14.4 Cardiac musculature

• Early development of the cardiac musculature
  
• Histological aspects
  

Early development of the cardiac musculature

The cardiac musculature stem from mesenchymal material that forms the cardiogenic plate in the early development before the prechordal plate 8.
This mesenchyme consists of a connected group of cuboid cells in the ventral mesoderm.

Fig. 19
The left diagram shows an overview of the embryonic plate In the middle a lateral view of the same embryo is shown.

On the right, the cross sections are shown that correspond to the levels indicated with the capital letters. At this stage the material for the cardiac anlage, the myocardial plate or cardiogenic plate, lies cranial to the prechordal plate.

In stage 9 the cardiac anlage lies medio-ventral to the pericardiac coelom. Through the development of the head fold, the situation of the heart is influenced. In the following two days the pericardiac coelom expands10 and now surrounds the endocardiac tube, the cardiac jelly secreted by the myocardium mantle, and the myocardium mantle itself. The myocardium mantle in stage 10 is already capable of contractions, implying a very early formation of contractive elements.

Fig. 20
The embryo is shown with its dorsally lying amniotic cavity and the ventrally lying umbilical vesicle. Due to the folding over of the embryos, the cardiac anlage and the material for the diaphragm undergo a rotation around 180 degrees. The pericardial cavity develops above the cardiac anlage. A part of the material for forming the diaphragm lies on the cranial side of the cardiac anlage. Furthermore, the notochord plate is visible. It replaces the hypoblast in the medial part.

Fig. 21
The embryo begins to fold over cranially and caudally. This is why the cardiac anlage and the material destined for the diaphragm undergo a rotation of 180 degrees. The pericardial cavity, which first develops above the cardiac anlage, comes to lie ventrally. Further on, in stage 10 the closure of the neural tube begins. In its cranial and caudal parts, though, it remains open (neuroporus cranialis and caudalis). The notochordal plate has also separated from the endoderm and formed the definitive notochord.

Fig. 22
The rotation of the cardiac anlage is complete; the pericardial cavity now lies ventral to the cardiac anlage. With the folding, the future intestines have arranged themselves into three sections: foregut, midgut and endgut. The neural tube is closed cranially. The diaphragm anlage, the transverse septum, now lies between the cardiac anlage and the umbilical vesicle, in front of the liver bud. With the folding over, the allantoic stalk is now found on the ventral side of the embryo. In this stage the oropharyngeal membrane is already open. A communication thus exists between the intestines and the amniotic cavity (not shown here).

Fig. 23
The folding over of the embryo is almost complete. Through the massive expansion of the brain the navel region with the umbilical vesicle and the allantois in the allantoic stalk become more compressed. Between the endoderm and the notochord the dorsal aorta has fused in the middle section. Above and below it is still paired.