16.1 Outer form and position of the heart

• Introduction
  
• First signs of cardiac development
  
• The formation of the venticular loop - the U-shaped heart
• Development of the pericardium
• Descent of the hearts

Introduction

In the young embryo epiblast cells wander via the primitive streak between epi- and hypoblast and form the mesoderm layer out of which various structures arise. Cells, which are destined for cardiogenesis, take up a position that is cranial to the forming neural tube. Cardiogenesis takes place via a complex series of steps:

1. Determination of mesoderm- and neural crest cells for heart formation
2. Growth and differentiation processes to become cardiomyocytes
3. Migration and transformation processes in order to form the heart

First signs of cardiac development

The cardiogenic plate is formed by a collection of mesoderm cells in the most anterior part of the embryo.
In the interactive diagram, please observe how the position of the pericardial cavity changes in relation to the cardiogenic plate due to the bending of the cranial end of the embryo.

Up to the early stage 9 (at roughly 25 - 27 days) 9 this cardiac anlage is still located in the visceral part of the splanchnic mesoderm (splanchnopleura) above the umbilical vesicle. Out of this mainly develops the myocardium, which is responsible for the very early contractile ability 9-10 of the embryonic heart. Through the rotation following the cranial folding of the embryo the pericardial cavity comes to lie ventrally from the cardiac anlage and, as things progress, will surround it (11).

The blood from the supplying vessels, the umbilical and omphalomesenteric veins, flows caudally over the inflow tract into the cardiac anlage and leaves it via the outflow tract and the aortic arches at the cranial end.

The cardiac tube itself consists of three layers: epicardium, myocardium and endocardium.
The outermost layer and boundary of the pericardial cavity is the epicardium. The myocardial mantle follows as the next inner layer. Together they form the myoepicardium. The considerable distance from the myocardial mantle to the endocardial tube is filled with cardiac jelly and the cardiac lumen is coated with endocardial cells.

Fig. 2a
The cranial part of the embryo is shown in a medial sagittal section. The cardiac anlage is surrounded by the pericardial cavity. The heart consists of the myoepicardial mantle, the cardiac jelly and the endocardium.
Fig. 2b
In this transversal section the heart finds itself at the level of the rhomboencephalon, i.e., still very cranial. Note the position of the pericardium in relation to the heart, the many layers of the cardiac wall and the provisional existence of a dorsal suspension (mesocardium).
(see also the development of the pericardium)