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.

picture of real heart

Urinary and Reproductive Systems

Urinary and Reproductive Systems

Introduction
The body must eliminate what it cannot absorb, and it must deliver or receive sex cells to reproduce. These bodily functions are intimate to our sense of well-being, so complications of these functions distress patients greatly. 
The urinary system maintains vital fluid balances and chemistry. As with the respiratory system, the gross anatomy of the organ (the kidney) and the tubing is unremarkable compared to its critical clinical significance. Also, as with the respiratory system, mastery of how the structures appear radiographically is a goal of studying the anatomy.
Sexual reproduction in animals is believed to be a derivation of an original design in which species were self-reproducing that is, of a design that did not involve male and femaleindividuals. To derive the anatomy of sexual reproduction, maleness and femaleness result from co-opting the same tissue zones, but selectively and to opposing degrees. The three basic tissue types grow toward a goal of projection or invagination, which enables the coupling between individuals that is necessary to execute reproductive behaviors.
Development Of the Kidney and Ureter
Recall that mesoderm first condenses into three zones paraxial, intermediate, and lateral plate . Paraxial mesoderm forms the axial skeleton and supporting musculature of the body. The lateral plate mesoderm helps to form the body wall that folds around and makes a trunk of the body. The mesoderm clump between the paraxial and lateral plate clusters, the intermediate mesoderm, is implicated in the growth of the urinary and reproductive systems.
In real space, the wedge of intermediate mesoderm lies beside the primitive aorta, just ventral to the somite columns that are becoming the vertebral bodies. The wedge bulges against the otherwise smooth continuity of the peritoneal membrane layer of cells, so it is referred to as the urogenital ridge. From the very beginning of formation, the ridge has a dedicated nephrogenic cord of cells for the urinary system and a gonadal ridge, or a genital ridge, for the reproductive system . They cooperate because of the convenience of an exit portal that is provided by the former and the intriguing consequences of sex differentiation in the latter.
Even though it cannot do so directly to the outside world, the developing fetus must eliminate waste fluid. For this purpose, an internal system of drainage tubes develops early (and is eventually replaced by a permanent organ and duct). The temporary system forms within the nephrogenic cord as a series of drainage ducts (glomeruli) that connect in a lattice-like fashion to form an exit tube, or mesonephric duct . The duct must go somewhere, and this is where the endodermal gut tube enters the picture. Remember that the urorectal septum of mesoderm bisected the cloacal end of the gut tube and separated it into a natural end to the gut tube (rectum) and a severed remnant with one end presenting to the outside world and the other end connected to the allantois. This severed remnant is well positioned for drainage to the outside world if only something would connect to it 


The severed remnant is a blind pouch of endoderm, formally called the urogenital sinus. The sinus will communicate with the outside world through what used to be the upper half of the cloaca. This will become the urinary and genital orifices of the body, and it now is clear how those portals came to be in front of, a or anterior to, the anal portal. If, however, internal to this portal the urogenital sinus is a blind pouch, then it has nothing to excrete. The mesonephric duct will take advantage of this opportunity and connect the body's fluid-filtering system to an exit way, but only temporarily . Soon after connection, the sinus induces the permanent urinary system organs to form.
The mesonephric duct forms while the embryonic body is still differentiating. This duct is a dynamic latticework of draining tubes, with some forming in the lower part of the body at the same time that older, more superior ones are disintegrating. While the latticework and original function of the duct system disappear, the duct itself persists on each side parallel to the vertebral column. It loses its waste fluid-filtering purpose but is available to be co-opted by the reproductive system.
Focus now on the bottom of the mesonephric duct, where it ports into the back of the urogenital sinus. As soon as this relationship is forged, the mesonephric duct grows an aggressive Buda or diverticulum, from the junction point . This bid is called the ureteric bud, or the metanephric blastema. The term ureteric Bud affirms that the ureter derives from this; the term metanephric blastema implies that this is a revised nephros, or kidney, system. It replaces the defunct mesonephric system.


The origin of the ureteric bud is the complicated part. Once it has appeared, further growth of the ureter and kidney is largely just expansion of the tube (the future ureter) and its ear-shaped cap called the metanephric blastema (the future kidney) . The developing kidney establishes a major vascular connection to the aorta as it becomes the central organ of urea filtration. As it expands, the mesonephric duct persists, but it no longer is connected to regional capillary beds by lattice-like glomeruli. Its upper end is open, and its lower end drains into the back of the urogenital sinus.
If the kidneys were located in the pelvis, this would be the end of the story regarding urinary system development. The kidneys are found more superior, however, overlapping with the lowest ribs and lining the posterior body wall at the same level as the duodenum and pancreas . They must get there from their beginnings deep in the pelvic cavity, but the means by which they ascend are not clear. There is no functional reason for the kidneys to be located where they are in the adult. In part, they probably just drift into the available lumbar gutter as the fetus elongates and the herniated midgut returns to the abdominal cavity. The ureter is now quite elongated, and it remains a slender, smooth muscle tube connecting the kidney to the back of the urogenital sinus. It travels superficial to some structures (common iliac vessels) and deep to others (gonadal vessels) to reach the urogenital sinus, which remains in the pelvis 
The gross anatomy of the adult kidney is appreciated best in a coronal section. The organ matures as five relatively independent clusters of functioning tubules, each with its own dedicated branch of the renal artery. A cortex, which is heavily invested in fascia (renal capsule), houses pyramidal colonies of tubules that drain toward the hilum of the kidney into dedicated calyces at the top of the ureter. Like a stream-and-river system, minor calyces merge into major calyces, which collectively form the renal pelvis, or delta. The conformation of calyces makes the inception of the ureter resemble a multitude of trumpets.

The ureter narrows considerably once it has received the output of the kidney. This narrowing complicates the transport of calcifications that can arise within the renal network (renal calculi, or kidney stones). Sharp edges of kidney stones may snag the inner lining of the ureter and become trapped, leading to smooth muscle spasming of the ureter, visceral pain, and partial restriction of urine drainage.
The right and left kidney ascend to different vertebral levels because of the physical barrier of the liver on the right side. Although both kidneys overlap the margins of the twelfth rib and the first lumbar vertebra, the right kidney is slightly more inferior, as seen in a frontal view, than is the left kidney. Both kidneys come to lie adjacent to the suprarenal glands, which, as their name implies, are found perched atop and alongside the superior pole of each kidney .The two structures are related spatially but are mostly independent functionally.
Development of the Bladder and Urethra
The proximal part of the urinary system processes body fluid and transports it to a storage organ that develops from the urogenital sinus. The storage organ (the urinary bladder) matures in the same manner in both males and females, but its distal continuation to the outside world (the urethra) modifies differently according to sex.
Whereas the mesonephric duct and the ureteric bud both developed from intermediate mesoderm, the urogenital sinus is an endodermal structure. As with other endodermal derivatives of the gut tube, it is enveloped by a layer of mesoderm that has contractile properties (smooth muscle). In the adult state, this smooth muscle wrap of the bladder is called the detrusor muscle.
The urogenital sinus has three distinct parts: vesical, pelvic, and phallic . At the top end, the allantois, that finger-like blind pouch now imprisoned at the base of the umbilical cord, balloons out as the vesical region to form the urinary bladder. It expands because the mesonephric duct and its associated ureteric bud invade the wall of the sinus . The tip of the allantois, however, stays tucked into the tight opening of the umbilical cord. It normally withers in place until it is just a fibrous band of tissue tethering the bladder wall to the inside of the abdominal wall (the urachus). If the allantois does not wither, however, then urine accumulating in the bladder can leak through the patency into a cyst near the umbilicus or even dribble out of the umbilical knot (a urachal cyst, or fistula) 
The pelvic part of the urogenital sinus is unremarkable. It simply connects the swelled bladder to the skin barrier, where the urinary orifice will be. In the mature state, this narrowed part of the sinus will be called the pelvic urethra and the membranous urethra. It is most notable clinically because in males it elaborates to form the secretory structures of the prostate gland. Prostatitis and prostate tumors, thus, impinge on the urethra, resulting in the major symptom of impaired micturition (difficulty voiding urine).
The phallic part of the urogenital sinus is quite remarkable. It has the same original configuration in all embryos, but it is radically altered in the presence of a Y chromosome (i.e., in males). The embryonic phallus is simply a bulge in the body wall just above where the gut tube exits the body . In fact, the underside of the phallus is formed by the upper portion of the cloacal membrane. When the urorectal septum separates the cloacal membrane into an anal orifice and a urogenital orifice, the urogenital orifice is the part that remains along the underside of the phallus.


The phallus transforms differently in males than in females. In females, the phallus does not elaborate, so even though it is relatively large in the fetus, it is relatively small in the mature state. The relationship of the urogenital orifice to the phallus barely changes in females, as displayed in an external view of fetal development . Indeed, the external structure of the mature female phallic region is a minimal modification of the fetal design. The phallic part of the urogenital sinus has very little length so the urethra in females is a short shunt from the bladder to the outside world.
In males, the phallus elongates and takes the phallic portion of the urogenital sinus with it. This means that the underside of the phallus is grooved by a long, slit-like orifice, which at this stage may be called the urethral groove . In the mature state, this groove closes over and gets swallowed up within the phallus, where it is called the penile urethra. Despite being closed up inside the phallus, it must still get to the outside world, and this is a very interesting part of genital development .
In summary, the body develops a filter for collecting fluid waste and uses a remnant of the gut tube as a route for this fluid to leave the body. The filter begins as a simple sink and tube design called the mesonephros (collecting tubules connected by a mesonephric duct). This temporary filter ultimately is replaced by its own bud, the metanephros. The metanephros expands to form a single filtering organ (the kidney), which is connected to the gut tube remnant by its own root (the ureteric bud, which is the future ureter). This filter empties fluid waste into the gut tube remnant, which by now has an expanded top end (the bladder) and a narrowed, tubular bottom end (the urethra). The outer opening of the urethra is the adult derivative of the urogenital membrane half of the original cloacal membrane. Its configuration differs between males and females.


Reproductive System
The anatomy of the reproductive system obviously differs between males and females, but how and why? The why part involves genetic coding for the production of hormones that influence how cell colonies differentiate. The how part is the story of what happens to the discarded mesonephric duct.
The relevant cells are in the intermediate mesoderm . One border of the mesoderm column is called the genital ridge, or the gonadal ridge, because germ cells cluster within it. The gonad will become the testis in the male and the ovary in the female, but they both arise in the back of the abdominal area within this gonadal ridge. Close to this ridge is the mesonephric duct, which runs vertically (longitudinally) down from the ridge and empties, as noted above, into the bladder. The important event to notice is how the peritoneal lining of the intermediate mesoderm escapes in, a or gets drawn into, the mesoderm column itself . It is as if the mesonephric duct wanted a partner, so it drew in the border of its own territory into a second, parallel tube. This incorporated sleeve, or tube, logically is called the paramesonephric duct.
These two ducts factor heavily in the differentiation of male from female, which is logical given that there are two different adult pathways (male and female) and two different usable ducts (mesonephric and paramesonephric). We now describe how the gonadal ridge uses one, but not the other, depending on the sex identity of the germ cells.


Female Reproductive Anatomy
Female identity results from an XX configuration of the twenty-third chromosome and subsequent formation of primordial germ cells in the yolk sac. These germ cells migrate toward the gonadal ridge. As they collect along the gonadal ridge, the ridge cells harbor them closely. Females develop a finite number of primary egg cells, or oocytes. They must be guarded closely and released sparingly (typically one per month for ~30“35 years). This may be why early in development the ridge cells form a capsule around the germ cells that prevents the germ cells from establishing a relationship with the nearby tubules of the mesonephric duct . Remember that the tubules disintegrate as the kidney and ureter mature, so with no ability to drain egg cells from the female gonad, the mesonephric tubules are resorbed. In the absence of anything draining into the mesonephric duct at its top end, the duct also dissolves.
The paramesonephric duct remains as the most likely escape route for egg cells that the female gonad periodically releases. Physically, however, it is not related to the developing ovary, so at best, it can be a kind of sink for collecting the expelled egg . This is one major challenge. The other major challenge is what to do with the egg once it has been collected by the open end of the paramesonephric duct. To meet this challenge, the paramesonephric duct distorts into a holding chamber called the uterus . This is no small feat.When the paramesonephric ducts first formed, they were just invaginations of the edge of a column of cells. The top end was open to the peritoneal cavity and so was the bottom end. If you were swimming about in the peritoneal cavity you could enter the top end of the duct and emerge, through the bottom end of the chute, in the very same peritoneal cavity. Likewise, if an egg cell is expressed through the peritoneal membrane by the ovary and gets drawn into the top end of the paramesonephric duct,it would end up back in the peritoneal cavity at the bottom of the abdomen unless the bottoms of the right and left paramesonephric ducts sealed themselves together and swelled into a holding chamber, which is precisely what happens .

The uterus thus forms in the midline of the body, with two uterine tubes (Fallopian tubes, oviducts, or salpinges) reaching up on either side toward the encapsulated ovary. The merger of the paramesonephric ducts at their bottom ends, however, creates a closed loop. The uterus must reach the outside world so that two important things can happen: First, the germinating cell from a male can reach the egg, and second, the resulting conceptus can be birthed. Toward this goal, the paramesonephric duct, which is now the uterus, does what the mesonephric duct also did it ports into the back of the urogenital sinus.
Unlike the mesonephric duct, which actually pokes into the sinus and opens a hole, the uterine union of the paramesonephric ducts only neighbors against the surface of the sinus . After all, the bottom ends of the ducts already have sealed together into the uterine chamber; at best, it can abut the sinus wall with its own wall. When the wall of the uterine chamber impacts the wall of the sinus, the uterine wall induces the sinus wall to stretch out, and eventually, a cavity forms within the wall of the sinus itself. This is the early stage of the vaginal cavity. The top of the cavity, which is formed by the original impact of the uterine wall and the sinus wall, erodes, but the bottom of the cavity remains an intact wall. This will be important later.
At this point, therefore, the uterine cavity is on the verge of breaching the urogenital sinus cavity (the future bladder). Rather than commingle the pathway for fluid waste and germ cell transport, however, the uterovaginal cavity establishes its own advent to the outside world. Using its intact lower wall as a guide, the uterovaginal cavity pivots and drives its lower wall directly toward the urogenital orifice underneath the phallus . It effectively extends itself into a canal parallel to the bladder and its urethra. This, of course, is the typical adult female configuration, in which the vaginal orifice is located behind, or inferior to, the urethra and the vaginal canal and uterus are located directly behind the bladder in the midline .


During this entire process, the lower wall of the vaginal canal remains intact, effectively shutting off the female reproductive system from the outside world. For some time after birth, this wall (the hymen) persists. It disintegrates at a variable point in time before onset of the first mensis, or reproductive cycle.
As the kidneys are ascending to their mature location, the gonads descend away from their original location. In the female, the ovary remains tightly shrink-wrapped by the peritoneal lining against which it grew. The ovary migrates inferiorly to the edge of the pelvic brim (the inner rim of the pelvic skeleton), guided by a cord of tissue called the gubernaculum. For reasons that still are not clear, the gubernaculum in the female is weaker than the gubernaculum in the male”to the extent that in the female, it fails to pull the gonad very far at all. The ovary migrates approximately halfway around the abdominal wall before coming to rest in the pelvic cavity just below the pelvic brim. The gubernaculum persists, but as a loose, fibrofatty band of tissue called the round ligament .
The uterine tubes remain physically apart from the ovary, with the tubes being separated from it by the layer of peritoneum that coats the ovary capsule. During ovulation, when the ovary releases an egg, the egg itself pierces the peritoneal lining on the ovary and, for the briefest of moments, enters the peritoneal cavity. Very nearby is the wide opening of the top of the uterine tube (formerly the paramesonephric duct), and the egg typically is sucked into, or flows into, the opening and travels down the tube into the uterine chamber. This is the one and only natural occurrence of peritoneal sac rupture.

The complicated part of the female reproductive system is now complete. The shapes of the external surfaces of the system barely differ from those of the early fetal stage . The female external genitalia are a maturation of the basic plan. The male genitalia, by contrast, transform the basic plan. This basic plan consists of the genital tubercle, or phallus; the folds of skin that form around it; and the urogenital orifice that runs under it. Two skinfolds are important to realize. The outer one is a swelling of the ectoderm and underlying fascia (loose mesoderm) of the abdominal wall called the labioscrotal swelling. In females, this fat-filled, pendulous pouch presses against the one from the other side as an effective, if passive, closure of the vestibule containing the openings to the urinary and reproductive systems.
Medial to these labioscrotal swellings are less pudgy folds of ectoderm that closely parallel the vestibule; they run the length of the original urethral groove, or urogenital orifice, along the underside of the phallus. These are called the urogenital folds. They have no fatty layer underneath them, but they do house a spongy body of tissue that can store a large amount of blood”it has the capacity to become turgid with fluid pressure. In females, this spongy body is called the bulb of the vestibule. In the mature female, the labioscrotal swellings become the labia majora, and the urogenital folds become the labia minora, both of which attend the vestibule that holds the now-separated openings to the urethra and the vaginal canal .

As mentioned above, the phallus in the female does not proliferate. In the mature state, it remains a bulb of ectoderm at the top of the vestibule. Like the labia minora, this part of the phallus houses a rich vascular bed that can swell with blood (the corpus cavernosum). The tip of this bed, the midline culmination of the genital tubercle, is the clitoris in the mature female.
Male Reproductive Anatomy

The transformation to maleness starts from the same basic design of mesonephric and paramesonephric ducts . As the germ cells accumulate along the gonadal ridge, however, no firm capsule encloses them. Males produce an almost infinite number of sex cells throughout their adult lives, so the imperative is not to harbor them closely but to release them. An avenue for this release is conveniently in place in the form of the original collecting tubules of the mesonephric duct . Thus, in the presence of male sex cells, the mesonephric duct remains intact as the most convenient portal for their transmission, because it is linked to a large number of tubules. The duct persists as the ductus deferens, connected to the gonad via the mature tubules, now called the rete testis. This induced preservation signals the death of the paramesonephric duct, which begins to degenerate at the same time.
In keeping with the theme of minimization of male internal reproductive anatomy, the released sex cells simply co-opt the urinary pathway to reach the outside world. The mesonephric duct does not have to manufacture a new facility for handling the released sex cells, because it already is ported into the back of where the bladder becomes the prostate. The real energy of development during the male transformation is spent in two other ways first, getting the gonad to descend to the very bottom of the trunk so that it can suspend away from the body, and second, growing a genital extension of the urethral groove.

Core body temperature is too hot for sperm cells housed in the gonad to survive. The gonad needs to free itself of this temperature trap, but how? The remarkable process of male gonadal descent bubbles out the bottom of the abdominal wall into a sac that is so thin the gonad it holds enjoys the cooling effect of being outside the body cavity and as exposed to the elements as the nose or the fingertips. This, of course, makes that part of the abdominal wall vulnerable to other internal pressures. Many of these manifest as hernias; thus, studying the anatomy of the abdominal wall must include an emphasis on the inguinal canal and the opening forged by the descending gonad.
As noted above, the gonads develop in the back of the abdominal region in the fetus. As noted below, the mesoderm of the abdominal wall forms a three-layered sandwich of muscles. The gonad develops in the space between the innermost layer of this sandwich and the peritoneal membrane . In males as well as females, a gubernaculum of fibrofatty tissue will direct the gonad around the abdominal wall. In males, however, the gubernaculum tows the gonad all the way around to the front of the abdominal wall. As it does so, it induces the wall to pouch outward such that the labioscrotal swelling becomes a true scrotal sac. In keeping with conservative embryologic patterning, all tissue layers between the gonad and the skin follow the pouch”the transversalis fascia, the internal abdominal oblique, the external abdominal oblique, the superficial fascia, and the skin . The innermost wall muscle (the transversus abdominis) does not extend inferiorly enough to be in the path of the inguinal canal, so it does not participate in the formation of the spermatic cord. Once pouched away from the abdominal wall, these tissue layers are called the internal spermatic fascia, the cremaster muscle, the external spermatic fascia, the dartos muscle, and the skin, respectively .

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The testis must still be connected to the inside world of the body, so a neurovascular bundle and a sperm duct still connect to it and travel between it and the body cavity through the inguinal canal. The testicular artery of the bundle remains a direct branch off of the abdominal aorta, which means that it runs a rather winding course down and around the body wall to keep track of the migrated gonad . The ductus deferens, which is a remnant of the mesonephric duct, connects the testis to the bladder as originally designed, so it curls into the pelvic cavity immediately on leaving the inguinal canal at the deep inguinal ring.
The point at which the scrotal sac first pouches away from the abdominal wall represents a significant weakening of the wall, because each tissue layer bubbles out and stretches considerably. This point of the abdominal wall is called the superficial inguinal ring, and it is the location of inguinal hernias when excessive internal pressure forces the gut tube either through the inguinal canal (indirect hernia) or through the remaining muscular wall of the abdomen between the superficial inguinal ring and the midline (direct hernia) .

If the gubernaculum successfully draws the testis into the scrotal sac, the gubernaculum then reduces to nothing more than a miniscule ligament that binds the testis to the bottom of the sac. Success is not guaranteed, however, and the testis may get trapped along the way and take up to a year after birth to descend completely. The condition of incomplete testis descent is called cryptorchidism. 
Remember that unlike the developing ovary, the maturing male gonad allies more closely with the collecting tubules than with the peritoneum that lines the gonadal ridge. The medulla of the testis becomes a production factory for male sex cells, and it remains interfingered with the mesonephric tubules. The other tubules of the mesonephric duct degenerate as part of the planned transfer of urinary function to the metanephros. The mesonephric duct, however, remains (and is now the ductus deferens, or vas deferens), and the junction of it and the metanephros with the bladder becomes a point of interest.
In the mature state, the ureter and the ductus deferens enter the back of the bladder in different places, but in the fetal state, they are two branches of a single trunk that is rooted into the back of the urogenital sinus . This trunk progressively burrows into the back of the sinus (now the bladder) such that it melds into the wall of the bladder itself in the shape of a triangle, or trigone. This burrowing absorbs the full extent of the shared tube between the mesonephric and metanephric ducts, and in the end, each tube empties into the bladder in a different place. This difference is affected by the descent of the gonad, which causes the mesonephric duct (ductus deferens) to bend around the perimeter of the bladder. This detailed moment of development is important because it explains two things about adult male anatomy first, why the ductus deferens appears to drape over the ureter, and second, why the ductus deferens empties into the prostate below the bladder (rather than the back of the bladder, where the process began) . This prostatic position of the sperm duct enables a basic separation of collected urine (in the bladder, trapped by a sphincter) and semen, which enters the urethra below the bladder.
Clinical Anatomy
 Cryptorchidism
The birth of a child is one of the most exciting, but also distressing, moments of life. When parents first see their baby, they naturally want everything to look normal. Development, however, can be interrupted, perturbed, or even just slightly out of sync. One of the most valuable roles you can play as a health care professional is to explain a newborn baby's condition to worried parents. For example, parents expecting a boy may be distressed at the sight of a scrotal area that looks asymmetric or flat. This may be a result of cryptorchidism, a condition in which the gonad has not descended completely into the scrotal sac.
In approximately 3% of newborn males, a testis or testes remain undescended at birth. Normally, the tardy gonad will descend during the first three months after birth. If the gonad remains sequestered, however, it may fail to mature properly, which can lead to infertility, renal problems, and testicular tumors.

To preserve the integrity of the sex cells in the common channel of the urethra, accessory organs of reproduction form near the base of the bladder. One is the seminal vesicle, which is a pouch of the bottom of the mesonephric duct itself, much like the original ureteric bud. The other is the prostate gland, which is a ballooning of the lining of the urethra itself, packaged within a coat of mesoderm . Anatomically, the prostate gland surrounds the initial part of the urethra. The ductus deferens receives the output of the seminal vesicle, at which point the duct is called the ejaculatory duct. This duct is embedded in the substance of the prostate, so the point at which male sex cells first enter the urinary system is in the prostatic urethra . Because the urethra is wholly enclosed in the prostate gland, enlargement of the prostate because of prostatitis or prostate cancer can constrict the flow of urine. The frequent urge to urinate followed by diminished flow is a primary symptom of prostate disease.


Male and female reproductive anatomy logically is complementary. Female internal anatomy is elaborate, but the external anatomy is little modified. Male internal anatomy is especially minimal, but external anatomy is greatly modified. Male anatomy seizes on the opportunity to use the phallus as an extender of the urethra. The original tissues available to be incorporated are the same as those for the female the genital tubercle, the labioscrotal swelling, and the urogenital folds bordering the urethral groove . In males, the genital tubercle drives the forward development of the reproductive system.
As described earlier, the underside of the tubercle is grooved by the urethra, which is the exit point for excretion of urine and ejaculation of sex cells. As the tubercle elongates in males, it draws the groove out along with it. The vascular bed of the tubercle is described as cavernous because it can retain a large volume of blood. We identify this major portion of the developing penis as the corpus cavernosum, or cavernous body, and this portion is homologous to the body of the clitoris . The urogenital folds also extend along the length of the penis because as in the female, they border the urethral orifice. At the top, or forward, end of the urethral groove, the urogenital folds come together to form a natural boundary to the groove. This boundary cap mushrooms into a kind of head to the penis. In the mature state, the urogenital fold in the male is called the corpus spongiosum, or spongy body, and its top end is called the glans penis.

Clinical Anatomy
: Hypospadias
If the spongy body of the penis fails to fuse over the urethral groove, the newborn's urethra will open along the underpart, or ventral part, of the shaft instead of at the tip of the head. Called hypospadias, this condition can range from a minor transposition of the urethral orifice to a major, slit-like opening in the shaft. The baby can still urinate, just not from where you might expect it. Indeed, many cases of hypospadias are clinically benign or go undetected and untreated until adolescence, when boys take a deeper interest in their own bodies. By knowing the way in which the penis develops, you can help to explain the treatment options for this simple, but strange-looking, condition to the parents.

The anatomic configuration now has the basic shape of a mature penis, but the long opening of the urethra looks very strange along the underside of the penile shaft. In normal development, the urogenital folds will zip together and close over this exposure. This solves the problem along the penile shaft, but it creates another problem the urethra now has no opening to the outside world. To solve this problem, the top, or forward, end of the spongy body, now the glans penis, cavitates, or bores a hole into itself . The glans penis thus develops a pit that tunnels inward until it reaches the enclosed urethra. Failure of this process to complete leads to a relatively common clinical anomaly of the male reproductive system called hypospadias.