The parietal layer is lined byMethothelium derived from somatic

Mesoderm. & the visceral layer is Covered by mesothelium derived from

Splanchnic mesoderm.

4 occipital &

8 cervical&

12 thoracic &

5 lumbar&

5 sacral&

8 – 10 coccygeal

Early in the 4th week, the intraembryonic coelom appears as a horseshoe-shaped cavity. The curve at the cranial end of the embryo represents the future pericardial cavity &

limbs indicate future pleural ;peritoneal cavities . The distal part of each limb of intraembryonic coelom is continuous with extraembryonic coelom at lateral edges of

embryonic disc .This communication is important to midgut development. At 6th W herniation occure withen the umblical cord. And at 10th week this connection is lost.

The intraembryonic coelom becomes the embryonic body cavity during 4th week ( 22-28 days ) 1. Pericardial cavity 2. two pericardioperitoneal canals 3. peritoneal cavity.

During formation of the head fold , the heart& pericardial

cavity move ventrocaudally, anter.

to the foregut As a result , the pericardial

cavity opens into pericardioperitoneal canals, which pass

dorsal to the foregut & on each side of it

( future esophagus ) and dorsal to septum transversum ( a thick plate of mesodermal

tissue ) that occupies the space between

thoracic cavity & yolk stalk (

Partitions is formed in each pericardioperitoneal canal that separate pericardial cavity from pleural cavities & pleural cavities from peritoneal cavity. These partition are pair of membranous ridges that is produced in the lateral wall of

each canal. The cranial ridges: pleuropericardial folds, located superior to lungs. Caudal ridges: pleuroperitoneal

folds are located inferior to developing lungs.

•An extention of the visceral peritoneum

covering an organ form the mesentery. So , the

mesentery is a double layer of the peritonium . It connect the organ to

the body wall & conveys vessels & nerves to it . The ventral and dorsal mesenteries divide the

cavity into R & L halves The venteral mesentere.

Disappears , except where it is attached to the caudal part of the

foregut ( primordium of terminal part of

esophagus , stomach & 1st part of duodenum) .

After this level the peritoneum cavity

becomes continuous space

•The venteral mesentery

degenerates , resulting in a

large embryonic peritoneal cavity

extending from the heart to the

pelvic region.

Pleuropericardial Mermbranes

They project into the cranial ends of the

pericardioperitoneal canal

The bronchial buds grow laterally from the caudal end

of the trachea into pericardioperitoneal canals

( future pleural canals). As the primordial pleural cavities

expand ventrally around the heart, they extend into the

body wall , spilitting the mesenchyme into : An outer layer that becomes the

thoracic wall &.

An inner layer (pleuropericardial

membrane), that becomes the fibrous pericardium .

) the outer layer of the pericardial sac enclosing the

heart (

They contain the common cardinal veins & phrenic Ns. They become mesentery like folds ( extending from the lateral wall of thoracic wall ). This is due to 1. Growth of

common cardinal v., descend of heart ; expansion of pleural cavities

By the 7th week , the pleuropericardial

membranes fuse with mesenchyme ventral to esophagus, forming the

primordium of mediastinum ,separating the pericardial

cavity from the pleural cavities .

The primordial mediastinum consists of a

mass of mesenchym ( embryonic connective

tissue ) that extendes from the sternum to the vertebral

column, separating the developing lungs .

The right pleuropericardial opening closes earlier than the left one , probably because the right common

cardinal vein is larger than the left one, and produces a larger pleuropericardial membrane.

Pleuroperitoneal MembraneAs the pleuroperitoneal folds enlarge ,they project into the pericardioperitoneal canals. Gradually ,the folds become

membranous.. They are attached dorsolaterally to abdominal wall and their crescentic free edges

project into the caudal ends of the canals .

During the 6th week these memberanes extend

ventromedially until their free edges fuse with the dorsal

mesentery of esophagus & septum transversum. This

process separates pleural cavities from peritoneal cavity

Closure of the pleuroperitoneal openings is assisted by the migration of myoblasts (primordial muscle cells ) into the membranes . The right opening closes before the left

one. This may be due to large size of the right lobe of the liver

Development of the Diaphragm

1 -septum transversum

2 -pleuroperitoneal membranes.

3 -Dorsal mesentery of esophagus

4 -Muscular ingrowth from lateral body walls

Septum Transversum

It is first identifiable at the end of the third week as a mass of

mesodermal tissue cranial to the pericardial cavity . After

head folds at 4th week, it forms a thick incomplete partition

between pericardial & abdominal cavities . There is a large opening on each side of

the esophagus ( pericardioperitoneal canal ) .

This septum is the primordium of the central tendon of the diaphragm. It does not separate the thoracic &

abdominal cavities completely. Because there is pericardioperitoneal canal . It expands and fuses with

the mesenchyme ventral to esophagus & pleuroperitoneal memberanes

The transverse septum grows dorsally from the ventrolateral body wall and

forms a semicircular shelf which separates the heart from the liver . A

large part of the liver is embeded in the septum .It is located caudal to the

pericardial cavity and partially separates it from the developing

peritoneal cavity

Pleuroperitoneal Memberanes

It forms the primordial diaphragm It forms large portion of fetal

diaphragm . But they form small portion of the newborn infant’s

diaphragm . At the end of the 6th week it fuse with the other 2

components( 9 to 12 ws ).

Dorsal Mesentry of esophagus

It forms the median portion of the diaphragm . The crura .( a leglike pair of muscle bundles that cross

in the median plane anterior to the aorta . It develop from

myoblasts that grow into the dorsal mesentery of esophagus .

As the 4 parts of the diaphragm fuse mesenchyme in the septum transversum extends into the other 3 parts It

forms myoblasts that differentiate into the skeletal muscle of the diaphragm.

muscular ingrowth from lateral body walls

During 10th to 12 th weeks, the lungs and

pleural cavities enlarge, into the lateral body walls .

During this process the body- wall tissue is split

into 2 layers : An external layer that

becomes part of the definitive abdominal wall.

An internal layer that contributes to peripheral parts of the diaphragm ,

external to the parts derived from the pleuroperitoneal

membranes.

Positional Changes

Durind the 4th week ( A ) & At 6 week it is at the level of the thoracic somites & by the beginning of the 8th week , the dorsal part of the diaphragm lies at the level of the 1st lumbar vertebra

Innervation of the diaphragm

During the 5th week , myoblasts from 3,4;5 somites migrate into the developing diaphragm ( S. T. ) bringing their nerve fibers with them ( phrenic n. ) which arise from the venteral primary rami of the

3rd, 4th; 5th cervical spinal nerves. The embryonic phrenic ns. Enter the diaphragm by passing through the pleuropericardial membranes . This explain why phrenic ns. Lie on the fibruos

pericardium. Hence it is motor to diaphragm and also sensory , but the costal rim receives sensory supply from the lower intercostal ns. Because of the origin of the peripheral part of the diaphragm

from the lateral body walls .

Posterolateral defect of the diaphragm is a

common congenital anomaly of it . It occurs about once in 2200. In

congenital diaphragmatic hernia ,CDH, herniation of stomach, spleen; most of

intestines herniate. Usually, unilateral CDH

results from devective formation and or fusion of

the pleuroperitoneal membrane with other 3 parts of the diaphragm. CDH, occurs on the left

side in 85 to 90 % of cases This may be due to earlier

closure of the right pleuroperitoneal opening . CDH, is the most common

cause of polyhydramnios & pulmonary hypoplasia

that because the lug is copressed

In severe lung hypoplasia , some primordial alveoli rupture, causing air to enter the pleural cavity ,

pneumothorax, Also, fetal lung maturation may be delayed.

Prenatal diagnosis of CDH: ultrasound& magnetic resonance imaging. It can repair prenatally between 22 &

28 Ws. But there is risk to fetus and mother .