Chapter 116. Pericardial Cavity Development and Primitive Heart Circulation - Review of Medical Embryology Book

THE FIRST SIGN OF HEART FORMATION is found at the end of week 3. The first heart-forming cells appear as irregular clusters and cords in the cephalic part of the embryo between the entoderm of the yolk sac and the splanchnic mesoderm. These cell clusters form solid strands across the midline in front of the neural plate and extend down on each side of the embryo by the time the first somites appear

As the head end of the embryo grows forward and folds off from the yolk sac, the 2 solid strands approach each other ventrally and also acquire a lumen lined by endothelial cells. Thus, the 2 endocardial tubes are formed
The lumen of each of the 2 tubes gradually extends cranially into the midline cell strands and finally the 2 meet
With further lateral folding of the embryo, the fusion of the 2 endocardial tubes then progresses from the cephalic point in a caudal direction, thus forming a single endocardial tube

SIMULTANEOUSLY, WITH LATERAL FOLDING and the medial migration and fusion of the tubes, the intracoelomic cavities, right and left, also approach each other in the midline. Initially, at the 4-somite state (about day 21), the primitive heart tubes are connected to the anterior and posterior walls, between the right and left coelomic cavities, by the dorsal and ventral mesocardium

Whereas the ventral part disappears immediately after its early formation, the dorsal mesocardium persists a little longer
As the heart tube elongates, bends, and loops, it slowly sinks into the dorsal wall of the pericardial cavity, which is formed from a fusion of the right and left intraembryonic coelomic cavities
Eventually, beginning at the cranial end, the dorsal mesocardium also breaks down and has entirely disappeared at the 16-somite stage; and the heart tube is then freely suspended in the pericardial cavity and is attached to the surrounding tissues only at its cephalic and caudal ends. The newly formed passage, dorsal to the primitive heart tube, is the future transverse sinus of the pericardial cavity

IN WEEK 5, THE INTRAEMBRYONIC COELOM consists of a thoracic and abdominal component, connected by a canal found on each side of the foregut. In the adult, the intraembryonic coelom is divided into 3 well-defined compartments: the pericardial cavity with the heart, the pleural cavities with the lungs, and the peritoneal cavity with the viscera below the diaphragm. The diaphragm forms the septum between the thorax and abdomen; the pleuropericardial membrane forms between the pericardial and pleural cavities (see discussion on coelomic cavities and mesenteries)

CONTRACTIONS OF THE HEART begin by day 22 and are of myogenic origin. The muscles of the atrium and ventricle are continuous, and contraction, in peristaltic waves, begins in the sinus venosus

The circulation through the embryo and heart, at first, is an ebb-and-flow type, but by the end of week 4, coordination of heart contractions creates a unidirectional flow

THE SINOATRIAL VALVES control the blood flow from the sinus venosus into the atrium, and the blood then passes through the atrioventricular canal into the ventricle. When the latter contracts, blood is pumped through the bulbus cordis and truncus arteriosus into the aortic sac and aortic arches of the branchial arches. Blood then passes to the dorsal aortae for distribution to the embryo, yolk sac, and the placenta