cord (A–C), it also rotates (counterclockwise, as seen from the front). To pack more tube into the same amount of space, the tube “squiggles” into tight coils, which persist in the adult state as the coils of the small intestine. The midgut rotation completes its final turn, and the midgut loop returns to the fetal abdomen. The 270 rotation explains why the cecum ends up in the right lower quadrant (D). The herniation reduces as the tube returns to the fetal abdominal cavity (E).

Hindgut

Compared to development of the midgut and foregut, development of the hindgut is simple. The hindgut must open to the outside world so that waste matter can be expelled, and this junction of the inner tube with the outer world is the focus of hindgut development. When the hindgut tube forms during lateral folding of the embryo, it opens to the outside world through an unmodified bottom end called a cloaca (Fig. 3.17A). Many animal species use this unmodified exit port to eliminate all forms of waste (both liquid and solid) and to expel eggs. Placental mammals modify the cloaca into separate tubes for solid and liquid waste and also make accommodations for the reproductive pathway. Concentrate on this aspect of embryology, because it explains the positional anatomy of the perineal region. Social norms discourage people from learning about this part of their own body despite its considerable clinical relevance. Knowing how it develops is the first step toward mastering its anatomy.

Division of the hindgut cloaca results from interference by mesoderm tissue. A mesoderm colony of cells termed the urorectal septum migrates from its formation point between the hindgut tube and the connecting stalk. Recall from Figures 1.23 and 3.11 that the allantois diverticulum is trapped up within the connecting stalk at this time, so the urorectal septum effectively sits between the blind pouch of the allantois and the hindgut tube proper. As its name implies, the urorectal septum will migrate toward the bottom end of the embryo, and in doing so, it will drive a wedge of mesoderm between the hindgut and the allantois (Fig. 3.17B,C).

FIGURE 3.17 A wedge of mesoderm divides the hindgut.
Recall that the allantois diverticulum is trapped in the connecting stalk (A). A migrating urorectal septum of mesoderm pinches the base of the diverticulum off of the hindgut (B and C). This results in two portals to the outside world, one of which is still connected to the gut tube (anal) and one of which is a blind pouch (urogenital). 

FIGURE 3.18 Hindgut derivatives.
The functional and structural transition between midgut and hindgut is subtle—along the distal portion of the transverse colon. The hindgut develops into the rest of the transverse colon, the descending colon, the sigmoid colon, and the rectum.

To reach the bottom end of the embryo from this position, however, the septum must push through the cloaca. The clump of mesoderm that drives through the endoderm of the cloaca and contacts the ectoderm is now called the perineal body. It divides the former cloacal membrane into a rear part, for what is left of the hindgut, and a front part, for the piece of the hindgut that is still connected to the allantois. This division now gives the body a dedicated outflow track for solid waste (the hindgut) and a blind pouch that terminates just above it as a urogenital sinus for fluid excretion and reproduction. Obviously, more change is in order for this blind pouch (see Chapter 5).

The adult derivatives of the hindgut are the final portions of the large intestine and the rectum. You have anticipated how the hindgut transitions from the transverse colon to the descending colon. As the descending colon reaches the well of the pelvis in the lower left quadrant of the abdomen, it actually lifts off the wall and is once again intraperitoneal. Think of it as laying a tube along the inside of a frame but ending up with more tube than frame. A relatively long stretch of tube must fit between the pelvic brim and the midline of the body, so it fans out just like the small intestine. This region is called the sigmoid colon, and it remains intraperitoneal (suspended by the sigmoid mesocolon) until it reaches the midline. Here, it falls back against the body wall and runs a straight course toward the outside world as the rectum (Fig. 3.18).

FIGURE 3.19 The anal canal meets the outside world.
The bottom of the gut tube is exposed to the outside world, but not without some protection. The ectodermal contact with the end of the tube curls inward, which pushes the endoderm approximately an inch superiorly into the anal canal. This enables voluntary sphincter muscles (see Chapter 7) to keep the anal orifice closed. In keeping with development, the endodermal portion of the anal canal is supplied by a gut tube artery and drains to the liver (via the inferior mesenteric vein), whereas the ectodermal portion is supplied by and drains back into the systemic circulation (via middle and inferior rectal vessels)

Development of the gut tube now is almost complete. The region that abuts (no pun intended) the bottom of the embryo will become the anal canal. This is the other region (besides the mouth) where endoderm meets ectoderm. As this junction develops, the part derived from ectoderm actually invaginates, or curls inward (Fig. 3.19). This protective step keeps the absorbent endodermal lining from constant exposure to the outside world. The bottom of the anal canal is, thus, composed of in-turned skin, which can press against itself through the action of sphincter muscles. This is one area in which shared venous drainage exists between vessels that lead back to the heart directly (caval) and those that lead back to the liver first (portal). The anal canal is, thus, said to be a region of portal-caval anastomosis.

digestive system

Digestive System

Introduction

At the top end of the endoderm (the oropharyngeal membrane), the ectoderm and mesoderm elaborate to protect the sensitive endoderm from direct contact with the outside world. Lips, teeth, and a large oral cavity grow as a kind of security post to guard the gut tube. They physically process what you acquire before you have to absorb it.From this point on or down,  as it were the endoderm conforms to a tube, and it absorbs both beneficial and harmful things alike. Some parts of the tube are more receptive to certain compounds, such as proteins, than to others, and some parts mostly reclaim water that the body has added to the mix to prevent dehydration. Entire organs develop from the tube to assist with the complex task of breaking down molecules before the long and winding intestinal road of absorption. The convoluted gut tube is suspended in the abdominal cavity in a sling of mesoderm, just as the early development of the embryo mandated. Like the lungs That derived from it, the gut tube presses against a closed sac in this case, the peritoneum. The parietal peritoneum is highly sensitive, and it often bears the consequence of gut tube disorders or at least traffics pain referred by them. The diagnosis and treatment of disease states related to the gut tube, however, rely primarily on blood chemistry data, which limit the applications of basic gross anatomy. The primary objectives for learning gut tube anatomy should be to understand the relative position of organs for physical diagnosis and clinical imaging and to master the position and informative qualities of the peritoneal sac against which the tube and its accessory organs grow.
Abdominal Cavity
When we last tracked the endoderm, it had rolled into a continuous tube and was virtually surrounded by a layer of mesoderm (now called visceral, or splanchnic, mesoderm). This arrangement is best appreciated in cross-section (Fig. 3.1). The endoderm develops primarily into the digestive system, which includes the tube itself plus the organs that bud off of it. The tube will expand, convolute, bud organs, and rotate as it becomes the adult gastrointestinal (GI) tract. Its contact with the visceral mesoderm leads to formation of a smooth muscle wall around the endoderm cells, which gives The gut tube a truly tubular appearance. It also enables a mechanical squeezing of the endodermal sleeve (peristalsis) that helps to move the processed food matter, or ingesta, along the system.




FIGURE 3.1 The endoderm folds into a tube.
This classic, cross-sectional view of the folding embryo shows how the endoderm layer of cells folds into a tube (A). As it becomes tubular, it maintains surface contact with the visceral layer of lateral plate mesoderm (B). The endodermal tube becomes suspended in a sling of visceral mesoderm. This sling is made up of two layers (one from each side of the body) and a potential space between them (C).

Note how the aorta is positioned such that it can send a branch to the gut tube between the two layers of the mesoderm that sling the gut tube. This construct of mesoderm is called a mesentery. Because the one shown here reflects off of the back wall of the body, it is called the dorsal mesentery. Remember that the endoderm gives rise only to the epithelium of the gut tube, not to the smooth muscle that acts on it. That smooth muscle is a derivative of mesoderm. When it contracts, the tube is squeezed, and this action facilitates the peristaltic effect of moving food particles along the production line. Part of the tube is found in the adult thorax, or thoracic cavity, and part is found in the abdomen, or abdominal cavity. These two cavities of the body are separated by the diaphragm, which, as you remember, was part of the transverse septum of mesoderm but became relocated during longitudinal folding. We should now finish the story of the cavities before we further examine the development of the tube itself. The lateral folding of the embryo creates a captured cavity, the intraembryonic coelom. This merged cavity space is continuous from top to bottom until the transverse septum cuts the single cavity in half during the longitudinal fold). The upper half becomes the thoracic cavity, and the piece of the coelom that remains there looks like a drooping arch. The heart grows against the bend of the arch, and the lungs grow against the limbs of the arch. Below the diaphragm, the merged limbs of the coelom become the peritoneal sac. The layer of mesoderm that completely lines and, thus, constitutes the peritoneal cavity is now called the peritoneum. Some of it coats the wall of the body (parietal peritoneum), and some of it coats the gut tube (visceral peritoneum). The highly elongated and organ-sprouting gut tube pushes against the peritoneum so much that very little cavity is left in the sac. Thus, fluid accumulation within the sac (ascites) quickly leads to discomfort and provokes medical attention.
Now that we have established how the peritoneal sac is formed, we can proceed to describe the gross anatomy of the digestive system. This relatively simple structural system of the body is incredibly complex physiologically. The clinical spectrum of complications in this system is vast, because its structure is in contact with the outside world and all its impurities. Major pathophysiologies, such as diabetes, cirrhosis, and colitis, result from dysfunctional behavior of this system consequent, in some cases, to consumption behavior. The clinical anatomy of these diseases is less apparent, so in studying the gross anatomy of the digestive system, the objectives are to master the names of its parts, to understand their nerve and blood supply, and to position the tube relative to the body wall that surrounds it.
Esophagus and Foregut The first part of the tube to consider is the section that connects the input hole (the mouth, or oral cavity) with the processing unit (the stomach, intestines, etc.). This part is called the esophagus, and it is located in the thorax. This section of the endodermal tube changes very little from its initial appearance (Fig. 3.2). It remains a flaccid tube surrounded by muscle. The muscle arises from the visceral mesoderm that coated the gut tube after lateral folding (see Fig. 1.16). When the muscles of the esophagus contract, they pulse whatever is inside the esophagus downward. This peristalsis is governed by parasympathetic fibers of the vagus nerve (cranial nerve X). Dysfunction of this process is increasingly common and can lead to gastroesophageal reflux disease (GERD).
The esophagus passes behind the diaphragm, but it projects forward just enough that the diaphragm collars it. The extent to which the diaphragm squeezes the transition between the esophagus and stomach (gastroesophageal junction) may lead to indigestion, reflux of food, and/or heartburn. Heartburn refers to the mistaken sense that the discomfort is in the nearby heart and not the esophagus, which, in turn, might lead the patient directly to the emergency room. The gastroesophageal junction also renders the diaphragm vulnerable to slackening, which could result in a herniation of the gut tube. A sliding hiatal hernia is one in which the entire junction and the upper part of the stomach slide up through the hiatus, creating an uncomfortable pinch of the stomach sac (Fig. 3.3).

FIGURE 3.2 The adult gut tube.
The esophagus is an unmodified tube, just a conduit between where food is initially processed (oral cavity) and where it is digested (stomach and beyond).

Anatomists describe the developing gut tube below the diaphragm as having three regions: a foregut, a midgut, and a hindgut. Each region draws a dedicated artery from the developing circulatory system, so this classification is somewhat logical. The foregut also is the part of the tube that buds off all the accessory organs, so the division of foregut and midgut is even more logical. The transition from midgut to hindgut is more arbitrary, in the sense that both have a similar function of absorption, their nerve supplies overlap, and the exact point at which the circulatory supply of one blends into the circulatory supply of the other is vague.

FIGURE 3.3 Hiatal hernia.
The relationship between the gut tube and the diaphragm is lax enough that the tube can herniate into the thorax, typically by sliding up the esophageal hiatus

The foregut region becomes the stomach, the accessory organs of digestion and the first part of the duodenal portion of the small intestine. All this makes sense considering what the digestive system must accomplish once the ingesta finally gets below the diaphragm. The foregut is the domain of the celiac trunk of arteries (Fig. 3.4), the first of the midline branches of the abdominal aorta. The foregut has one more distinguishing feature. When the septum transversum arrived to divide the thorax from the abdomen, it actually bridged the space from the foregut to the ventral body wall. As the cranial portion of the septum transversum developed into the diaphragm, the caudal portion thinned into a ventral mesentery. Only the foregut has a ventral mesentery (Fig. 3.5). This ventral mesentery, which is exactly similar in design to the dorsal mesentery that runs the entire length of the gut tube, is available to sandwich anything that might bud off from the foregut.


FIGURE 3.4 A dedicated branch of the aorta serves each gut tube region.

The celiac trunk serves the foregut region and the organs that bud from it. The superior mesenteric artery serves the midgut region, and the inferior mesenteric artery serves the hindgut region. Note that the term mesenteric is used here. This implies that the arteries are located within the mesentery between the body wall and the gut tube.

FIGURE 3.5 Formation of a ventral mesentery in the foregut region.
The accessory organs of digestion (liver, pancreas, and gallbladder) derive from the foregut only. Like the gut tube, they rest in a sling of mesoderm, but because the gut tube already occupies the dorsal mesentery, these organs need a mesentery of their own. The ventral mesentery appears to form from thinning of the overlying mesoderm of the septum transversum.

The first task of the foregut is to store the ingesta, and the first structure of the foregut is an inflated part of the tube called the stomach. Structurally, the stomach is simply an expansion of the gut tube to form a larger pouch. Functionally, the stomach secretes a variety of strong acids to reduce the ingesta even more. These acids work effectively on protein compounds.
The stomach is not centered in the middle of the body, which is where it starts out as part of the endodermal gut tube. Indeed, the stomach rotates as it forms (Fig. 3.6). The dorsal border of the foregut expands first, creating a greater curvature along that border and a lesser curvature along the ventral border. At the same time, the tube spins 90 on its own axis because of the rapid growth of the liver (see below). This positions the greater curvature facing the left side. Eventually, this expanded greater curvature sags down so that it points inferiorly, and this is the final position of the normal stomach, the dominant organ in the left upper quadrant of the abdomen (Fig. 3.7).


FIGURE 3.6 The dorsal mesentery of the stomach warps.
The stomach part of the gut tube balloons posteriorly but not anteriorly, resulting in a surface of greater curvature and a surface of lesser curvature. The stomach also rotates (A, B) to accommodate rapid growth of the neighboring liver (not shown). One result is a longer apron of dorsal mesentery, which is known as the greater omentum (C).

Remember that like all parts of the gut tube, the foregut suspends from the vertebral column within the sling of dorsal mesentery created by the visceral layer of mesoderm. The expansion, twisting, and sagging of the stomach region affects this mesentery as well. It follows the position of the greater curvature such that it greatly elongates and folds down like an apron by the end of growth. This apron of mesentery is called the greater omentum (see Fig. 3.7).
Each gut tube region is served by a dedicated artery. The blood supply to the stomach must be from the artery of the foregut, the celiac trunk. A larger point, however, is at play here. Note that the gut tube began as a midline structure running parallel to, but in front of, the vertebral column. The only structure between the two is the dorsal aorta. The shortest possible route for blood to reach the gut tube is as a direct branch of the aorta that runs between the two layers of mesoderm that droop off of the body wall to sling the gut tube (see Fig. 3.1). In the case of the stomach, the arteries are branches of the celiac trunk. However, because the dorsal mesentery of the stomach elongates so much as the foregut expands and rotates, it would not be economical for the blood supply to elongate and hang down like an apron as well. Instead, the blood supply to the stomach approaches from the top of the dorsal mesentery (to catch the very top of the greater and lesser curvatures), or it shuttles in at the bottom of the stomach expansion (to catch the bottom of the greater and lesser curvatures) .

FIGURE 3.7 The stomach and its mesenteries.
The dorsal mesentery persists as the remarkable greater omentum, a double-layer fold of fat-rich connective tissue (A). This unwieldy expanse of mesentery eventually incorporates the transverse part of the colon (B). It has been called the abdominal policeman because of its perceived role in defending the peritoneum by adhering to sites of inflammation, absorbing bacteria and other contaminants, and providing leukocytes for a local immune response. The ventral mesentery persists as the lesser omentum. It cordons a lesser part of the peritoneal sac posteriorly and ensheaths the ducts that connect the accessory organs back to the gut tube.

The portion of foregut distal to the stomach will form the proximal part of the duodenum (Fig. 3.8). This C-shaped tube marks the transition toward the absorbing portion of the gut tube; it also marks the end of the accessory organs that are attached to the tube (see below). Developmentally, the part of the duodenum that forms from the foregut region of the tube is indicated by the persistence of a ventral mesentery (see Fig. 3.7). All subsequent parts of the tube have only a dorsal mesentery.
The duodenum demonstrates a key principle of digestive system anatomy. The gut tube greatly elongates during growth, reaching a linear distance of approximately 20 feet. To package all of that in the small volume of the adult abdominal cavity, the tube must curl and ball up, much like trying to put a long hose in a small box. All this accommodation distorts the relationship of the tube to the dorsal mesentery. The possible outcomes are illustrated in Figure 3.9.


FIGURE 3.8 Regional anatomy of the duodenum.
The duodenum is the C-shaped continuation of the gut tube beyond the stomach (A). It rests in a key region of the abdomen, near each of the accessory organs of digestion and the kidneys, spleen, inferior vena cava, and aorta (B


FIGURE 3.9 The tube pushes against the peritoneum to varying degrees.
Because of cramped spacing in the abdominal cavity, the relationship of the gut tube to the dorsal mesentery distorts during growth. In some regions, the gut tube is pushed back against the body wall, effectively removing the dorsal mesentery. This condition is called retroperitoneal, and the gut tube is essentially fixed in space against the back of the abdomen. In other regions the dorsal mesentery expands and twists dramatically to give the gut tube maximum flexibility and mobility.


Remember that the mesentery is really just two layers of mesoderm with a space between them. Part of this space is occupied by the gut tube, and part of it is empty except for the blood vessels and nerves that must serve the tube and its coating. Sometimes, the tube pulls farther away from the vertebral column, thus stretching the dorsal mesentery. This gives the tube the property of being very bendable and movable in the abdominal cavity, because it is swinging more freely from the support post of the vertebral column. The jejunum and ileum of the small intestine are examples of this condition. Because it appears as though the tube is completely surrounded by the visceral mesoderm, this condition is called intraperitoneal. The tube is not inside the peritoneal sac, but you will appreciate this best if you follow the developmental possibilities (see Fig. 3.9). Parts of the tube are pushed back against the body wall by pressure from other organs. This condition is called retroperitoneal, because the whole tube appears to be behind the visceral mesoderm that forms the lining of the peritoneal sac. These parts of the tube are fixed in position, and they are only blanketed by the tangent peritoneal membrane. The duodenum has both an intraperitoneal part and a retroperitoneal part (Fig. 3.10). The first part of the duodenum, derived from the foregut, is intraperitoneal; the remaining two-thirds of the duodenum are retroperitoneal. The duodenum is really at the mercy of the developing stomach and the large liver. This means that as the stomach spins on its long axis and sags to the left, the duodenum is kicked up to the right, and in the end, the convexity of the C in the C-shaped duodenum lies to the right of the vertebral column (see Fig. 3.10). The position of the duodenum across the level of the first few lumbar vertebrae will prove to be a very busy area of the abdominal cavity.