Body Cavities (Embryonic)
From a
tightly packed, flat trilaminar disc of cells the body cavities must form. This
is initiated around 21 days in the lateral plate mesoderm, which splits into
splanchnic and somatic divisions. Between these mesodermal divisions vacuoles
form and merge creating a U‐shaped cavity in the embryo. This is the intra‐embryonic
cavity (Figure
19.1) and initially has open communication with the extra‐embryonic cavity (or
chorionic cavity).
When the
embryo folds the connection with the chorionic cavity is lost resulting in a
cavity from the pelvic region to the thoracic region of the embryo.
Of the two
layers of lateral plate mesoderm that divided, a somatic layer lines the
intra‐embryonic cavity and a splanchnic layer covers the viscera.
The septum
transversum divides the cavity into two: the thoracic and abdominal
(peritoneal) cavities. The division is not complete and there remains
communication between these cavities through the pericardioperitoneal canals
(Figure 19.2).
Membranes
develop at either end of these canals. These membranes separate the thoracic
cavity into the pericardial cavity and pleu- ral cavities and are called pleuropericardial
folds (Figures 19.2 and 19.3). The folds carry the phrenic nerves and
common cardinal veins and as the position of the heart changes inferiorly, the
folds fuse. The pleuropericardial folds will formthefibrouspericardium (Figure
19.3).
The
diaphragms of the septum transversum, pleuroperitoneal folds, some oesophageal
mesentery and a little muscular ingrowth from the dorsal and lateral body walls
(Figure 19.4).
The septum
transversum originates around day 22 at a cervical level, but caudal to the
developing heart. It receives innervation from spinal nerves C3–C5, the
beginning of the phrenic nerve. With growth of the embryo the position
alters to rest at the level of the thoracic vertebrae.
The septum
transverum is a boundary between the abdominal cavity and the thoracic cavity.
There are two connections between these cavities as mentioned above; the
pericardioperitoneal canals. The pleuroperitoneal folds arise from the
dorsal body wall and eventually close off the pericardioperitoneal canals and
prevent communication between the abdominal and thoracic cavities.
The
pleuroperitoneal folds fuse with the septum transversum, the oesophageal
mesentery and the muscular ingrowth from the body walls to form the diaphragm.
Muscle cells from the septum transversum and the body wall invade the folds
forming the muscular part of the diaphragm (Figure 19.4). The septum transversum
forms the central tendon and the mesentery of the oesophagus merges into the
central tendon, thus allowing passage of the aorta, vena cava and oesophagus.
Clinical relevance
In a congenital diaphragmatic
hernia,
caused by a failure of the diaphragm to form completely, the abdominal contents
herniate into the thoracic cavity negatively affecting lung development, leading
to pulmonary hypoplasia and hypertension. Generally survival rates are about
50%, but if the liver is unaffected they are nearer 90%. Treatment involves
mechanical ventilation and extracorporeal membrane oxygenation (ECMO) to
perform gas exchange, and even a lung transplant has been successfully
reported.
Gastroschisis is also a herniation of
the bowel, but caused by an anterior abdominal wall defect, usually just to the
right of the umbilicus. Viscera are not covered with peritoneum or amnion, and
it is not associated with the same level of other abnormalities (unlike
omphalocoele). Surgical is required and generally survival rates are good.
