138 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Development of Stomach
During the fourth week, a slight dilation of the tubular foregut indicates the site of the primordial stomach. It first appears as a fusiform enlargement of the caudal part of the foregut that is oriented in the median plane (see Fig. 12-2B). The primordial stomach enlarges and broad- ens ventrodorsally. Its dorsal border grows more quickly than its ventral border. This site of rapid growth demar- cates the greater curvature of the stomach (see Fig.
12-2D).
Rotation of Stomach
As the stomach enlarges, it rotates 90 degrees in a clock- wise direction around its longitudinal axis. The effects of rotation on the stomach are (see Fig. 12-2 and Fig. 12-3):
•
The ventral border (lesser curvature) moves to the right and the dorsal border (greater curvature) moves to the left (see Fig. 12-2C to F).•
Before rotation, the cranial and caudal ends of the stomach are in the median plane (see Fig. 12-2B).•
During rotation and growth of the stomach, its cranial region moves to the left and slightly inferiorly, and its caudal region moves to the right and superiorly (see Fig. 12-2C to E).•
After rotation, the stomach assumes its final position, with its long axis almost transverse to the long axis of the body (see Fig. 12-2E). This rotation and growth explains why the left vagus nerve supplies the anterior wall of the adult stomach, and the right vagus nerve innervates its posterior wall.These foregut derivatives, other than the pharynx, lower respiratory tract, and most of the esophagus, are supplied by the celiac trunk, the artery of the foregut (see Fig. 12-1 and Fig. 12-2A).
Development of Esophagus
The esophagus develops from the foregut immediately caudal to the pharynx (see Fig. 12-1). Initially, the esoph- agus is short, but it elongates rapidly and reaches its final relative length by the seventh week. Its epithelium and glands are derived from the endoderm. The epithelium proliferates and, partly or completely, obliterates the esophageal lumen; however, recanalization normally occurs by the end of the eighth week. The striated muscle of the esophagus is derived from mesenchyme in the fourth and sixth pharyngeal arches (see Chapter 10, Figs.
10-1 and 10-5B). The smooth muscle, mainly in the infe- rior third of the esophagus, develops from the surround- ing splanchnic mesenchyme.
Figure 12–1 Drawing of median section of a 4-week embryo, showing the early alimentary system and its blood supply.
Septum transversum Omphaloenteric duct and vitelline artery Allantois Anal pit
Cloacal membrane
Cloaca Hindgut
Inferior mesenteric artery
Superior mesenteric artery to midgut
Primordium of liver Celiac trunk Gastric and duodenal regions Esophageal region Aorta
Heart Pharynx Stomodeum
10
Blockage (atresia) of the esophageal lumen occurs in approx- imately 1 in 3000 to 1 in 4500 neonates. Approximately one third of affected infants are born prematurely. Esophageal atresia is frequently associated with tracheoesophageal fistula (see Chapter 11, Fig. 11-5 and Fig. 11-6). The atresia results from deviation of the tracheoesophageal septum in a posterior direction (see Chapter 11, Fig. 11-2 and Fig. 11-6);
as a result, separation of the esophagus from the laryngotra- cheal tube is incomplete. In some cases, the atresia results from failure of esophageal recanalization during the eighth week of development. A fetus with esophageal atresia is unable to swallow amniotic fluid, resulting in polyhydram- nios, the accumulation of excessive amniotic fluid.
C H A P T E R 12 ALIMENTARY SYSTEM 139
Figure 12–2 Illustrations of the development and rotation of the stomach and formation of the omental bursa and greater omentum. A, Median section of the abdomen of a 28-day embryo. B, Anterolateral view of the embryo shown in A. C, Embryo of approximately 35 days.
D, Embryo at approximately 40 days. E, Embryo of approximately 48 days. F, Lateral view of the stomach and greater omentum of an embryo of approximately 52 days. G, Sagittal section, showing the omental bursa and greater omentum. The arrows in F and G indicate the site of the omental foramen.
A
B C
D E
F G
Brain
Primordial dorsal mesogastrium Stomach
Dorsal abdominal wall Dorsal aorta
Dorsal aorta
Esophagus
Esophagus
Proximal part of stomach
Spleen
Primordial dorsal mesogastrium
Celiac trunk
Primordial ventral mesogastrium
Posterior abdominal wall Aorta
Dorsal aorta
Stomach
Stomach
Omental bursa (lesser sac)
Dorsal mesogastrium Omental
foramen
Omental bursa
Dorsal abdominal wall Greater omentum
Omental bursa (area indicated by broken line)
Omental foramen
Greater omentum Level of
section on right
Plane of section on right Greater omentum Greater omentum
Stomach Duodenum
Duodenum
Liver
Pancreas Dorsal pancreatic bud
Spleen
Right gastro-omental artery Greater curvature
of stomach
Foregut artery (celiac trunk) Pharyngeal arch
arteries
Pharynx (cranial part of foregut)
Celiac trunk
Septum transversum Spinal cord
Superior mesenteric artery Inferior mesenteric artery Midgut
Heart
Cloaca (caudal part of hindgut) Omphaloenteric duct
140 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 12–3 Development of stomach and mesenteries and formation of omental bursa.
A, Embryo of 5 weeks. B, Transverse section showing clefts in the dorsal mesogastrium. C, Later stage after coalescence of the clefts to form the omental bursa. D, Transverse section showing the initial appearance of the omental bursa. E, The dorsal mesentery has elongated and the omental bursa has enlarged. F and G, Transverse and sagittal sections, respectively, showing elongation of the dorsal mesogastrium and expansion of the omental bursa. H, Embryo of 6 weeks, showing the greater omentum and expansion of the omental bursa. I and J, Transverse and sagittal sections, respectively, showing the inferior recess of the omental bursa and omental foramen. The arrows in E, F, and I indicate the site of the omental foramen. In J, the arrow indicates the recess of the omental bursa.
A B C D
E F G
H I J
Dorsal aorta Dorsal aorta
Superior recess of omental bursa
Stomach Stomach
Level of section B
Level of section F Plane of section G
Plane of section G
Plane of section J
Dorsal mesogastrium
Omental bursa
Omental bursa Inferior recess
of omental bursa Outline of
omental bursa
Dorsal abdominal wall
Aorta
Omental foramen (entrance to omental bursa)
Omental bursa
Stomach
Greater omentum
Gastric artery
Dorsal abdominal wall
Gastric artery
Gastric artery Stomach
Greater omentum Dorsal aorta
Omental
foramen Omental bursa
Clefts in primordial dorsal
mesogastrium
Primordial ventral mesogastrium
Level of section D
Primordial dorsal mesogastrium
Plane of section J
Level of section I
Entrance to omental bursa
C H A P T E R 12 ALIMENTARY SYSTEM 141
caudal part of the foregut early in the fourth week (see Fig. 12-4A and Fig. 12-6A). Wnt/β-catenin signaling is involved in the induction of the hepatic diverticulum.
The diverticulum extends into the septum transversum (see Fig. 12-6B), a mass of splanchnic mesoderm between the developing heart and midgut. The diverticulum enlarges and divides into two parts as it grows between the layers of the ventral mesogastrium (see Fig. 12-4A).
The larger cranial part of the diverticulum is the primor- dium of the liver; the smaller caudal portion becomes the gallbladder. The proliferating endodermal cells give rise to interlacing cords of hepatocytes (parenchymal liver cells) and to the epithelial lining of the intrahepatic part of the biliary apparatus. The hepatic cords anastomose around endothelium-lined spaces, the primordia of the hepatic sinusoids. The fibrous and hematopoietic tissue and Kupffer cells of the liver are derived from mesen- chyme in the septum transversum. The liver grows rapidly from the 5th to 10th weeks and fills a large part of the upper abdominal cavity (see Fig. 12-4 and Fig. 12-6C and D).
Hematopoiesis (formation and development of various types of blood cells) begins in the liver during the sixth week. By the ninth week, the liver accounts for approxi- mately 10% of the total weight of the fetus. Bile forma- tion by the hepatic cells begins during the 12th week.
The small caudal part of the hepatic diverticulum becomes the gallbladder and the stalk forms the cystic duct (see Fig. 12-4B and C). Initially, the extrahepatic biliary apparatus is occluded with epithelial cells. The stalk connecting the hepatic and cystic ducts to the duo- denum becomes the bile duct; this duct attaches to the ventral aspect of the duodenal loop. As the duodenum grows and rotates, the entrance of the bile duct is carried to the dorsal aspect of the duodenum (see Fig. 12-4C and D). The bile entering the duodenum through the bile duct after the 13th week gives the meconium (first intestinal discharges of neonate) a dark green color.
Mesenteries of Stomach
The stomach is suspended from the dorsal wall of the abdominal cavity by the primordial dorsal mesogastrium (see Fig. 12-2B and C and Fig. 12-3A to E). This mesen- tery, originally located in the median plane, is carried to the left during rotation of the stomach and formation of the omental bursa. The primordial ventral mesogastrium attaches to the stomach, duodenum, liver, and ventral abdominal wall (see Fig. 12-2C and Fig. 12-3A and B).
Omental Bursa
Isolated clefts develop in the mesenchyme forming the dorsal mesogastrium (see Fig. 12-3A and B). The clefts soon coalesce to form a single cavity—the omental bursa (lesser peritoneal sac)—a large recess of the peritoneal cavity (see Fig. 12-2F and G and Fig. 12-3C and D).
Rotation of the stomach pulls the dorsal mesogastrium to the left, thereby enlarging the bursa. The pouch-like bursa facilitates movements of the stomach.
The omental bursa lies between the stomach and pos- terior abdominal wall. As the stomach enlarges, the bursa expands and hangs over the developing intestines (see Fig.
12-3J). This part of the bursa is the greater omentum (see Fig. 12-3G to J and Fig. 12-13A). The two layers of the greater omentum eventually fuse (see Fig. 12-13F). The omental bursa communicates with the main part of the peritoneal cavity through a small opening—the omental foramen (see Fig. 12-2D and F and Fig. 12-3C and F).
Development of Duodenum
Early in the fourth week, the duodenum begins to develop from the caudal part of the foregut and the cranial part of the midgut (Fig. 12-4A). The developing duodenum elongates, forming a C-shaped loop that projects ventrally (see Fig. 12-4B to D). As the stomach rotates, the duode- nal loop rotates to the right and lies retroperitoneal (exter- nal to peritoneum). Because of its derivation from the foregut and midgut, the duodenum is supplied by branches of both the celiac and superior mesenteric arteries (see Fig.
12-1). During the fifth and sixth weeks, the lumen of the duodenum is temporarily obliterated because of prolifera- tion of its epithelial cells; normally the lumen is recana- lized by the end of the embryonic period (8 weeks).
Development of Liver and Biliary Apparatus
The liver, gallbladder, and biliary duct system arise as a ventral outgrowth—hepatic diverticulum—from the
10
10
Partial occlusion of the duodenal lumen—duodenal stenosis—
is usually caused by incomplete recanalization of the duode- num, resulting from defective vacuolization. Most stenoses involve the horizontal (third) and/or ascending (fourth) parts of the duodenum. Because of the stenosis, the stomach’s contents (usually containing bile) are often vomited.