Whereas the superior two thirds of the neural tube give rise to the brain, the inferior third forms the spinal cord. In the embryo, the spinal cord extends along the entire vertebral column length, but after that it lags behind vertebral growth. Ossiication of the entire sacrum is visible sonographically by approximately 21 weeks (Chap. 10, p. 191). By 24 weeks, the spinal cord extends to SIiat birth to L3, and in the adult to LI.
Spinal cord myelination begins at midgestation and continues through the irst year of life. Synaptic function is suiciently developed by the eighth week to demonstrate flexion of the neck and trunk (Temiras, 1968). During the third trimester, integration of nervous and muscular function proceeds rapidly.
The embryology of the heart is complex. At its earliest stages of formation, the fetal heart undergoes molecular programming, and more than a hundred genes and molecular factors are integral to its morphogenesis. To summarize, the straight cardiac tube is formed by the 23rd day during an intricate morphogenetic sequence, during which each segment arises at a unique time. The tube then undergoes looping, and the chambers then fuse and form septa (Manner, 2009). The valves develop, and the aortic arch forms by vasculogenesis. For a complete description, refer to Chapter 9 in Hursts he Heart (Keller, 2013).
his unique circulation is substantially diferent from that of the adult and functions until birth, when it changes dramatically. For example, because fetal blood does not need to enter the pulmonary vasculature to be oxygenated, most of the right ventricular output bypasses the lungs. In addition, the fetal heart chambers work in parallel, not in series, which efectively supplies the brain and heart with more highly oxygenated blood than the rest of the body.
Oxygen and nutrient materials required for fetal growth and maturation are delivered from the placenta by the single umbilical vein (Fig. 7-9). The vein then divides into the ductus venosus and the portal sinus. The ductus venosus is the major branch of the umbilical vein and traverses the liver to enter the inferior vena cava directly. Because it does not supply oxygen to the intervening tissues, it carries well-oxygenated blood directly to the heart. In contrast, the portal sinus carries blood to the hepatic veins primarily on the left side of the liver, and oxygen is extracted. The relatively deoxygenated blood from the liver then lows back into the inferior vena cava, which also receives more deoxygenated blood returning from the lower body. Blood flowing to the fetal heart from the inferior vena cava, therefore, consists of an admixture of arterial-like blood that passes directly through the ductus venosus and less well-oxygenated blood that returns from most of the veins below the level of the diaphragm. The oxygen content of blood delivered to the heart from the inferior vena cava is thus lower than that leaving the placenta.
Foramen ovale Umbilical v. Hypogastricaa. Ductus Aorta _ --
FIGURE 7-9 The intricate nature of the fetal circulation is evident. The degree of blood oxygenation in various vessels differs appreciably from that in the postnatal state. aa = arteries; LA = left atrium; LV = left ventricle; RA = right atrium; RV = right ventricle; v = vein.