Ligamentum teres hepatis
- Descending aorta
Superior vesical artery
- Descending aorta
Superior vesical artery
Figure 11.48 Human circulation after birth. Note the changes occurring as a result of the beginning of respiration and interruption of placental blood flow. Arrows, direction of blood flow.
The lymphatic system begins its development later than the cardiovascular system, not appearing until the fifth week of gestation. The origin of lymphatic vessels is not clear, but they may form from mesenchyme in situ or may arise as saclike outgrowths from the endothelium of veins. Six primary lymph sacs are formed: two jugular, at the junction of the subclavian and anterior cardinal veins; two iliac, at the junction of the iliac and posterior cardinal veins; one retroperitoneal, near the root of the mesentery; and one cisterna chyli, dorsal to the retroperitoneal sac. Numerous channels connect the sacs with each other and drain lymph from the limbs, body wall, head, and neck. Two main channels, the right and left thoracic ducts, join the jugular sacs with the cisterna chyli, and soon an anastomosis forms between these ducts. The thoracic duct then develops from the distal portion of the right thoracic duct, the anastomosis, and the cranial portion of the left thoracic duct. The right lymphatic duct is derived from the cranial portion of the right thoracic duct. Both ducts maintain their original connections with the venous system and empty into the junction of the internal jugular and subclavian veins. Numerous anastomoses produce many variations in the final form of the thoracic duct.
The entire cardiovascular system—heart, blood vessels, and blood cells—originates from the mesodermal germ layer. Although initially paired, by the 22nd day of development the two tubes (Figs. 11.3 and 11.4) form a single, slightly bent heart tube (Fig. 11.6) consisting of an inner endocardial tube and a surrounding myocardial mantle. During the 4th to 7th weeks the heart divides into a typical four-chambered structure.
Septum formation in the heart in part arises from development of endocardial cushion tissue in the atrioventricular canal (atrioventricular cushions) and in the conotruncal region (conotruncal swellings). Because of the key location of cushion tissue, many cardiac malformations are related to abnormal cushion morphogenesis.
Septum Formation in the Atrium. The septum primum, a sickle-shaped crest descending from the roof of the atrium, begins to divide the atrium in two but leaves a lumen, the ostium primum, for communication between the two sides (Fig. 11.14). Later, when the ostium primum is obliterated by fusion of the septum primum with the endocardial cushions, the ostium secundum is formed by cell death that creates an opening in the septum primum. Finally, a septum secundum forms, but an interatrial opening, the oval foramen, persists. Only at birth, when pressure in the left atrium increases, do the two septa press against each other and close the communication between the two. Abnormalities in the atrial septum may vary from total absence (Fig. 11.19) to a small opening known as probe patency of the oval foramen.
Septum Formation in the Atrioventricular Canal. Four endocardial cushions surround the atrioventricular canal. Fusion of the opposing superior and inferior cushions divides the orifice into right and left atrioventricular canals. Cushion tissue then becomes fibrous and forms the mitral (bicuspid) valve on the left and the tricuspid valve on the right (Fig. 11.17). Persistence of the common atrioventricular canal (Fig. 11.20) and abnormal division of the canal (Fig. 11.21 B) are well-known defects.
Septum Formation in the Ventricles. The interventricular septum consists of a thick muscular part and a thin membranous portion (Fig. 11.25) formed by (a) an inferior endocardial atrioventricular cushion, (b) the right conus swelling, and (c) the left conus swelling (Fig. 11.23). In many cases these three components fail to fuse, resulting in an open interventricular foramen. Although this abnormality may be isolated, it is commonly combined with other compensatory defects (Figs. 11.28 and 11.29).
Septum Formation in the Bulbus. The bulbus is divided into (a) the trun-cus (aorta and pulmonary trunk), (b) the conus (outflow tract of the aorta and pulmonary trunk), and (c) the trabeculated portion of the right ventricle. The truncus region is divided by the spiral aorticopulmonary septum into the two main arteries (Fig. 11.22). The conus swellings divide the outflow tracts of the aortic and pulmonary channels and with tissue from the inferior endocardial cushion close the interventricular foramen (Fig. 11.23). Many vascular abnormalities, such as transposition of the great vessels and pulmonary valvular atresia, result from abnormal division of the conotruncal region; they may involve neural crest cells that contribute to septum formation in the conotruncal region.
The aortic arches lie in each of the five pharyngeal arches (Figs. 11.35). Four important derivatives of the original aortic arch system are (a) the carotid arteries (third arches); (b) the arch of the aorta (left fourth aortic arch); (c) the pulmonary artery (sixth aortic arch), which during fetal life is connected to the aorta through the ductus arteriosus; and (d) the right subclavian artery formed by the right fourth aortic arch, distal portion of the right dorsal aorta, and the seventh intersegmental artery (Fig. 11.35B). The most common vascular aortic arch abnormalities include (a) open ductus arteriosus and coarctation of the aorta (Fig. 11.37) and (b) persistent right aortic arch and abnormal right subclavian artery (Figs. 11.38 and 11.39), both causing respiratory and swallowing complaints.
The vitelline arteries initially supply the yolk sac but later form the celiac, superior mesenteric, and inferior mesenteric arteries, which supply the foregut, midgut, and hindgut regions, respectively.
The paired umbilical arteries arise from the common iliac arteries. After birth the distal portions of these arteries are obliterated to form the medial umbilical ligaments, whereas the proximal portions persist as the internal iliac and vesicular arteries.
Venous System. Three systems can be recognized: (a) the vitelline system, which develops into the portal system; (b) the cardinal system, which forms the caval system; and (c) the umbilical system, which disappears after birth. The complicated caval system is characterized by many abnormalities, such as double inferior and superior vena cava and left superior vena cava (Fig. 11.46).
Changes at Birth. During prenatal life the placental circulation provides the fetus with its oxygen, but after birth the lungs take on gas exchange. In the circulatory system the following changes take place at birth and in the first postnatal months: (a) the ductus arteriosus closes; (b) the oval foramen closes; (c) the umbilical vein and ductus venosus close and remain as the ligamentum teres hepatis and ligamentum venosum; and (d) the umbilical arteries form the medial umbilical ligaments.
Lymphatic System. The lymphatic system develops later than the cardiovascular system, originating as five sacs: two jugular, two iliac, one retroperitoneal, and one cisterna chyli. Numerous channels form to connect the sacs and provide drainage from other structures. Ultimately the thoracic duct forms from anastomosis of the right and left thoracic ducts, the distal part of the right thoracic duct, and the cranial part of the left thoracic duct. The right lymphatic duct develops from the cranial part of the right thoracic duct.
1. A prenatal ultrasound of a 35-year-old woman in her 12th week of gestation reveals an abnormal image of the fetal heart. Instead of a four-chambered view provided by the typical cross, a portion just below the crosspiece is missing. What structures constitute the cross, and what defect does this infant probably have?
2. A child is born with severe craniofacial defects and transposition of the great vessels. What cell population may play a role in both abnormalities, and what type of insult might have produced this effect?
3. What type of tissue is critical for dividing the heart into four chambers and the outflow tract into pulmonary and aortic channels?
4. A patient complains about having difficulty swallowing. What vascular abnormality or abnormalities might produce this complaint? What is its embryological origin?
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.