Muscles of the ventrolateral thorax and abdominal wall, diaphragm
With the development of the ribs, which proceed out of the associated ventral sclerotome process and grow ventrally in an appositional way, premyoblasts from the thoracic dermatomyotome of the associated somites also get into the thoracic wall. They develop to become intercostal musculature. Their innervation also proceeds in a strictly segmental way. Secondarily, muscles from the arm buds reach the thorax (pectoral muscle and M. serratus anterior, both of which are innervated via the brachial plexus). The caudal section of the thoracic somites form premyoblasts in the same form as in the rib region. These premyoblasts also migrate ventrally (stages 14-16) and, on both sides, form compact premuscle masses that are soon, though, riddled with somatopleura cells. Initially this mass of muscle still lies in the posterior section of the lateral abdominal wall. Out of this the 4 abdominal wall muscle blastemas differentiate more and more with the M. rectus abdominis , M. obliquus internus and externus as well as the M. transversus abdominis (stages 19-20). This process is regulated by the somatopleura cells. Finally the muscle blastemas reach their definitive positions in the ventral region of the abdomen.
In the region below the navel the muscles have various origins: on the one hand myoblasts stem from the caudal eminence (eminentia caudalis) region and arrive at locations around the cloacal membrane on the ventral side. There they form a part of the abdominal as well as portions of the bladder musculature. From the same mesenchyma stem also the musculature of the cloacal sphincter, as well as the intestinal musculature. On the other hand, the lowest thorax somites deliver parts of the lower abdominal musculature. At the navel ring cells of ectodermal origin enter under the ectoderm and form their part of this musculature.
The diaphragm is formed to a great extent from mesenchyma of the pleuroperitoneal fold (PPF). This is pyramid-shaped, extends into the cervical level to the mesenterium of the esophagus and unites ventrally with the transverse septum.
In the early development (stage 10) the transverse septum is a mesenchymal bridge above the somites between the pericardium and navel vesicle and forms the transition from the parietal to visceral mesoderm of the lateral plate. With the folding of the embryo around a transverse axis through the prechordal plate the heart makes a rotation of 180 degrees. Thereby the transverse septum and the PPF again come to lie caudally in a thoracic position (compare descent of the heart). A hint as to its original position, though, is provided by the innervation of the diaphragm by the n. phrenicus that derives from spinal nerves C3-C5.
Besides the material from the transverse septum with myoblasts from the cervical region, the diaphragm musculature has still other sources. These are the splanchno- and somatopleura and the pleuro-peritoneal fold (PPF) or pleuro-peritoneal membrane, which forms the dorso-lateral section of the transverse septum and thus a fold for the v. cardinalis communis.