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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.