The Cardiac Primordium - The heart of mammals does not arise wholly by the simple fusion, side-by-side, of bilateral halves. A cardiogenic plate lies in front of the head - in the splanchnic mesoderm, beneath the single pericardial chamber located there (See Figure 2.01A). With the forward growth of the head (chiefly neural plate) there is a reversal of this portion of the blastoderm, and the region thus turned under becomes the floor of the fore-gut (See Figure 2.01B). In this process, the heart primordium is necessarily reversed end-for-end, with respect to its original orientation. Also, it then lies above, instead of below the pericardial coelom, and in the splanchnic mesoderm that is situated beneath the fore-gut. The now caudal (tail) end of the heart is continuous with the mass of mesoderm, just cephalad of the anterior intestinal portal, that forms the septum transversum. Here it receives the several veins that enter the septum to drain blood into the heart.
The Cardiac Tube - The earliest identifiable cardiac primordia are aggregates of splanchnic mesodermal cells that appear in the cardiogenic plate, beneath the coelom. They arrange themselves side-by-side as two longitudinal strands, each of which gains a cavity just as primordial vessels do elsewhere in the embryo. These thin-walled, endothelial tubes lie within corresponding longitudinal folds of the splanchnic mesoderm (See Figure 2.01D). In the cranial (head) half of the future heart the two endothelial vessels quickly fuse into a single tube and their individual mesodermal folds merge into a single, trough-like fold which encloses them (See Figure 2.01C). Traced caudad from the short, common pericardial cavity, where these events have occurred, separate tubes and folds are still seen. This is because they necessarily follow the course of the two lateral coelomic canals which continue the common chamber of the pericardial cavity caudad (See Figure 2.01D). As the anterior intestinal portal retreats in a caudal direction to elongate the fore-gut, opportuinity is offered for these paired cardiac primordia to join the median, unpaired portion, already formed. This they do in pace with the enlarging pericardial cavity which progressively incorporates the lateral coelomic canals. Thus, the paired cardiac primordia merge progressively until the entire heart is a single organ.
In embryos with but few somites, the heart is a simple tube within a tibe (See Figure 2.01C). The internal, endothelial component is destined to become the actual lining layer of the endocardium; the external, thick covering gives rise to the myocardium and epicardium. Such a heart is suspended by a mesentery attachment where the lateral margins of the mesodermal folds are reflected upon the ventrolateral sides of the fore-gut. This mesentery, named the dorsal mesocardium, is only temporary. It is lost before the heart has advanced greatly. A peculiarity of the mammalian heart, in contrast to other vertebrates, is that there is no ventral mesocadium (See Figure 2.01C). This is because the coelom arises very early from the coalescence of separate spaces and forms a complete cavity in the region of the heart before the head-fold and heart, as such, begin to differentiate.
Cardiac Regions - Even while the bilateral cardiac halves are merging, they each bear alternate dilatations and constrictions which indicate the future atrium, ventricle and bulbus. The union of the bulbar and ventricular halves is complete in embryos with some nine somites, but the atria are still paired sacs. Such a heart shows at first three divisions (See Figure 2.02A):
Within the next day or two a 4th division, the sinus venosus becomes recognizable as aregion distinct from the caudal end of the atrium (See Figure 2.02B). At this period it lies within the septum transversum and is a center of confluence for the veins draining the body (cardinals), placental (umbilical), and the yolk sac (vitelline). As in fishes, it serves as a reservoir during atrial contraction. At the cranial end of the heart, it is convenient to distinguish a 5th division, the truncus arteriosus, where the bulbus continues as a conduction canal into the aortic sac (See Figure 2.02A). Internally, a pair of sinus valves (right and left) guard the entrance into the atrium and prevent backflow during atrial contraction; swollen endocardial cushions (dorsal and ventral) narrow the heart locally into an atrio-ventricular canal, while elongate bulbar ridges (dorsal and ventral) course into the bulbus and truncus (See Figure 2.02B)
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