Download File

1
Body Cavities
Formation of the Intraembryonic Cavity
At the end of the third week the lateral plate mesoderm acquires a cleft that splits it into
two layers. The somatic mesodermal layer takes a course towards the amniotic wall
while the splanchnic mesoderm layer becomes continuous with the wall of the yolk sac.
Because of its intimate relationship with the yolk sac through the yolk stalk, the gut can
be divided into a foregut an opened (with yolk sac) bottom midgut, and a hindgut. The
pharyngeal gut or pharynx extends from the buccopharyngeal membrane to where the
tracheobronchial diverticulum will form. The foregut lies caudal to the pharyngeal tube
and extends down to where the outgrowth of the liver will form. The midgut begins
caudal to the liver and extends to the mid-portion of the transverse colon. The hindgut
extends from the transverse colon to the cloacal membrane
As the lateral mesoderm of the early embryo splits, the somatic portions fold and
merge, ventrally. The space between the somatic and the splanchnic layers of mesoderm
becomes the common intraembryonic coelom or intraembryonic (body) cavity. The
same folding process that results in the completion of the ventral body wall and the
separation of the intraembryonic coelom from the extraembryonic coelom also brings the
two layers of splanchnic mesoderm together around the newly formed gut as the primary
or common mesentery. The primary mesentery suspends the gut from the dorsal body
wall as the dorsal mesentery and also attaches the gut to the ventral body wall as the
ventral mesentery. This placement divides the coelom into right and left components.
Soon, however, most of the ventral mesentery breaks down and causes a convergence of
the right and left halves of the coelom.
In the region of the developing stomach and liver ventral mesentery persists, forming
the ventral mesogastrium and the falciform ligament of the liver. Cranially, the
tubular primordium of the heart is similarly supported by a dorsal mesocardium and
briefly by a ventral mesocardium, which soon breaks down.
Serous Membranes
Cells of the somatic mesoderm lining the intraembryonic cavity become mesothelial and
form the parietal layer of the serous membranes. The parietal layer lines the
peritoneal, pleural, and pericardial cavities. Cells of the splanchnic mesoderm form the
visceral layer of the serous membranes covering the abdominal organs, lungs, and
heart. Parietal and visceral layers are continuous with each other at the dorsal mesentery.
Formation of the Septum Transversum and Pleural Canals
A major component in division of the common coelom into thoracic and abdominal
components is the septum transversum. The septum grows from the body wall as a
semicircular shelf, separating the heart from the developing liver. During its early
development, a major portion of the liver is embedded in the septum transversum.
Eventually, the septum transversum constitutes a significant component of the
diaphragm, namely its central tendon.
The expanding septum transversum serves partly separates the pericardial and
peritoneal portions of the coelom. For a while, two short channels located on either side
of the foregut will connect the coeloms above and below the septum transversum.
2
Initially, these canals, the pleural (pericardioperitoneal) canals, represent the spaces
into which the developing lungs grow. The pleural canals enlarge greatly as the lungs
increase in size and ultimately form the pleural cavities.
The pleural canals are partially bounded by two paired folds of tissue: the
pleuropericardial and pleuroperitoneal folds. The pleuropericardial folds are ridges of
tissue associated with the common cardinal veins, which bulge into the dorsolateral wall
of the coelom. The common cardinal veins are continuous with the sinus venosus of the
heart. Extensions of the folds are the pleuropericardial membranes which, in the adult,
form the fibrous (parietal) layer of the pericardium.
Associated with the pleuropericardial folds are the phrenic nerves. These arise from
joined branches of cervical roots 3, 4, and 5 and supply muscles of the diaphragm.
At the caudal ends of the pleural canals the pleuroperitoneal folds become prominent
as the expanding lungs push into the mesoderm of the body wall. The pleuroperitoneal
folds fuse with the septum transversum and mesentery of the esophagus. This process
obliterates the pleural canal, hence, eliminating the connection between the abdominal
and thoracic cavity.
Formation of the Diaphragm
The diaphragm, which separates the thoracic from the abdominal cavity in adults, is a
composite structure derived from four embryonic components. The large ventral element
of the diaphragm arises from the (1) septum transversum, which fuses with the ventral
part of the esophageal mesentery and forms the central tendon of the diaphragm.
Converging on the esophageal mesentery from the dorsolateral sides are the (2)
pleuroperitoneal folds. The septum transversum and pleuroperitoneal folds form the bulk
of the diaphragm. As the lungs continue to grow, their caudal tips excavate additional
space in the body wall. (3) Mesenchyme that is separated from the body wall by this
process becomes another component of the diaphragm, forming a thin rim of tissue along
the diaphragm's dorsolateral borders. (4) Dorsal mesentery of the esophagus will develop
into the crura of the diaphragm. Except for the central tendon, the rest of the diaphragm
is predominately muscular.
Clinical Correlations
Cleft sternum – This results from lack of fusion of bilateral bars of mesoderm
responsible for this defect. In some cases the heart protrudes through the defect to
produce ectopia cordis.
Omphalocele – This is herniation of abdominal viscera through an enlarged umbilical
ring and the protruding organs are covered by parietal peritoneum and amnion. The
omphalocele occurs when the organs fail to return into the abdominal cavity at about 10
weeks of gestation.
Gastroschisis – This is herniation of abdominal contents through the body wall directly
into the amniotic cavity. Here, the abdominal contents are not covered by parietal
peritoneum or amnion. Usually, it occurs to the right side of the umbilicus. Unlike the
omphalocele, gastroschisis is not associated with chromosome abnormalities or other
3
severe defects and survival rates are excellent. Both omphalocele and gastroschisis result
in elevated α-fetoprotein in amniotic fluid.
Diaphragmatic hernias – A congenital diaphragmatic hernia is one of the more common
malformations in the newborn (1/2000). It is most frequently caused by failure of one or
both the pleuroperitoneal membranes, or folds, to close the pericardioperitoneal canals.
The abdominal viscera enter into the chest and compress the heart and lungs.
Esophageal hernia – This is thought to be due to a congenital shortness of the
esophagus. The stomach gets constricted by the diaphragm.
Questions
1.
The ________ extends from the liver to the mid part of the transverse colon.
a. foregut
b. midgut
c. hindgut
c. vitelline gut
2.
Splanchnic mesoderm is continuous with the wall of the ____________.
a. amnion
b. chorionic plate
c. epiblast
d. yolk sac
3.
The falciform ligament is derived from the ________.
a. ventral mesentery
b. buccopharyngeal membrane
c. dorsal mesentery
d. cloacal membrane
4.
Somatic mesoderm will form the _________.
a. visceral layers of serous membranes
b. septum transversum
c. parietal layers of serous membranes
d. ventral mesocardium
5.
The __________ separates the thoracic coelom form the abdominal coelom.
a. visceral layers of serous membranes
b. septum transversum
c. parietal layers of serous membranes
d. ventral mesocardium
4
6. Two short canals on either side of the foregut temporarily connecting the thoracic
and
abdominal coeloms are the _____________, spaces for the developing lungs.
a. pericardioperitoneal canals
b. umbilical ring
c. pleuropericardial folds
d. allantois
7. The common cardinal veins are located in the __________________.
a. pericardioperitoenal canals
b. umbilical ring
c. pleuropericardial folds
d. allantois
8.
The pleuropericardial folds contain the ___________ and eventually fuse to enclose
the heart in the fibrous pericardium.
a. lungs
b. phrenic nerves
c. ribs
d. stomach
9.
The central tendon of the diaphragm is mostly derived from the ______________.
a. pleuroperitoneal
b. body wall mesoderm
c. septum transversum
d. pleuropericardial folds
10. Herniation of abdominal viscera not covered by amnion or peritoneal membranes is
called _________.
a. ectopia cordis
b. omphalocele
c. gastroschisis
d. diaphragmatic hernia