1. Lecture 6: Development of Body cavities

2. Learning Objectives
By the end of the session the students should be able to:
a. Discuss formation of body cavities.
b. Describe formation of serous membranes.
c. Identify clinical applications
Reference: Langman's Medical Embryology, 12th Edition Page 87– 95

3. Body cavities
Three:
Pleural
Pericardial
Peritoneal
All these cavities are formed within the intra-embryonic ceolom.
But how come this intra-embryonic ceolom comes into existence.

4. Intra-embryonic mesoderm differentiates into:
At the end of Week 3:
Intra-embryonic mesoderm differentiates into:
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Soon after small clefts appear within lateral plate mesoderm which coalesce to split the solid layer into two:
Parietal (somatic) intra-embryonic mesoderm
Visceral (splanchnic) intra-embryonic mesoderm

5. The space between these two layers is the primitive body cavity (intra-embryonic ceolom)
It is initially a large continuous space extending from the thoracic to the pelvic regions.
To form adult pericardial, pleural and peritoneal cavities, two partitions to develop:
A pair of pleuro-pericardial membrane
Diaphragm

8. We stop it here and go first to the SEROUS MEMBRANES.

9. Cells of the somatic and visceral mesoderm lining the intra-embryonic cavity become mesothelial and form:
the parietal layer of serous membrane (lining the outside of the peritoneal, pleural and pericardial cavities)
and visceral layer(covering abdominal organs, lungs and heart.
Both these layers are continuous with each other as the dorsal mesentry which suspends gut tube in peritoneal cavity and is running from the caudal limit of the foregut to end of the hindgut.

10. Ventral mesentry exists only from the caudal limit of the foregut to upper portion of duodenum.
Mesentries are double layers of peritoneum that provide a pathway for blood vessels, nerves and lymphatics to the organs.

11. Coming back to the cavities again
Thoracic cavity:
The septum transversum-a thick plate of mesoderm does not separate thoracic and abdominal cavities completely and leaves large openings peicardio-peritoneal canals on each side of foregut.
Lung buds growing and expanding in the canals making them smaller-then extending into body wall
Pleuropericardial folds appear as small ridges and project into undivided thoracic cavity.

12. contd With further expansion of lungs, body wall mesoderm splits into:
Definitive wall of the thorax
Pleuropericardial membranes (extension of P.P.folds)
Pleuropericardial membranes fuse with each other and with the root of the lungs, and the thoracic cavity is divided into pericardial cavity and two pleural cavities.

16. Pleural cavities and the pericardial cavities have been formed and what is left?
PERITONEAL CAVITY

17. Peritoneal cavity:
Pleural cavities are separate from pericardial cavity but are in open communication with peritoneal cavity because of incomplete diaphragm.
At week 5, pleuro-peritoneal folds appear, extend into the gap, fuse with the septum transversum and mesentry of the oesophagus (week 7)
The connection between the pleural and the peritoneal portion of the body cavity due to incomplete diaphragm is closed by pleuro-peritoneal membranes.

19. Clinical Correlates Body wall defects Diaphragmatic hernias
Cleft sternum
Omphalocele
Gastroschisis
Diaphragmatic hernias
Congenital