THE FETAL AND NEONATAL CIRCULATION 



l62 ; 



monkey, leave little doubt that there is a blood flow 

 mechanism in these placentas which approaches the 

 efficiency of the countercurrent methods (31). 



EARLY DEVELOPMENT OF THE 

 CARDIOVASCULAR SYSTEM 



Streeter's label for the fetus as a whole, "Open 

 for business during alterations'" is most readily ex- 

 tended to the cardiovascular system: this is the first 

 organ system to reach a functional state in the embryo, 

 it supplies all the embryonic tissues and undergoes 

 rapid and extensive alterations during the develop- 

 ment of the organs. 



Peripheral Circ illation 



Again, Hamilton et al. provide a detailed account 

 of the morphology of the development of the mam- 

 malian cardiovascular system (102). But, what deter- 

 mines the appearance of isolated endothelial cords, 

 first in the yolk sac area, and then in the embryo, 

 with the eventual formation of diffuse plexuses? 

 Why do lumina develop in these cords and why do 

 larger channels form? What is responsible for the 

 elaboration of the neighboring mesenchyme into 

 the tunica media and adventitia? Why does the heart 

 form and become differentiated to direct the blood 

 through these channels? None of these questions can 

 be fully answered but, following the study of the his- 

 togenesis of the arteries in the chick embryo, Hughes 

 discusses the many factors which can influence the 

 development of blood vessels (113). The primitive 

 endothelial network is formed before the circulation 

 begins and is determined by genetic factors: in con- 

 trast, the development of the main vessels within the 

 capillary network is dependent upon a circulation 

 and the dynamic relationship between the structure 

 of vessels and the rate, direction and pressure of 

 blood within them is probably acquired early in em- 

 bryonic life. There are no hemodynamic measure- 

 ments with which to substantiate this statement, but 

 the classical relation between function and structure 

 is to be found in Benninghof and Spanner's descrip- 

 tion of the acardiac fetus with a normal twin (35); 

 all the arteries of the acardiac fetus, including the 

 aorta and common carotid, possessed the structure 

 of peripheral muscular vessels because they were 

 physiologically peripheral arteries of the normal 



Premature- 30w«(« 9* 



Full Term 3e-do««ks 3 months 



x COLLAGEN • ELASTIN oELASTIN+COLLAGEN 

 fig. 4. The pattern of fibrous protein distribution in the 

 major vessels of the human infant at 30 weeks gestation, full 

 term, and 3 months post partum (Cleary, unpublished). 



twin, whose heart circulated the blood in both fetuses. 

 The mammalian ductus arteriosus, on the other 

 hand, is a particular example of a muscular artery 

 joining two adjacent elastic arteries. The adult pattern 

 of fibrous protein distribution is present by 30 weeks 

 in the major vessels of the human fetus (fig. 4) ; elastin 

 exceeds collagen in the thoracic aorta but the propor- 

 tion of each is reversed in the abdominal aorta. The 

 percentage of elastin increases to a maximum 3 

 months after delivery. 



The capillary networks are coarse in young em- 

 bryos and become more delicate and numerous dur- 

 ing development, but at different times in the various 

 tissues (142, 152) and it would be instructive to cor- 

 relate the degree of vascularization with the oxygen 

 requirements of the organs. The richness of distribu- 

 tion of the capillary bed will be of special importance 

 in the lungs, brain and cardiovascular system of the 

 prematurely born and postmature young. 



The Heart 



Ebert et al. describe the initial phases in heart 

 formation, from experimental evidence in the chick 

 and rat embryos (87, 96). In the prestreak embryo 

 the capacity for heart formation is widely distributed 

 and pulsating cardiac muscle may develop in tissue 

 culture of peripheral and posterior regions of the 

 blastoderm: later, this capacity is more restricted and 

 is finally limited to two definite regions which sub- 

 sequently fuse in the head process stage embryo. 

 In vitro these cells will develop into a rounded mass 

 of cardiac muscle and the onset of contractility is 

 swift and associated with the appearance of glycogen, 

 but not with definite mvofibrils or cross striations. 



