Download Development of Body Cavities

EMBRYOLOGY:
Development of Body Cavity, Serous Membranes and Gut (I)
Recommended reading:
Animations:
Larsen (3rd ed.) , pp. 133-143, 235-239
Syllabus, 9 pages
http://www.med.uc.edu/embryology/chapter6/animations/contents.htm
OBJECTIVES: Following this lecture and having read the assigned text, the student should be
able to:
I.
1.
Explain the formation of the coelomic cavity (intra-embryonic coelom) and the
role of embryonic folding in this process.
2.
Explain the origin of the septum transversum and how it assumes its definitive
position.
3.
Correlate the 3 principal body cavities of the adult with various regions of the
embryonic coelomic cavity.
4.
Account for separation of the pleural and pericardial cavities.
5.
Account for separation of the thoracic and abdominal cavities by development of
the diaphragm.
6.
Explain the formation of the primitive gut tube, its division into foregut, midgut,
and hindgut, and its relationship to primitive mesenteries.
7.
List the adult derivatives of the 3 regions of the primitive gut.
8.
Define: pericardioperitoneal canal, pleuropericardial fold and membrane,
pleuroperitoneal fold and membrane, dorsal mesentery, ventral mesentery,
mesentery of esophagus.
Formation of the intra-embryonic coelom
A.
The intra-embryonic mesoderm (3rd germ layer) is established during gastrulation.
1.
At the end of week 3, the mesoderm on each side of the midline
differentiates into 3 continuous parts:
a.
Paraxial mesoderm
b.
Intermediate mesoderm
c.
Lateral plate mesoderm
2.
Intercellular clefts form in the lateral plate mesoderm and divide it into two
layers:
a.
Somatic mesoderm layer
b.
Splanchnic mesoderm layer-continuous with the mesoderm covering
the wall of the yolk sac
Adapted from: Langman's Medical Embryology
B.
The intra-embryonic coelom is the space bordered by the somatic and splanchnic
mesoderm layers.
1.
The intra-embryonic coelom is initially open into the extra-embryonic
coelom on both sides.
2.
The intra-embryonic coelom becomes closed off during week 4 by a folding
of the originally flat embryo in both lateral and cephalo-caudal directions.
a.
The intra-embryonic coelomic cavity extends from the thoracic to
the pelvic regions of the embryo.
b.
The mesoderm lining the coelomic cavity forms a serous membrane
or mesothelium.
(1)
Somatic mesoderm forms the parietal layer of mesothelium,
which lines the body wall surfaces of the pleural, pericardial,
and peritoneal cavities.
(2)
Splanchnic mesoderm forms the visceral layer of
mesothelium, which lines the surfaces of the lungs, heart,
and abdominal organs.
(3)
At sites of contact between the body wall and organs, the
parietal and visceral layers of mesothelium are continuous.
(4)
Two-sided elongations of mesothelium between that lining
the body wall and that lining the abdominal organs are called
mesenteries.
Adapted from: Langman's Medical Embryology
II.
Division of the coelom
A.
The diaphragm, the partition that separates the thoracic and abdominal cavities in
the adult, develops from the following four embryonic structures:
1.
Septum transversum
a.
It is the first structure that subdivides (partially) the coelom into
primitive thoracic and abdominal cavities.
b.
It is formed by mesoderm cranial to the pericardial cavity at the end
of week 3.
c.
As the head folds under during week 4, it is relocated into a ventral
position between the primitive pericardial cavity and the yolk stalk.
d.
It then fuses with mesenchyme ventral to the esophagus.
2.
e.
It does not completely separate the thoracic and abdominal cavities,
since there are two large posterior openings, known as the
pericardioperitoneal canals, on either side of the foregut.
f.
It becomes the central tendon of the adult diaphragm.
Pleuroperitoneal membranes
a.
These begin as crescent-shaped pleuroperitoneal folds at the caudal
ends of the pleural cavities.
b.
The folds grow medially and ventrally
pericardioperitoneal canals and by week 7 fuse with:
c.
3.
4.
(1)
The dorsal portions of the septum transversum
(2)
The dorsal mesentery
mesoesophagus)
of
the
across
esophagus
the
(dorsal
After fusion, the folds become the pleuroperitoneal membranes,
which close off the pericardioperitoneal canals and complete the
separation of thoracic and abdominal cavities.
Dorsal mesoesophagus
a.
It suspends the esophagus from the dorsal body wall.
b.
After fusion with the pleuroperitoneal folds, it forms the median
portion of the diaphragm.
c.
Muscle grows into the dorsal mesoesophagus to form the crura of
the adult diaphragm.
Lateral and dorsal body wall
a.
From weeks 9-12, the pleural cavities enlarge into the body wall.
b.
Muscle tissue from the body wall is excavated by the expanding
pleural cavities and becomes the peripheral muscle of the adult
diaphragm.
Embryo of approximately 5 weeks.
Adapted from: Langman's Medical Embryology
B.
Innervation of diaphragm
1.
2.
C.
Phrenic nerves-provide motor and central sensory innervation
a.
Arise from 3rd, 4th and 5th cervical segments supplying septum
transversum in its original cervical location.
b.
Pass through the pleuropericardial folds, which become the fibrous
pericardium of the adult.
Lower thoracic nerves-provide sensory innervation to peripheral diaphragm
Division of the thoracic cavity
1.
Pleuropericardial membranes
a.
Ridges of mesenchyme (containing the veins returning systemic
blood to the heart) form pleuropericardial folds.
b.
The folds elongate medially to form membranes, which partially
separate pericardial and pleural cavities.
2.
Within the pericardioperitoneal canals, the lung buds rapidly grow and
excavate the mesenchyme of the body wall in dorsal, lateral, and ventral
directions, thus enlarging the pleural cavities.
3.
The root of each developing lung fuses with the free edge of each
pleuropericardial membrane, thereby completing the separation of the
pericardial cavity from the two pleural cavities.
Adapted from: Langman's Medical Embryology
D.
III.
Diaphragmatic hernia
1.
Usually congenital (Bochdalek’s hernia) and if extensive may be associated
with severe hypoplasia of lungs that is fatal at birth
2.
More common on the left side because the pericardioperitoneal canal tends
to close later on the left.
Development of the gut and mesenteries (part I)
A.
The general structural plan of the definitive gut
1.
A continuous series of tubular segments with associated organs that began
as diverticula
2.
Lined by simple columnar epithelium
3.
Reinforced externally by splanchnic (visceral) mesoderm
4.
Suspended in the peritoneal cavity by dorsal and (in some segments) ventral
mesenteries
Adapted from: Langman's Medical Embryology
B.
Formation of primitive gut tube from endoderm
1.
The endoderm originally lines a cavity (not the body cavity), which forms
the primitive gut, yolk sac, and allantois.
a.
During the cephalocaudal and lateral folding of the embryo, part of
the endoderm lined cavity is incorporated into the embryo to form
the primitive gut.
b.
The yolk sac and allantois temporarily remain outside the embryo,
but retain their continuity with the gut.
2.
C.
a.
Foregut-the cephalic tube extending from the buccopharyngeal
membrane to the anterior intestinal portal (the opening of foregut
into midgut)
b.
Hindgut-the caudal tube extending from the posterior intestinal
portal (the opening of midgut into hindgut) to the cloacal membrane
c.
Midgut-the central region, which opens into the yolk sac via the
vitelline duct or yolk stalk
Formation of primitive mesenteries
1.
The foregut, midgut, and hindgut are initially in direct contact with the
dorsal body wall.
2.
The developing abdominal cavity enclosing the gut is lined with
mesothelium, which in the abdominal cavity is called peritoneum.
3.
The space lined by peritoneum is called the peritoneal cavity.
4.
The dorsal tissue between the foregut. midgut, and most of the hindgut
narrows until the tube is suspended from the body wall by a dorsal
mesentery.
5.
D.
The primitive gut acquires blind ending tubes in the cephalic and caudal
ends of the embryo, but in the middle remains open into the yolk sac.
a.
The mesentery consists of two layers of peritoneum, between which
are blood vessels, lymphatics, nerves, and fat.
b.
The peritoneum on each side of the mesentery connects parietal
peritoneum on the body wall with visceral peritoneum on the gut or
gut-associated organ.
A ventral mesentery is transiently present, but persists only in the foregut
region.
Adult derivatives of primitive gut regions
1.
The primitive foregut (supplied by the celiac artery) gives rise to the:
a.
Pharynx and lower respiratory system
b.
Esophagus
2.
c.
Stomach
d.
Cranial portion (upper half) of the duodenum, with 3 appendages:
Liver
(2)
Gall bladder
(3)
Pancreas
The primitive midgut (supplied by the superior mesenteric artery) gives rise
to the:
a.
Caudal portion (lower half) of the duodenum
b.
Jejunum
c.
Ileum
d.
Ascending colon and two appendages:
e.
3.
(1)
(1)
Cecum
(2)
Appendix
Proximal 2/3 of the transverse colon
The primitive hindgut (supplied by the inferior mesenteric artery) gives rise
to the:
a.
Distal 1/3 of the transverse colon
b.
Descending colon
c.
Rectum
d.
Upper 2/3 of the anal canal
(1)
To the level of the cloacal membrane
(2)
In adults, to the pectinate line
e.
Allantois
f.
Urogenital sinus