markedly reduced lung volume. Many infants with a con- genital diaphragmatic hernia die of pulmonary insufficiency, despite optimal postnatal care, because their lungs are too hypoplastic to support extrauterine life.
LUNG HYPOPLASIA
C H A P T E R 11 RESPIRATORY SYSTEM 135.e1
Answers to Chapter 11 Clinically Oriented Questions
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137
C H A P T E R
Alimentary System
12
Foregut 137
Development of Esophagus 138 Development of Stomach 138 Development of Duodenum 141 Development of Liver and Biliary Apparatus 141
Development of Pancreas 143 Development of Spleen 143
Midgut 144
Rotation of Midgut Loop 145 Cecum and Appendix 145 Hindgut 153
Cloaca 153 Anal Canal 155
Clinically Oriented Questions 160
he alimentary system (digestive system) is the digestive tract from the mouth to the anus with all its associative glands and organs. The primordial gut (earliest stage of development) forms during the fourth week as the head, caudal eminence (tail), and lateral folds incorporate the dorsal part of the umbilical vesicle (yolk sac) (see Chapter 6, Fig. 6-1).
The primordial gut is initially closed at its cranial end by the oropharyngeal membrane (see Chapter 10, Fig. 10-1B), and at its caudal end by the cloacal membrane (Fig. 12-1). The endoderm of the primordial gut gives rise to most of the gut, epithelium, and glands. The epithelium at the cranial and caudal ends of the alimentary tract is derived from the ectoderm of the stomodeum and anal pit (proctodeum), respectively (see Fig. 12-1).
The muscular and connective tissue and other layers of the wall of the alimentary tract are derived from the splanchnic mesenchyme surrounding the primordial gut. For descriptive purposes, the gut is divided into three parts: foregut, midgut, and hindgut. The regional dif- ferentiation of the primordial gut is established by sonic and Indian hedgehog genes (SHH and IHH) that are expressed in the endoderm and the surrounding mesoderm. The endoder- mal signaling provides temporal and positional information for the development of the gut.
FOREGUT
The derivatives of the foregut are the:
•
Primordial pharynx and its derivatives•
Lower respiratory system•
Esophagus and stomach•
Duodenum, just distal to the opening of the bile duct•
Liver, biliary apparatus (hepatic ducts, gallbladder, and bile duct), and pancreasT
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.