The abdominal wall surrounds the anterolateral aspect of the abdominal cavity, where many important organs are located.
Chief layers of the abdominal wall include:
The major functions of the abdominal wall include:
At around the second week of life, the embryo starts to grow and fold upon itself. During this time, there are specific events controlled by growth factors that are occurring that include cellular differentiation, multiplication, migration, and deposition of cells at different sites. Several of these events coincide with the formation of the abdominal wall, and it is at this point that certain abdominal wall defects occur.
The embryo comprised of the ectoderm is a flat structure within the umbilical ring; it will become the surface epithelium and the endoderm, which will become the inner epithelium of the organs in the digestive tract.
As the embryo changes shape, the mesoblast layer starts to form and plays a vital role in the development of the basic abdominal wall structures. At about 3 to 4 weeks, the abdominal and thoracic cavities are distinct, and the folding of the embryo starts to develop along two perpendicular axes but in four directions: caudal, cranial, and laterally on each side. The cranial fold will contain the embryonic derivatives that eventually will come from the epigastric and thoracic walls. The caudal fold will lead to the development of the bladder, hindgut, and hypogastrium. The lateral folds will lead to the development of the lateral abdominal walls and midgut.
The ectoderm will remain in continuity with the amnion and ultimately will form the umbilical ring around a yolk sac which is quickly dissolving. The umbilical ring contains the umbilical vessels, allantois, vitelline duct, extraembryonic coelom, and associated vitelline vessels.
The next stage is the development of the abdominal wall from the 6 to 10 weeks. As the midgut grows rapidly, it is at this stage that congenital defects of the abdominal wall appear.
In the presence of normal development, there is a rotation of the midgut which returns to the abdominal cavity around weeks 10 to 12. If the abdominal wall development is not proper, the intestinal contents will remain herniated inside the umbilical cord. By the twelfth week, there is the obliteration of the extraembryonic space and the intestine returns to the abdominal cavity. At the same time, the vitelline duct, allantois, and other vessels are obliterated, leaving just two umbilical arteries and one umbilical vein covered by amnion.
The anterior abdominal wall is supplied by the following:
There are thoracoepigastric veins running longitudinally which connect the lateral and superficial epigastric veins.
Nerves of the anterior abdominal wall include:
The five muscles in the abdominal wall are divided into two groups: (1) two vertical muscles situated near the midline of the body and (2) three flat muscles located laterally and stacked on top of each other. The three flat muscles include the external oblique, internal oblique, and transversus abdominis. The flat muscles flex and rotate the trunk. Because the fibers of these muscles criss-cross and interlink with each other, they also strengthen the abdominal wall and reduce the risk of herniation.
The Rectus Sheath is an aponeurosis formed by the five muscles of the abdomen. It has an anterior and posterior wall for most of its length. The anterior wall is formed by the aponeurosis of the external oblique and half of the internal oblique. The posterior wall is formed by the aponeurosis of the half of the internal oblique and transversus abdominis. About midway between the umbilicus and pubic symphysis, there is only the anterior wall of the rectus sheath and no posterior sheath. At this junction, the rectus abdominis muscle is in direct contact with the transversalis fascia. The area of transition at which the posterior wall disappears is the arcuate line.
Surgeons need to understand the anatomy of the abdominal wall so that the correct incision is made when entering the abdominal cavity.
The major pathology of the anterior abdominal wall are hernias that include the following: