Development of the pericardium

In the early part of stage 9 (ca. 25th day) 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 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.

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 (stage 9) being transiently formed on the dorsal side of the cardiac loop.