DEVELOPMENT OF COELOM OR BODY CAVITIES Lecture - Anatomy Lectures - MBBS Lectures
Learning Objectives:
•By the end of this session the students should be able to:
Body Cavities
THE PRIMITIVE COELOM
Originally the coelom of the animals was used as a temporary reservoir for excretory wastes.
But this function has been superseded in vertebrates so that it now serves as a large bursa to permit frictionless movement of the heart, lungs and abdominal viscera.
The coelom permits the visceral organs to grow and shift position without hindrance.
The heart, lungs and abdominal organs of mammmals occupy separate coelomic compartments, whose respective linings are named pericardium, pleura and peritonium.
The first occurrence of a body cavity in early human stages is in the extra-embryonic mesoderm which lies between the embryo proper and the primitive chorionic capsule.(11th -12th DAY)
INTRAEMBRYONIC COELOM
•In the end of 3rd week intraembryonic mesoderm on each side of midline differentiates into paraxial portion, an intermediate portion and a lateral plate.
•When intercellular clefts appear in the lateral mesoderm, the plates are divided into two layers ;the somatic mesoderm layer and the splanchnic mesoderm layer.
The future peritoneal cavity communicates broadly with the extra-embryonic coelom of each side.
As the embryo continues its folding and elongation, the peritoneal chamber is separated progressively from the extra-embryonic coelom; the last region of closure is at the site of the developing umblical cord.
As the gut & the abdominal wall folds off, the primitive ventral mesentry is lost and the right and left temporary cavities become a single, common chamber.
At the end of this early period the coelomic system thus consists of a single pericardial cavity & a single peritoneal cavity, interconnected by a pair of pleural canals.
A: HORSESHOE-SHAPED INTRAEMBRYONIC COELOM
Serous membrane
•Cells of somatic mesoderm lining the intraembryonic cavity become mesothelial & form the parietal layer of the serous membranes lining the outside of the peritoneal ,pleural and pericardial cavities.
•Cells of the splanchnic mesoderm layer form the visceral layer of the serous membranes covering the abdominal organs ,lungs and heart
DIVISIONS OF PRIMITIVE COELOM
ØThe division of the continuous, primitive coelom into separate, permanent cavities is accomplished through the development of the three sets of partitions:
•SEPTUM TRANSVERSUM
•PLEURO-PERICARDIAL MEMBRANES
•PLEURO-PERITONEAL MEMBRANES
SEPTUM TRANSVERSUM
Unsplit mass of mesoderm
Transverse partition between the pericardial & abdominal cavities.
Occupies space between the gut, yolk stalk & ventral body wall.
The septum does not separate the thoracic and abdominal cavities completely but leave large openings the pericardioperitoneal canals on each side of forgut
• Because of the growth of the bronchial buds (primordia of bronchi and lungs) into the pericardioperitoneal canals, a pair of membranous ridges is produced in the lateral wall of each canal :
● The cranial ridges—pleuropericardial folds—are located superior to the developing lungs.
● The caudal ridges—pleuroperitoneal folds—are located inferior to the lungs.
PLEURO-PERICARDIAL MEMBRANES
In a 4mm. Embryo the lungs begin to develop within the medial mass of mesenchyme that separates the two pleural canals.
As a result of the rapid growth of the lungs, the pericardioperitoneal canals become too small, & the lungs begin to expand into the mesenchyme of the body wall dorsally, laterally, & ventrally, & soon bulge into them.
The canals thereby become the potential pleural cavities.
Ventral & lateral expansion is posterior to the pleuro-pericardial folds.
At first, these folds appears as small ridges projecting into the primitive undivided thoracic cavity.
With expansion of the lungs, mesoderm of the body wall splits into two components:
A) the definitive wall of the thorax
B) The fibrous pericardium
Subsequently, the heart decends & the positional changes of the sinus venosus shift the common cardinal veins toward the midline.
THE PERICARDIUM
The pleuro-pericardial membranes are drawn out in a mesentry-like fashion.
Finally, they fuse with each other & with the root of the lungs.
The thoracic cavity is divided into the definitive pericardial cavity & two pleural cavities.
In the adult, the pleuro-pericardial membranes form the fibrous pericardium.
PLEURO-PERITONEAL MEMBRANES (formation of diaphragm)
Although the pleural cavities are separate from the pericardial cavity, they remain in open communication with the abdominal (peritoneal) cavity.
During further development, the opening between them is closed by cresent-shaped folds, the pleuro-peritoneal folds, which project into the caudal end of the pericardio-peritoneal canals.
Gradually, the folds extend medially & ventrally, so that by the 7th week, they fuse with the mesentry of the esophagus & with the septum transversum.
Hence, the connection between the pleural & peritoneal portions of the body cavity is closed by the pleuro-peritoneal membranes.
Congenital Anomalies
•Further expansion of the pleural cavities relative to mesenchyme of the body wall add a peripheral rim to the pleuroperitoneal membranes.
•Once rim is established myoblasts originating in the body wall penetrates the membranes to form the muscular part of diaphragm.
THE DIAPHRAGM
Diaphragm is derived from four sources, but the limits of these several contributions cannot be set exactly.
1.septum transversum forms the central tendon of the diaphragm.
2.Small intermediate portions from the paired pleuro-peritoneal membranes.
3.Muscular components from the lateral and dorsal body wall.
4.Mesentry of the esophagus in which crura of the diaphragm develop
•Herniations through the sternocostal hiatus (foramen of Morgagni)-the opening for the superior epigastric vessels in the retrosternal area may occur
The septum transversum lies opposite cervical segments during the 4th week
By the 6th week, the developing diaphragm is at the level of the thoracic somites.
The repositioning of the diaphragm is caused by rapid growth of the dorsal part of the embryo (vertebral column), compared with that of the ventral part.
By the beginning of the 3rd month, some of the dorsal bands of the diaphragm originate at the level of the 1st lumber vertebra.
POSITIONAL CHANGES OF THE DEVELOPING DIAPHRAGM
NERVE SUPPLY OF THE DIAPHRAGM
PHRENIC NERVE: Motor & sensory innervation
LOWER INTERCOSTAL (THORACIC) NERVES: Sensory fibers to the peripheral part of the diaphragm
Mesentery Development
Mesentery is a double layer of peritoneum that enclose an organ and connect it to the body wall
Provide pathways for vessels, nerves and lymphatics
Parts of developing GIT
•Pharyngeal gut
•Fore gut
•Mid gut
•Hind gut
Mesentery Development
•During embryonic development digestive tract and accessory organs are suspended in peritoneal cavity by:
–dorsal mesentery
Øventral mesentery
THE DORSAL MESENTERY
•Extends from lower end of esophagus to the cloacal region of the hind gut
•Derived from splanchnic mesoderm
•In region of stomach- The dorsal mesogastrium or greater omentum
•Duodenum-The dorsal mesoduodenum
•Jejunum & ileum- mesentery proper
•Colon- The dorsal mesocolon
Ventral mesentery
Mesenteries of the Stomach
Primitive Dorsal Mesogastrium
Appearance of the Omental Bursa
Development of Spleen
Elongation of the Dorsal mesogastrium and expansion of the Omental bursa
Formation of Greater Omentum
Mesentery of duodenum
Mesentery of Liver
Mesentery of Pancreas
Mesenteries of mid gut
•Rotation of intestinal loop cause twist in dorsal mesentry.
•Ascending colon becomes Retro perotoneal.
•Transverse Mesocolon fuses with posterior wall of Greater Omentum
Mesentery of Hindgut
•Dorsal Mesentry forms Sigmoid Mesocolon.
•Rectum is only partially covered by the peritoneum.
Mesenteries of Digestive Organs
REFERENCES:
Developmental Anatomy, A Textbook and Laboratory Manual of Embryology By LESLIE BRAINERD AREY, Revised 7th Edition
LANGMAN’S Embryology 10th Edition, By T.W.SADLER
The Developing Human, Clinically Oriented Embryology by KEITH L. MOORE, 8th Edition
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BODY WALL DEFECTS
•Cleft sternum
•Cantrell pentalogy
•Omphalocele
•Gastroschisis
•Bladder exstrophy
•Cloacal exstrophy
CLEFT STERNUM
Ventral body wall defect
Lack of fusion of the bilateral bars of mesoderm
Heart protudes through a sternal defect
Absence of the lower third of the sternum
Ectopia cordis
CANTRELL PENTALOGY
ECTOPIA CORDIS
CLEFT STERNUM
OMPHALOCELE
GASTROSCHISIS
CONGENITAL DIAPHRAGMATIC HERNIA
A: HERNIATION OF LIVER
B: HERNIATION OF STOMACH & BOWEL
C: CHEST RADIOGRAPH
PARASTERNAL HERNIA
Small part of muscular diaphragm fails to develop.
Hernia may remain undiscovered until the child is several years old.
A small peritoneal sac containing intestinal loops may enter the chest between the sternal & costal portions of the diaphragm.
ESOPHAGEAL HERNIA
Congenital shortness of the esophagus
Upper portion of the stomach retained into the thorax
Stomach is constricted at the level of the diaphragm
REFERENCES:
Developmental Anatomy, A Textbook and Laboratory Manual of Embryology By LESLIE BRAINERD AREY, Revised 7th Edition
LANGMAN’S Embryology 10th Edition, By T.W.SADLER
The Developing Human, Clinically Oriented Embryology by KEITH L. MOORE, 8th Edition
