EARLY DEVELOPMENT OF CARDIOVASCULAR SYSTEM At the end of the second week, embryonic nutrition is obtained from the maternal blood by diffusion through the extraembryonic coelom and umbilical vesicle. At the beginning of the third week, blood vessel formation
begins in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion (Fig. 4-11). Embryonic blood vessels begin to develop approximately 2 days later. The early formation of the cardiovascular system is correlated with the urgent need for blood vessels to bring oxygen and nourishment to the embryo from the maternal circulation through the placenta. During the third week, a primordial uteroplacental circulation develops (Fig. 4-12). Vasculogenesis and Angiogenesis The formation of the embryonic vascular system involves two processes,
vasculogenesis and angiogenesis
Vasculogenesis is the formation of new vascular channels by assembly of individual cell precursors (angioblasts). Angiogenesis is the formation of new vessels by budding and branching from preexisting vessels. Blood vessel formation in the embryo and extraembryonic membranes during the third week (see Fig. 4-11) begins when mesenchymal cells differentiate into endothelial cell precursors, or angioblasts (vessel-forming cells). Angioblasts aggregate to form isolated angiogenic cell clusters, or blood islands, which are associated with the umbilical vesicle or endothelial cords within the embryo. Small cavities appear within the blood islands and endothelial cords by confluence of intercellular clefts. The angioblasts flatten to form endothelial cells that arrange themselves around the cavities in the blood islands to form the endothelium. Many of these endothelium-lined cavities soon fuse to form networks of
endothelial channels (vasculogenesis). Additional vessels sprout into adjacent areas by endothelial budding (angio- genesis) and fuse with other vessels. The mesenchymal cells surrounding the primordial endothelial blood vessels differentiate into the muscular and connective tissue elements of the vessels. Blood cells develop from specialized endothelial cells (hemangiogenic epithelium) of vessels as they grow on the umbilical vesicle and allantois at the end of the third week (see Fig. 4-1 IE and F) and later in specialized sites along the dorsal aorta. Progenitor blood cells also arise directly from hemangiopoietic stem cells. Blood formation (hematogenesis) does not begin in the embryo until the fifth week. It occurs first along the aorta and then in various parts of the embryonic mesenchyme, mainly the liver and later in the spleen, bone marrow, and lymph
nodes. Fetal and adult erythrocytes are derived from hematopoietic progenitor cells. Primordial Cardiovascular System The heart and great vessels form from mesenchymal cells in the cardiogenic area (see Figs. 4-9A and 4-11B). Paired, longitudinal endothelial-lined channels, or endocardial heart tubes, develop during the third week and fuse to form a primordial heart tube (see Fig. 4-12). The tubular heart joins with blood vessels in the embryo, connecting the stalk, chorion, and umbilical vesicle to form a primordial cardiovascular system. By the end of the third week, the blood is circulating, and the heart begins to beat on the 21st or 22nd day.
begins in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion (Fig. 4-11). Embryonic blood vessels begin to develop approximately 2 days later. The early formation of the cardiovascular system is correlated with the urgent need for blood vessels to bring oxygen and nourishment to the embryo from the maternal circulation through the placenta. During the third week, a primordial uteroplacental circulation develops (Fig. 4-12). Vasculogenesis and Angiogenesis The formation of the embryonic vascular system involves two processes,
vasculogenesis and angiogenesis
Vasculogenesis is the formation of new vascular channels by assembly of individual cell precursors (angioblasts). Angiogenesis is the formation of new vessels by budding and branching from preexisting vessels. Blood vessel formation in the embryo and extraembryonic membranes during the third week (see Fig. 4-11) begins when mesenchymal cells differentiate into endothelial cell precursors, or angioblasts (vessel-forming cells). Angioblasts aggregate to form isolated angiogenic cell clusters, or blood islands, which are associated with the umbilical vesicle or endothelial cords within the embryo. Small cavities appear within the blood islands and endothelial cords by confluence of intercellular clefts. The angioblasts flatten to form endothelial cells that arrange themselves around the cavities in the blood islands to form the endothelium. Many of these endothelium-lined cavities soon fuse to form networks of
endothelial channels (vasculogenesis). Additional vessels sprout into adjacent areas by endothelial budding (angio- genesis) and fuse with other vessels. The mesenchymal cells surrounding the primordial endothelial blood vessels differentiate into the muscular and connective tissue elements of the vessels. Blood cells develop from specialized endothelial cells (hemangiogenic epithelium) of vessels as they grow on the umbilical vesicle and allantois at the end of the third week (see Fig. 4-1 IE and F) and later in specialized sites along the dorsal aorta. Progenitor blood cells also arise directly from hemangiopoietic stem cells. Blood formation (hematogenesis) does not begin in the embryo until the fifth week. It occurs first along the aorta and then in various parts of the embryonic mesenchyme, mainly the liver and later in the spleen, bone marrow, and lymph
nodes. Fetal and adult erythrocytes are derived from hematopoietic progenitor cells. Primordial Cardiovascular System The heart and great vessels form from mesenchymal cells in the cardiogenic area (see Figs. 4-9A and 4-11B). Paired, longitudinal endothelial-lined channels, or endocardial heart tubes, develop during the third week and fuse to form a primordial heart tube (see Fig. 4-12). The tubular heart joins with blood vessels in the embryo, connecting the stalk, chorion, and umbilical vesicle to form a primordial cardiovascular system. By the end of the third week, the blood is circulating, and the heart begins to beat on the 21st or 22nd day.
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