CHAPTER 13 Early embryonic circulation

The early embryonic circulation is symmetrical (Fig. 13.1). It is modified throughout development to produce a functioning fetal circulation which is connected to the placenta, and changes rapidly at birth to accommodate disconnection from the placenta and the start of gaseous exchange in the lungs. Major restructuring of early vessels occurs as the embryo grows: anastomoses form and then disappear, capillaries fuse and give rise to arteries or veins, and the direction of blood flow may reverse several times before the final arrangement of vessels is completed.

Fig. 13.1 The early, symmetrical blood vascular system. A, Ventrolateral view of the endothelial profile of the heart, the first aortic arch arteries and the dorsal aorta shown in relation to the major epithelial populations. B, Ventrolateral view of the main venous channels shown in relation to the major epithelial populations. C, Left lateral view of the blood vascular system of a stage 11 human embryo; only the endothelial lining of the heart tube is shown. At this stage the arteries and veins are in the process of development and no true circulation is yet established.

The earliest circulatory components develop by vasculogenesis in the extraembryonic tissues. The endothelial heart tubes, dorsal aortae, umbilical and early vitelline vessels arise by vasculogenesis within the embryo. Further vessel development occurs by a process of angiogenesis in which angioblasts, arising in splanchnic and somitic tissues, add endothelial sprouts and branches to earlier vessels. None of the main vessels of the adult arises as a single trunk in the embryo. A capillary network is first laid down along the course of each vessel; the larger arteries and veins are defined by selection and enlargement of definite paths in this network. Lymphatic vessels develop after the main arteries and veins are formed: they arise initially by angiogenesis from the cardinal veins and subsequently by proliferation of lymphangioblasts to form lymphatic capillaries.

Early blood vessels are initially surrounded by a fibronectin-rich matrix that is later incorporated into the endothelial basal lamina along with laminin, a particularly early constituent. Several layers of fibronectin-expressing cells are seen around larger vessels, such as the dorsal aortae. The endothelium does not synthesize a basal lamina in those regions where remodelling and angiogenesis is active, and the mesenchyme around such endothelium does not express α-actin or laminin until branching has stopped and differentiation of the tunica media begins (after a stable vascular pattern has formed). It is not known how differentiation of pericytes and smooth muscle cells is induced; the majority of arteries accumulate medial smooth muscle from the surrounding mesenchyme.

In early development, the arteries of the embryo are disproportionately large and their walls consist of little more than a single layer of endothelium. The cardiac orifices are also relatively large and the force of the cardiac contraction is weak, and consequently the circulation is sluggish, despite the rapid rate of contraction of the developing heart. However, the tissues are able to draw nourishment not only from the capillaries but also from the large arteries and the intraembryonic coelomic fluid.

It has been suggested that the rapidly expanding cardiovascular system is filled with plasma by the movement of fluid from the intraembryonic coelom to the veins. In general, the wall of the intraembryonic coelom is composed of proliferating cells which produce the splanchnopleuric and somatopleuric mesenchymal populations. However, the walls of a portion of the pericardioperitoneal canals are thinner, and possibly more permeable to coelomic fluid, at the time when the canals surround the hepatocardiac channels (veins). The latter lie between the hepatic plexus and the sinus venosus; the hepatocardiac channel on the right side is more developed and on the left it is more plexiform, with only a transitory connection to the sinus venosus. The differentiation of this specific coelomic region occurs just in advance of the expansion and filling of the right and left atria, at about stage 12.

As the heart muscle thickens, compacts and strengthens, the cardiac orifices become both relatively and absolutely reduced in size, the valves increase their efficiency, and the large arteries acquire their muscular walls and undergo a relative reduction in size. From this time onwards, the embryo is dependent for its nourishment on the expanding capillary beds, and the function of the larger arteries becomes restricted to that of controllable distribution channels to keep the embryonic tissues constantly and appropriately supplied.

The heart starts to beat early, before the development of the conduction system, and a circulation is established before a competent valvular mechanism has formed. Cardiac output increases in proportion with the weight of the embryo and cardiac rate increases with development. However, most of the increase in cardiac output results from a geometric increase in stroke volume. When dorsal aortic blood flow is matched to embryonic weight, blood flow remains constant over a more than 150-fold change in mass of the embryo.

After head folding, the embryo has bilateral primitive aortae, each consisting of ventral and dorsal parts that are continuous through the first embryonic aortic arches (see Ch. 35). The ventral aortae are fused and form a dilated aortic sac. The dorsal aortae run caudally, one on each side of the notochord. In the fourth week they fuse from about the level of the fourth thoracic to that of the fourth lumbar segment to form a single definitive descending aorta (Figs 13.1A,C, 13.2B). In general, more mature endothelial channels are seen in the rostral, more advanced regions of the embryo, whereas more caudally, a changing capillary plexus constantly remodels until it becomes confluent with the vascular channels of the connecting stalk. The dorsal continuation of the primitive dorsal aortae directs blood into an anastomosing network around the allantois which will form the umbilical arteries. Blood is channelled back to the developing heart from the allantois via umbilical veins, from anastomoses in the primitive yolk sac via the vitelline veins, and from the body via pre- and post-cardinal veins that join to form the common cardinal veins (Figs 13.1B,C, 13.2A).