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.
Clinical relevance
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.
