The heart tube is a rudimentary, tube-like structure that represents the earliest form of the heart in a developing embryo. It is the first organ to form and begin functioning, establishing the circulatory system. This primitive structure is the precursor to the complex, four-chambered heart that will eventually pump blood throughout the body.
Formation of the Heart Tube
The heart tube begins to develop around day 18 to 19 after fertilization, originating from mesodermal cells in the embryonic disc. These cells, specifically within the splanchnic mesoderm, form the cardiogenic area near the embryo’s head. Within this region, two endothelial strands, called angioblastic cords, canalize to form two distinct endocardial tubes.
By the end of the third week, these two endocardial tubes migrate towards the midline of the embryo and fuse, creating a single, primitive heart tube. This fusion is facilitated by lateral embryonic folding, which brings the bilaterally located tubes together. The newly formed heart tube is initially a straight structure with an inner endothelial lining, a middle layer of extracellular matrix called cardiac jelly, and an outer muscular layer.
Remodeling into a Four-Chambered Heart
The heart tube undergoes morphological changes to transform into the adult four-chambered heart. Around day 22-23, the heart tube begins cardiac looping, where it elongates and bends into a C-shape and then an S-shape. This looping positions the future chambers in their correct anatomical relationships, with the primitive ventricle moving anteriorly and to the right, and the primitive atrium shifting dorsally, cranially, and to the left.
Following looping, septation begins between weeks four and eight, dividing the single tube into distinct chambers. The interatrial septum forms in the common atrium, and the interventricular septum develops in the ventricles, having both muscular and membranous components that fuse. Simultaneously, endocardial cushions within the atrioventricular canal grow and fuse, separating the canal into left and right components and contributing to the formation of the atrioventricular valves. The outflow tract, initially a single vessel, also undergoes septation and spiraling, forming the aorta and pulmonary artery, occurring around the fifth week.
Early Circulation and Function
The heart tube is the first functional organ in the embryo, beginning to beat and pump blood around day 21 or 22 of gestation. This early pumping action is necessary because the embryo’s increasing nutritional and oxygen demands can no longer be met by simple diffusion alone by the beginning of the fourth week. The heart tube, despite initially lacking valves, generates unidirectional blood flow.
Blood enters the heart tube at its caudal end, specifically into the sinus venosus, from early embryonic veins. Contractions then propel this blood cranially, through various primitive regions of the tube, and out through the aortic arches. This rudimentary circulation delivers nutrients and oxygen to the developing tissues and removes waste products.
Significance of Proper Heart Tube Development
The precise and timely development of the heart tube is important for the healthy formation of the entire cardiovascular system. The heart’s early function as a pump is necessary to meet the rapidly increasing metabolic demands of the growing embryo. Errors during this sensitive period can lead to congenital heart defects (CHDs), which are among the most common birth defects, affecting approximately one in every 100 babies.
Malformations can arise from disruptions in various stages, including abnormal cardiac looping, which can cause conditions like transposition of the great arteries, where the aorta and pulmonary trunk are incorrectly connected. Issues during septation, such as incomplete fusion of the septa, can result in holes in the heart walls, like atrial or ventricular septal defects. Defects in the outflow tract development can lead to abnormalities in the great vessels such as coarctation of the aorta or Tetralogy of Fallot.