Chapter 114. Development of The Heart: Cardiac Tube Development - Review of Medical Embryology Book

Development of the heart is first indicated at day 18 or 19, in the cardiogenic area

During the stage of gastrulation, angiogenetic cell clusters from the splanchnic mesoderm layer of the late presomite embryo migrate from the primitive streak to the area of the oropharyngeal membran They unite with comparable migrating mesoderm from the opposite side to form a horseshoe-shaped plexus, the cardiac primordium or cardiogenic cords

Initially, the central portion of the horseshoe-shaped plexus is found anterior to the prochordal plate and neural plat With closure of the neural plate and formation of the brain vesicles, the CNS grows so rapidly in a cephalic direction that it extends over the central cardiogenic area and the future pericardial cavity
During this growth, the expanding brain pulls the prochordal plate (future buccopharyngeal membrane) and the central part of the cardiogenic plate forward, rotating the plate and pericardial part of the intraembryonic coelomic cavity so that the central portions of the cardiogenic plate and pericardial cavity, initially rostral to the buccopharyngeal plate, are now ventral and caudal to it

Cleavage of the lateral plate by the coelom reaches this region, resulting in the differentiation of the splanchnopleure and somatopleur These form the walls of the future pericardial cavity
Islands appear in the splanchnic mesoderm after day 20, and then by confluence, the cords become 2 thin-walled endothelial tubes which are called the endocardial heart tubes
As the embryo undergoes cephalocaudal flexion, the endocardial tubes approach each other in the midlin Closure of the foregut moves both tubes to a ventral position
The 2 endocardial tubes come together about day 22 and fuse, beginning at the cephalic end of the original horseshoe-shaped structure and extending in a caudal direction. Thus, a single endocardial tube is formed

The developing primitive heart tube, located in the splanchnic mesoderm of the pericardial cavity, bulges gradually more and more into the pericardial cavity and continues until the heart tube, with its investing layer, lies completely in the cavity

The fusion of the 2 tubes is followed by disappearance of the ventral mesocardium and a temporary attachment to the dorsal side of the pericardial cavity by a fold in mesodermal tissue, the dorsal mesocardium

As the heart tubes fuse, the mesenchyme around them thickens to form the myoepicardial mantle, which at first is separated from the endothelial wall of the tube by the cardiac jelly (gelatinous connective tissue substance). The jelly later is invaded by mesenchymal cells
The inner endocardial tube will become the internal endothelial lining of the heart, the endocardium; the myoepicardial mantle gives rise to the myocardium (heart muscle) and epicardium or visceral pericardium
The embryo is now about 23 days old, has 7 somites, and is 2 mm long

The time between the first appearance of the intraembryonic vessels and the heart tube formation is about 3 days. The resulting single median endocardial tube begins to beat about day 22
True embryonic circulation is established between days 27 and 29