Basic Science Gastro 43 Abdominal Wall Peritoneum Mesenteries Omentum and Retroperitoneum

• 
• [@ MPhIL]

Lateral plate - intraembryonic coelom appear

The ventral mesentery of the foregut is called the ventral mesogastrium, and the dorsal mesentery is called the dorsal mesogastrium.

The ventral mesogastrium becomes divided by the developing liver into a ventral part and a dorsal part.

• - The ventral part forms the ligaments of the liver, namely, (a) the falciform ligament, (b) the right and left triangular ligaments, and (c) the superior and inferior layers of the coronary ligament.
• - The dorsal part of the ventral mesogastrium forms the lesser omentum.

The fate of the dorsal mesogastrium is as follows. (1) The greater or caudal part of the dorsal mesogastrium becomes greatly elongated and forms the greater omentum. (2) The spleen develops in relation to the cranial part of the dorsal mesogastrium, and divides it into dorsal and ventral parts. The ventral part forms the gastrosplenic ligament while the dorsal part forms the lienorenal ligament. (3) The cranial most part of the dorsal mesogastrium forms the gastrophrenic ligament. The midgut and hindgut have only a dorsal mesentery, which forms the mesentery of the jejunum and ileum, the mesoappendix, the transverse mesocolon and the sigmoid mesocolon. The mesenteries of the duodenum, the ascending colon, the descending colon and the rectum are lost during development.

?

Lesser Sac or Omental Bursa is a large recess of the peritoneal cavity behind the stomach, the lesser omentum and the caudate lobe of the liver. It is closed all around, except in the upper part of its right border where it communicates with the greater sac through the epiploic foramen.

Boundaries

The anterior wall is formed by: (1) The caudate lobe of the liver; (2) the lesser omentum; (3) the stomach; and (4) the anterior two layers of the greater omentum.

The posterior wall is formed by: (1) Structures forming the stomach bed; and (2) the posterior two layers of the greater omentum.

The right border is formed by: (1) Reflection of peritoneum from the diaphragm to the right margin of the caudate lobe along the left edge of the inferior vena cava; (2) the floor of the epiploic foramen; (3) the reflection of peritoneum from the head and neck of the pancreas to the posterior surface of the first part of the duodenum (medial to the gastroduodenal artery); and (4) the right free margin of the greater omentum where the 2nd and 3rd layers of the omentum become continuous with each other.

The left border is formed by : (1) The gastrophrenic ligament; (2) the gastrosplenic and lienorenal ligaments enclosing the splenic recess of the lesser sac; and (3) the left free margin of the greater omentum, where again the 2nd and 3rd layers of the greater omentum become continuous.

The upper border is formed by the reflection of the peritoneum to the diaphragm from the oesophagus, the upper end of the fissure for the ligamentum venosum and the upper border of the caudate lobe of the liver.

The lower border is formed by continuation of the 2nd and 3rd layers of the greater omentum at its lower margin. However, in the adult the part of the sac below the level of the transverse colon is obliterated by fusion of the 2nd and 3rd layers.

• Clinical importance -
• A posterior gastric ulcer may perforate into the lesser sac. The leaking fluid passes out through the epiploic foramen to reach the hepatorenal pouch. Sometimes in these cases the epiploic foramen is closed by adhesions. Then the lesser sac becomes distended," and can be drained by a tube passed through the lesser omentum.

Epiploic foramen or foramen of Winslow  is the passage of communication, or foramen, between the greater sac and the lesser sac.

• It has the following borders:
• Anterior: the free border of the lesser omentum, known as the hepatoduodenal ligament. This has two layers and within these layers are the common bile duct, hepatic artery, and hepatic portal vein. 
• Posterior: the peritoneum covering the inferior vena cava
• Superior: the peritoneum covering the caudate lobe of the liver
• Inferior: the peritoneum covering the commencement of the duodenum and the hepatic artery, the latter passing forward below the foramen before ascending between the two layers of the lesser omentum.
• Left lateral: gastrosplenic ligament and splenorenal ligament. 

• Boundaries :
• - Anteriorly: (1) The inferior surface of the right lobe of the liver, and (2) the gall bladder.
• - Posteriorly : (1) The right suprarenal gland; (2) the upper part of the right kidney; (3) the second part of the duodenum; (4) the hepatic flexure of the colon; (5) the transverse mesocolon; and (6) a part of the head of the pancreas.
• - Superiorly; The inferior layer of the coronary ligament.
• - Inferiorly, It opens into the general peritoneal cavity

• Clinical anatomy - 
• This space is of considerable importance as it is the most dependent (lowest) part of the abdominal cavity proper when the body is supine. Fluids tend to collect here.
• This is the commonest site of a subphrenic abscess, which may be caused by spread of infection from the gall bladder, the appendix, or other organs in the region.

• The liver is invested in peritoneum except for the gallbladder fossa, porta hepatis, and posterior aspect of the liver on either side of the IVC in two wedge-shaped areas.
• The region of liver to the right of the IVC is called the bare area of the liver.
• The peritoneal duplications on the liver surface are referred to as ligaments.
• The diaphragmatic peritoneal duplications are referred to as the coronary ligaments, whose lateral margins on either side are the right and left triangular ligaments.
• From the center of the coronary ligament emerges the falciform ligament, which extends anteriorly as a thin membrane connecting the liver surface to the diaphragm, abdominal wall, and umbilicus.
• The ligamentum teres (the obliterated umbilical vein) runs along the inferior edge of the falciform ligament from the umbilicus to the umbilical fissure.
• On the posterior surface of the left liver, running from the left portal vein in the porta hepatis toward the left hepatic vein and the IVC is the ligamentum venosum (obliterated sinus venosus) that also runs in a fissure.

• The abdominal wall begins to develop in the earliest stages of embryonic differentiation from the lateral plate of the embryonic mesoderm.
• The mesoderm becomes divided by clefts on each side of the lateral plate that ultimately develop into somatic and splanchnic layers.
• The somatic layer contributes to the development of the abdominal wall.
• As the embryo enlarges and the abdominal wall components grow toward one another, the ventral open area, bounded by the edge of the amnion, becomes smaller. This results in the development of the umbilical cord as a tubular structure containing the omphalomesenteric duct, allantois, and fetal blood vessels, which pass to and from the placenta. By the end of the third month of gestation, the body wall has closed, except at the umbilical ring.

• Clinical significance 
• 1. Umbilicus is the meeting point of the four (two lateral, head and tail) folds of embryonic plate.
• 2. This is also the meeting point of three systems, namely the digestive (vitellointestinal duct), the excretory (urachus), and vascular (umbilical vessels).

• Exstrophy of the bladder is a ventral body wall defect in which the bladder mucosa is exposed.
• Epispadias is a constant feature, and the open urinary tract extends along the dorsal aspect of the penis through the bladder to the umbilicus.
• Exstrophy of the bladder may be caused by a lack of mesodermal migration into the region between the umbilicus and genital tubercle, followed by rupture of the thin of ectoderm.