SEM_07_02_Circulatory pathways. Arterial system. Venous system. Lymphatic system_PARTE2.docx

Development of the arterial system Initially, when the heart begins to develop as a pair of endocardial tubes, the blood is pumped out into a pair of dorsal aortas, which lie along the roof of the embryonic cavity. First, the dorsal aortas comprise a pair of longitudinal vessels in which the cranial...

Development of the arterial system Initially, when the heart begins to develop as a pair of endocardial tubes, the blood is pumped out into a pair of dorsal aortas, which lie along the roof of the embryonic cavity. First, the dorsal aortas comprise a pair of longitudinal vessels in which the cranial ends are connected to the heart and the caudal parts are linked to vitelline arteries. The formation of the adult unpaired aorta is a complex process associated with the fusion of the two endocardial tubes into a single cardiac tube. The two dorsal aortas also merge to form a single dorsal aorta (also known as descending aorta) which run all along the roof of the thoracic cavity (thoracic aorta) and abdominal cavity (abdominal aorta). The most caudal portions of the dorsal aorta remain paired forming the origin of the internal and external iliac arteries. When the body is folded, the cranial portion of the dorsal aorta, which lies dorsal to the foregut, form a series of pair aortic arches, which curved disposition bows from dorsal to ventral on each side of the foregut. The first appearance of the aortic arches coincides with an early stage of the embryo development characterised by the development of transient branchial structures. At this stage, a series of successive parallel aortic arches connect the dorsal and ventral aorta on each side of the foregut. This results in the formation of, in principle, six aortic arches located inside the mesenchyme of each branchial arch. In fish, the six aortic arches remain functional forming the arteries of the gill. In embryos of air-breathing animals, these aortic arches keep developing during a so-called branchial stage, but along with other branchial structures, they eventually undergo a massive transformation into some of the major components of the head and neck. At an early stage, the aortic arches come together ventrally to the foregut in pair of ventral aortas which are continuous with the arterial outlet from the primitive heart, the truncus arteriosus. Later, the two ventral aortas fuse to form a single ventral aorta (also known as ascending aorta or aortic sac) which keep connected with the aortic arches by a dilated portion known as the aortic sac. https://sway.office.com/VKE6vIU0DRejS3Dg#content=9Vmie5XJkxZMMt - Development of the aorta. In domestic mammals, the development of the aorta takes place about the third week of gestation. Initially, a pair of primitive aortas are connected to the heart through the first pair of aortic arches. Subsequently, the ventral portions of the paired aorta fuse to form the ascending aorta or aortic sac. The same happens to the dorsal portions of the aorta to form the descending aorta. At this stage, the dorsal and ventral parts of the aorta are connected by a set of six pairs of aortic arches, the so-called branchial arch arteries. Derivatives of Aortic Arches In air-breathing animals, with the remodelling of the branchial structures, the pairs of aortic arches are transformed into some of the major arteries that supply the head, neck and thorax in the adult. First and second pair of aortic arches. Although they may contribute to the formation of some arteries of the face and neck, they remain largely rudimentary in most mammals. Third pair of aortic arches. Each third aortic arch becomes a common carotid artery which further divides into the internal carotid and external carotid arteries. Consequently, the arteries derived from the third arch supply the neck and head whereas the fourth-derived arteries spread toward more caudal regions. Fourth pair of aortic arches. The fourth left aortic arch will become the definitive arch of the aorta. The distal portions of the right aortic arch contribute to the formation of the right subclavian artery but the proximal part of the fourth right aortic arch degenerates and disappears. If the definitive aortic arch develops as a double arch or on the right side, rather than its normal side on the left, it forms an abnormal vascular ring that traps the oesophagus in the middle. Fifth pair of aortic arches. They either remain rudimentary or never develop in birds and mammals. Sixth pair aortic arches. The proximal part of the sixth aortic arches become the pulmonary arteries. The distal part degenerates on the right side but persists on the left side forming the ductus arteriosus. https://sway.office.com/VKE6vIU0DRejS3Dg#content=gHhi1MwHVqL8aI - Aortic arches. These embryonic arches connect the aortic sac to the dorsal aorta. Each pair of aortic arches travel in the centre of each pharyngeal arch, embedded in the mesenchyme. Initially, there are five pairs of arches, but these undergo structural changes and differentiation to form the definite vascular patterns for the head and neck, aorta, and pulmonary circulation. - Schematic diagram depicting the embryological development of the aortic arches and their branches. Vascular ring anomalies Vascular ring anomalies occur when a congenital abnormality of the heart's blood vessels result in the oesophagus being compressed at the level of the base of the heart. This, in turn, prevents solid food from being able to pass properly past the compression as well as the dilatation of the oesophagus in front of the compressed area. This is termed megaesophagus. Because food is not moved properly through the oesophagus, regurgitation occurs. These congenital defects may arise if the development or dissolution of the aortic arches is disrupted. Multiple vascular ring anomalies have been reported in dogs, including the double aortic arch, persistent right aortic arch, left aortic arch with right ligamentum arteriosum, and retroesophageal subclavian arteries. The persistence of the abnormal right aortic arch is the most common oesophageal vascular ring anomaly seen in dogs. The setting of the vascular ring is completed by the branching of the brachiocephalic vessels and the location of the ductus arteriosus. If a double aortic arch is formed the oesophagus is trapped between the right and left aortic arches https://sway.office.com/VKE6vIU0DRejS3Dg#content=NiuWcdaMVuCCg3 - Vascular ring anomalies Abnormal location or development of the aortic arches may result in pressure on adjacent organs. An abnormal double aortic arch or a right aortic arch results in a ring around the oesophagus, leading to a constriction that can become a serious condition. - Oesophagus completely encircled by a double aortic arch Aortic segmental arteries From the descending aorta, blood is distributed to the body by segmental arteries in a metameric arrangement. The dorsal aorta gives off dorsal, lateral, and ventral segmental branches, some of which will persist as adult vessels. Dorsal segmental arteries are paired and emanate between individual somites from the most cranial segment back to the sacral region. Most of the embryonic dorsal segmental arteries will persist in the thoracic and lumbar region forming the intercostal arteries and lumbar arteries, respectively. Lateral segmental arteries supply organs that develop from the intermediate mesoderm. When the urogenital system develops, these arteries will be the ovarian, testicular, renal and adrenal arteries. Ventral segmental arteries are initially the paired arteries associated with the yolk sac and allantois. Further in development, they become single visceral branches for the stomach and intestines, the adult coeliac artery and mesenteric arteries. https://sway.office.com/VKE6vIU0DRejS3Dg#content=9u54h4fHKDYycu - Aortic segmental arteries. The segmental (intersegmental) arteries are branches of the dorsal aorta and extend out between the somites. They contribute to the formation of the arterial blood supply to the viscera (ventral and lateral segmental arteries) and to the body wall (dorsal segmental arteries) Development of the venous system From the beginning, the embryonic sinus venosus of the heart receives the return of blood from different parts of the body. It receives the vitelline veins, which drain the yolk sac, the umbilical veins, which drain the allantois, and the cardinal venous system, which return the deoxygenated blood from all the embryonic tissues. In contrast with the vitelline and allantois veins that only serve the foetus, the cardinal venous system is the origin of the definitive venous system, which major vessels are the cranial vena cava and caudal vena cava. The remodelling of the primitive cardinal venous system into the adult venous pattern is a gradual and complex process that sometimes involves the formation of transient intermediate veins. Cardinal veins In the embryo, the primitive venous system consists of four cardinal veins none of which are retained in the adults. The left and right cranial cardinal veins drain blood from the head, neck region and the cranial portion of the thorax. The left and right caudal cardinal veins return blood from the caudal portion of the body where the mesonephros is the main functional organ at this stage. Before entering the heart, the cranial and caudal cardinal veins fuse on each side of the embryo to form the left and right common cardinal veins which drain into the sinus venosus of the heart. When the body of the embryo increases in complexity, the paired cardinal system is gradually replaced by two unpaired adult veins that return the blood to the heart: the cranial vena cava and caudal vena cava. - The cardinal system The cardinal veins form as the basis for the intraembryonic venous part of the circulatory system. Initially, the cardinal system consists of symmetric, paired cranial and cardinal veins draining into the sinus horns via short common cardinal veins. Remodeling of the cranial cardinal veins. Cranially to the heart, the cranial cardinal veins will be transformed in the adult internal jugular veins. The much larger external jugular and subclavian veins arise by budding from the cranial cardinal vein. Near the heart, the blood from the left cranial cardinal vein is shifted toward the right side by an anastomotic vein, the left brachycephalic vein. Thus, all blood from the head, neck and thoracic limbs is carried toward the terminal segment of the right cranial cardinal vein which becomes the cranial vena cava. Failure of the anastomotic vein to develop results in a double cranial vena cava, which is a normal condition in rats and mice. https://sway.office.com/VKE6vIU0DRejS3Dg#content=IKYfxYb6ZxRtHR - Changes in the cranial cardinal veins. The cranial cardinal system drains the head, neck and thoracic limbs. They are in the origin of the main veins draining those regions which return the blood toward the cranial vena cava. Remodeling of the caudal cardinal veins. Formation of the supracardinal and subcardinal veins The initial system of caudal cardinal veins is temporary and end up being replaced by two new sets of paired veins, the supracardinal veins and the subcardinal veins. The subcardinal venous networks, located in the intermediate mesoderm, drain the blood ventrally from the embryonic kidneys and gonads. After forming extensive anastomoses, the right subcardinal vein gives rise to most of the caudal vena cava. The supracardinal veins develop to drain the dorsal region of the body and they will be transformed into the azygos veins in the adult. https://sway.office.com/VKE6vIU0DRejS3Dg#content=drm4kMLwOHCgT5 - The caudal cardinal veins mostly degenerate after being superseded by the supracardinal and subcardinal veins. The right subcardinal vein forms the most caudal part of the caudal vena cava into which the left cardinal vein will then drain. The supracardinal veins contribute to form the azygos veins. https://sway.office.com/VKE6vIU0DRejS3Dg#content=iG6MkS0MLVPSMo - Ventral view of the development of the venous system Lymphatics Lymphatic vessels develop from lymph sacs that arise from developing veins, which are derived from mesoderm. The first lymph sacs to appear are the paired jugular lymph sacs at the junction of the internal jugular and subclavian veins. From the jugular lymph sacs, lymphatic capillary plexuses spread to the thorax, thoracic limbs, neck, and head. Four more lymph sacs are formed to collect the lymph from the rest of the trunk and pelvic extremities of the body. These are the retroperitoneal lymph sac, the cisterna chyli, and the paired caudal lymph sacs. Lymph nodes are produced by localised mesodermal invaginations that divide the lumen of the vessels into sinusoids. Then, the mesoderm develops a reticular framework within which lymphocytes accumulate. The spleen and haemal nodes (in ruminants) develop in a similar way. - Development of the lymphatic system