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C H A P T E R
Body Cavities, Mesenteries, and Diaphragm
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92 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
the septum transversum—a thick plate of mesoderm that occupies the space between the thoracic cavity and the omphaloenteric duct (see Fig. 9-4A and B).
The septum transversum is the primordium of the central tendon of the diaphragm. Partitions form in each pericardioperitoneal canal, separating the pericardial cavity from the pleural cavities, and the pleural cavities from the peritoneal cavity (see Fig. 9-3A). Because of the growth of the bronchial buds (primordia of bronchi and lungs) into the pericardioperitoneal canals (Fig. 9-5A), a pair of membranous ridges is produced in the lateral wall of each canal. The cranial ridges—the pleuropericardial folds—are located superior to the developing lungs, and the caudal ridges—the pleuroperitoneal folds—are located inferior to the lungs.
Pleuropericardial Membranes
As the pleuropericardial folds enlarge, they form parti- tions that separate the pericardial cavity from the pleural cavities. These partitions—pleuropericardial membranes
—contain the common cardinal veins (see Fig. 9-5A and B), which drain the venous system into the sinus venosus of the primordial heart (see Chapter 14). Initially the bronchial buds are small relative to the heart and peri- cardial cavity (see Fig. 9-5). They grow laterally from the caudal end of the trachea into the pericardioperitoneal canals (future pleural canals). As the primordial pleural cavities expand ventrally around the heart, they extend into the body wall, splitting the mesenchyme into two layers: (1) an outer layer that becomes the thoracic wall and (2) an inner layer (pleuropericardial membrane) that becomes the fibrous pericardium, the outer layer of the pericardial sac that encloses the heart (see Fig. 9-5C and D).
The pleuropericardial membranes project into the cranial ends of the pericardioperitoneal canals (see Fig.
9-5B). With subsequent growth of the common cardinal The peritoneal cavity is connected to the extraembry-
onic coelom at the umbilicus (Fig. 9-4C and D). The peritoneal cavity loses its connection with the extraem- bryonic coelom during the 10th week as the intestines return to the abdomen from the umbilical cord (see Chapter 12).
During formation of the head fold, the heart and peri- cardial cavity are relocated ventrocaudally, anterior to the foregut (see Fig. 9-2A, B, D, and E). As a result, the pericardial cavity opens into the pericardioperitoneal canals, which pass dorsal to the foregut (see Fig. 9-4B and D). After embryonic folding, the caudal parts of the foregut, midgut, and hindgut are suspended in the peri- toneal cavity from the dorsal abdominal wall by the dorsal mesentery (see Fig. 9-2F and Fig. 9-3B to E).
Mesenteries
A mesentery is a double layer of peritoneum that begins as an extension of the visceral peritoneum that covers an organ. The mesentery connects the organ to the body wall and conveys its vessels and nerves. Transiently, the dorsal and ventral mesenteries divide the peritoneal cavity into right and left halves (see Fig. 9-3C). The ventral mesentery soon disappears (see Fig. 9-3E), except where it is attached to the caudal part of the foregut (primordium of stomach and proximal part of duodenum). The peritoneal cavity then becomes a continuous space (see Fig. 9-3A and Fig.
9-4D). The arteries supplying the primordial gut—celiac arterial trunk (foregut), the superior mesenteric artery (midgut), and inferior mesenteric artery (hindgut)—pass between the layers of the dorsal mesentery (see Fig. 9-3C).
Division of Embryonic Body Cavity
Each pericardioperitoneal canal lies lateral to the proxi- mal part of the foregut (future esophagus) and dorsal to
Figure 9–1 A, Dorsal view of a 22-day embryo, showing the outline of the horseshoe-shaped intraembryonic coelom. The amnion has been removed and the coelom is shown as if the embryo were translucent. The continuity of the intraembryonic coelom, as well as the com- munication of its right and left limbs with the extraembryonic coelom, is indicated by arrows.
B, Transverse section through the embryo at the level shown in A.
Neural folds
Amnion (cut edge) Neural tube Somatic mesoderm layer Amniotic cavity
Wall of umbilical vesicle
Splanchnic mesoderm layer Intraembryonic coelom
Extraembryonic coelom
Level of section B Somite
Notochord
B A
Future pleural cavity
Future peritoneal cavity
Umbilical vesicle
Future pericardial cavity
C H A P T E R 9 BODY CAvITIES, MESENTERIES, AND DIAPHRAGM 93
(Fig. 9-6B and C). Eventually, these membranes separate the pleural cavities from the peritoneal cavity. The pleu- roperitoneal membranes are produced as the developing lungs and pleural cavities expand and invade the body wall. They are attached dorsolaterally to the abdominal wall and their crescentic free edges initially project into the caudal ends of the pericardioperitoneal canals.
During the sixth week, the pleuroperitoneal mem- branes extend ventromedially until their free edges fuse with the dorsal mesentery of the esophagus and the septum transversum (see Fig. 9-6C). This membrane sep- arates the pleural cavities from the peritoneal cavity.
Closure of the pleuroperitoneal openings is completed by the migration of myoblasts (primordial muscle cells) into the pleuroperitoneal membranes (see Fig. 9-6D and E).
The pleuroperitoneal opening on the right side closes slightly before the left one.
veins, positional displacement of the heart, and expansion of the pleural cavities, the pleuropericardial membranes become mesentery-like folds extending from the lateral thoracic wall. By the seventh week, the pleuropericardial membranes fuse with the mesenchyme ventral to the esophagus, separating the pericardial cavity from the pleural cavities (see Fig. 9-5C). The primordial mediasti- num consists of a mass of mesenchyme that extends from the sternum to the vertebral column, separating the devel- oping lungs (see Fig. 9-5D). The right pleuropericardial opening closes slightly earlier than the left one and pro- duces a larger pleuropericardial membrane.
Pleuroperitoneal Membranes
As the pleuroperitoneal folds enlarge, they project into the pericardioperitoneal canals. Gradually, the folds become membranous, forming the pleuroperitoneal membranes
Figure 9–2 Embryonic folding and its effects on the intraembryonic coelom and other structures. A, Lateral view of an embryo (approximately 26 days). B, Schematic sagittal section of the embryo, showing the head and tail folds. C, Transverse section at the level shown in A, indicating how fusion of the lateral folds gives the embryo a cylindrical form. D, Lateral view of an embryo (approximately 28 days). E, Schematic sagittal section of the embryo, showing the reduced communication between the intraembryonic and extraembryonic coeloms (double- headed arrow). F, Transverse section, as indicated in D, showing the formation of the ventral body wall and the disappearance of the ventral mesentery. The arrows indicate the junction of the somatic and splanchnic layers of the mesoderm. The somatic mesoderm will become the parietal peritoneum lining the abdominal wall, and the splanchnic mesoderm will become the visceral peritoneum covering the organs (e.g., stomach).
Later al
fo
ld
Amnion
Heart
Pericardial cavity
Umbilical vesicle
Aorta
Embryonic coelom
Umbilical vesicle Extraembryonic coelom
Connecting stalk Amnion
Amnion Midgut
Midgut
Dorsal mesentery
Splanchnic mesoderm Somatic mesoderm
Ventral body wall Ventral mesentery
disappearing
Neural tube Peritoneal cavity Foregut
Septum transversum
Umbilical cord
Hindgut Heart prominence
A B C
D E F
Head
fold
Tail fold
Level of section C
Level of section F
94 BEFORE WE ARE BORN ESSENTIALS OF EMBRYOLOGY AND BIRTH DEFECTS
Figure 9–3 Mesenteries and body cavities at the beginning of the fifth week. A, Schematic sagittal section. Note that the dorsal mesentery serves as a pathway for the arteries that supply the developing gut. Nerves and lymphatics also pass between the layers of this mesentery.
B to E, Transverse sections through the embryo at the levels shown in A. The ventral mesentery disappears except in the region of the terminal esophagus, stomach, and first part of the duo- denum. Note that the right and left parts of the peritoneal cavity, which are separate in C, are continuous in E.
B
C
D
E
Notochord Dorsal aorta
Neural tube
Esophageal part of tracheoesophageal tube
Dorsal mesocardium
Dorsal mesentery Midgut
Hindgut Dorsal mesentery
Umbilical vesicle Pericardioperitoneal canal
Tracheal part of tracheoesophageal tube
Heart Pericardial cavity
Foregut artery
(future celiac arterial trunk) Stomach
Liver
Ventral mesentery Peritoneal cavity
Peritoneal cavity
Peritoneal cavity
Splanchnic mesoderm Neural tube
Dorsal aorta
Somatic mesoderm Ventral abdominal wall
A
Forebrain
Heart
Liver
Allantois
Plane of section B
Plane of section C
Plane of section D
Septum transversum
Dorsal mesentery
Dorsal mesentery
Midgut artery (superior mesenteric)
Plane of section E Ureteric bud
(primordium of permanent kidney)
C H A P T E R 9 BODY CAvITIES, MESENTERIES, AND DIAPHRAGM 95
transversum expands and fuses with the mesenchyme ventral to the esophagus and the pleuroperitoneal mem- branes (see Fig. 9-6C).
Pleuroperitoneal Membranes
These membranes fuse with the dorsal mesentery of the esophagus and the septum transversum (see Fig. 9-6C).
This fusion completes the partition between the thoracic and abdominal cavities and forms the primordial dia- phragm. The pleuroperitoneal membranes represent relatively small portions of the neonate’s diaphragm (see Fig. 9-6E).
Dorsal Mesentery of Esophagus
The septum transversum and pleuroperitoneal mem- branes fuse with the dorsal mesentery of the esophagus.
This mesentery becomes the median portion of the dia- phragm. The crura of the diaphragm—a pair of diverging muscle bundles that cross in the median plane anterior to the aorta (see Fig. 9-6E)—develop from myoblasts (pri- mordial muscle cells) that grow into the dorsal mesentery of the esophagus.