Circulatory System: Blood Vessels
Vasculogenesis is the formation of new blood vessels from cells that were not blood vessels before. As if by magic, blood cells and vessels appear in the early embryo. In fact, mesodermal cells are induced to differentiate into haemangioblasts, which further differentiate into both haematopoietic stem cells and angioblasts.
Haematopoietic stem cells will form all the blood cell types, and angioblasts will build the blood vessels. Separate sites of vasculogenesis may merge to form a network of blood vessels, or new vessels may grow from existing vessels by angiogenesis. When the liver forms it will be the primary source of new haematopoietic stem cells during development.
Angiogenesis is the development of new blood vessels from existing vessels. Endothelial cells detach and proliferate to form new capillaries. This process is under the influence of various chemical and mechanical factors. Although important in growth this also occurs in wound healing and tumour growth, and as such angiogenesis has become a target for anti‐cancer drugs.
Near the end of the third week blood islands form through vasculogenesis on either side of the cardiogenic field and the notochord (see Chapter 27). They merge, creating two lateral vessels called the dorsal aortae (Figure 29.1). These blood vessels receive blood from three pairs of veins, including the vitelline veins of the yolk sac (a site of blood vessel formation external to the embryo), the cardinal veins and the umbilical veins (Figure 29.1).
Blood flows from the dorsal aortae into the umbilical arteries and the vitelline arteries. Branches of the dorsal aortae later fuse to become the single descending aorta in adult life.
The heart tube will form where veins drain to the dorsal aortae. The aortic arches within the pharyngeal arches form here, linking the outflow of the primitive heart to the dorsal aortae. Blood flow begins during the fourth week.
Five pairs of aortic arches form between the most distal part of the truncus arteriosus and the dorsal aortae. They develop within the pharyngeal arches during weeks 4 and 5 of development and are associated with other struc from the pharyngeal arches in the head and neck.
The aortic arches grow in sequence and therefore are not all present at the same time. One little mystery in embryology is that the fifth aortic arch (and pharyngeal arch) either does not form or it grows and then regresses. For that reason the five aortic arch arteries that do develop are named I, II, III, IV and VI (Figure 29.2). The truncus arteriosus also divides and develops into the ventral part of the aorta and pulmonary trunk. Its most distal part forms left and right horns that also contribute to the brachiocephalic artery.
The five aortic arches and paired dorsal aortae combine and develop into a number of vessels of the head and neck (Figure 29.3):
Aortic arch I Maxillary artery Aortic arch II Stapedial artery (rare)
Aortic arch III Common carotid artery and internal carotid artery (external carotid artery is an angiogenic branch of aortic arch III)
Aortic arch IV Right side, right subclavian artery (proximal portion) Left side, aortic arch (portion between the left common carotid and subclavian arteries)
Aortic arch VI Right side, right pulmonary artery Left side, left pulmonary artery and ductus arteriosus
Aortic arch VI forms as a link between the truncus arteriosus and the left dorsal aorta (Figure 29.2); this link persists until birth as the ductus arteriosus. This vessel allows blood flow to bypass the lungs as it connects the pulmonary trunk with the aorta. Foetal pulmonary vascular resistance is high and most blood from the right ventricle (85–90%) passes through the ductus arteriosus to the aorta. Blood flow to the lungs is minimal during gestation and they are protected from circulatory pressures during development. This shunt also allows the wall of the left ventricle to thicken.
The blood supply to the tissue of the heart has been considered to form by angiogenesis from the walls of the right and left aortic sinuses (bulges in the aorta that occur just superior to the aortic valve). This may be influenced by specific tension in the walls of the heart. Vessels form that link with a plexus of epicardial vessels on the surface of the heart. The reverse may be true, however, and these arteries may grow from the epicardial plexus into the aorta and right atrium to initiate their function. Recently, cells from the sinus venosus have been tracked as angiogenic sprouts that migrate over the myocardium and form both coronary arteries and veins and these cells may, in fact, be the source of all the coronary blood vessels.
Coarctation of the aorta is a narrowing of the aorta sometimes found distal to the point from which the left subclavian artery arises. It may be described as preductal or postductal depending upon its location relative to the ductus arteriosus. With postductal coarctation, a collateral circulation develops linking the aorta proximal to the ductus arteriosus with inferior arteries. With a preductal coarctation the route of blood flow through the ductus arteriosus to inferior parts of the body is lost with birth causing hypoperfusion of the lower body.
Aberrations in aortic arch development may give anomalous h as a right arch of the aorta or a vascular ring around he trachea and oesophagus.