Embryology of the Vascular System

It is quite evident that the vascular apparatus does not independently and by itself “unfold” into the adult pattern. On the contrary, it reacts continuously in a most sensitive way to the factors of its environment, the pattern in the adult being the result of the sum of the environmental influences that have played upon it throughout the embryonic period. We thus find that this apparatus is continuously adequate and complete for the structures as they exist at any particular stage as the environmental structures progressively change; the vascular apparatus also changes and thereby is always adapted to the newer conditions. Furthermore, there are no apparent ulterior preparations at any time for the supply and drainage of other structures which have not yet made their appearance. For each stage it is an efficient and complete going-mechanism, apparently uninfluenced by the nature of its subsequent morphology. George L. Streeter (1918)

This observation made more than 80 years ago exemplifies the finest tradition of the working scientist: years of attention to the most minute details of a subject, which eventuate in the broadest and most comprehensive view of the fundamental issues. In this statement, Streeter summarizes all that needs to be said and virtually all that can be said about the development of the vascular system, save for some specific details that would only embellish the theme he has laid out.

The story of the development of the vascular system encompasses the life span of the organism. This system retains the ability to grow, change, regenerate, and add on in response to the changing needs of the tissues, from the earliest stages of embryonic life to the final breath. Thus, it supports normal growth, wound healing, and revascularization of tissues endangered by restricted flow in existing vessels, just as it supports the new growth of tumors and transiently develops a highly efficient transport and exchange system through the uteroplacental circulation during pregnancy. All this is accomplished by the opening and enlarging of preexisting vessels and the budding of new vascular growth from preexisting stem vessels. That it may eventually fail to respond to adequately supply the myocardium or the central nervous system is not as remarkable as the fact that it responds so well for so long. It seems likely that, in the embryonic and fetal history of the vascular system, there would be clues to the mysteries that surround this responsiveness throughout life. Furthermore, in the prenatal unfolding of the vascular system lie the origins of the various cardiovascular malformations to which the human organism is subject. We do not yet know whether the mechanisms of growth and the stimuli to vascularization of the embryo and fetus are the same as those that encourage and sustain the responsiveness of the vasculature in the postnatal organism.

This chapter does not attempt to review the enormous literature on the subject, and many exciting details are omitted in the interest of providing a simple narrative exposition of the high points. The organizational scheme first discusses a short history of the heart, which is simply a greatly modified blood vessel, followed by descriptions of the development of the large arteries and veins. The chapter concludes with some comments on the growth of small vessels, which, like acorns, must appear and flourish first to produce the mighty trunk and branches of the vascular tree.

Early History

An organism of a cubic millimeter or so in volume (depending on the surface area and other factors related to the effectiveness of diffusion) may thrive without a vascular system. The human embryo enjoys the elaboration of a vascular system from its earliest stages, almost as if it can anticipate that its bulk will soon require a highly sophisticated transport system. As the embryonic disk becomes recognizable, blood islands rapidly accumulate around the periphery of the disk. These isolated “puddles” begin to coalesce and communicate with one another until the embryo resembles a bloody sponge. Most prominent is the precephalic region, where the seemingly random coalescence of blood islands forms a network in the region soon to be identified as the cardiogenic plate ( Fig. 2.1 A ).

In these earliest stages of development, the vascular system manifests some of its greatest mysteries: to what extent is the developmental pattern dictated by tissue needs and demands (possibly through the release of angiogenic factors or through stimuli provided by metabolic products), and to what extent is it dictated by factors such as extravascular pressures restricting flow in one set of possible blood channels and forcing the enlargement of adjacent alternative routes of blood flow? To what extent is the overall pattern dictated genetically? The similarity of the vascular tree from one individual to another favors the speculation that there is a detailed genetic code. The variability from one to another—each pattern seemingly equally efficient in supporting tissues and organs—argues for development according to need and use and based on mechanical and other adventitious factors.

In the case of the heart, a detailed genetic code is surely the guiding factor. Here, curiously, we begin with a parallel pair of cardiac tubes that fuse into one large tube; the latter then divides internally into the right and left hearts. At first glance, this seems inefficient. Why not simply have each original tube of the pair form a right or left heart? The reason is clear when examining the details of internal division of the heart, in which the single outflow tract is divided in such a way as to connect the right heart to the primitive vessels supplying the pulmonary circuit and to connect the remaining members of the branchial arch arteries to the left heart.