16.1 Outer form and position of the heart

Development of the pericardium

In the early part of stage 9 (ca. 25th day) 9 the embryo is shaped like a foot sole and consists of ectoderm, mesoderm and endoderm cell layers. In the extraembryonic region the lateral plate mesoderm becomes split. The visceral layer, covering the umbilical vesicle, forms the splanchnopleura together with the adjacent endoderm. The parietal layer covers the amniotic cavity and together with the adjacent ectoderm is named somatopleura. In the lateral plate mesoderm itself, probably as a result of uneven growth, dehiscences (splits open), thereby creating small, fluid-filled cleavages. These spaces fuse in the area of the head and form the pericardial cavity that corresponds to the cranial part of the U-shaped intraembryonic coelom. During the course of stages 9 (ca. the 25th day) and 10 (ca. the 28th day) the formation of the head fold occurs via the cranial flexion. Thereby, the future outflow tract (arterial pole), which at the start of the cardiac formation 9 lies caudally to the inflow tract (venous pole), comes to lie cranially due to this cranial flexion of the embryo through a 180 degree rotation and the relative shrinking of the umbilical vesicle.

Fig. 4 - Position of the pericardium
before the rotation
Fig. 5 - Position of the pericardium
after the rotation

Pericardial cavity
Cranial end of the embryo

Arterial part (outflow tract)
Venous part (inflow tract)

Fig. 4
The pericardial cavity as well as the cardiogenic plate lie cranially to the embryonic anlage. The pericardial cavity is found dorsal to the cardiogenic tissue. Further, one can distinguish the venous (inflow tract) from the arterial (outflow tract) parts of the cardiac anlage. The venous part of the later cardiac anlage is found more cranially than the arterial.
Fig. 5
After the 180 degree rotation the pericardial cavity is found ventral to the cardiac anlage. Through the rotation, the venous part (inflow tract) of the cardiac anlage now lies caudal of the arterial pole (outflow tract).

The pericardial cavity expands on both sides of the cardiac anlage and invaginates the myocardiac mantle with the cardiac loop. This results in the mesocardium 9 being transiently formed on the dorsal side of the cardiac loop.

Overview of the illustrations

Meer info

The anatomic relationships of the adult heart can be better understood when one follows the embryonic development of the pericardium further.
(Interactive diagram).

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