21. Week 3 of Development: Cardiovascular System Development

  1. Introduction: early cardiovascular system formation can be related to the absence of a significant amount of yolk material in the ovum and yolk sac with the need for vessels to carry both nutrient materials and oxygen
  2. Angiogenesis or blood formation begins in the extraembryonic mesoderm of the yolk sac, connecting stalk, and chorion during days 13-15. The embryonic vessels begin to develop approximately 2 days later
    1. MESENCHYMAL CELLS or angioblasts aggregate to form isolated masses and cords called blood islands
      1. Spaces accumulate in the islands, angiob1asts arrange themselves around the cavities to form the primitive endothelium, then isolated vessels fuse to form networks of endothelial channels
      2. Vessels continue to extend into adjacent areas by endothelial budding and fusion with other vessels being formed independently
  3. Blood cells and primitive plasma develop from the endothelial cells as the vessels develop on the allantois and yolk sac
    1. BLOOD FORMATION begins in week 5, occurring in various portions of the embryonic mesenchyme, particularly in the liver, then later in the spleen, bone marrow, and lymph nodes
    2. MESENCHYMAL CELLS around the primitive endothelial vessels differentiate into the muscles and connective tissue of the blood vessels
    3. THE PRIMITIVE ENDOTHELIAL CARDIAC TUBES form from mesenchymal cells in the cardiogenic area
      1. Longitudinally paired endothelial channels, the heart tubes, develop before the end of week 3 and begin to fuse into the primitive heart tube
      2. By day 21, the paired tubes link up with blood vessels in the embryo, connecting stalk, chorion, and yolk sac to form a primitive cardiovascular system. The cardiovascular system is the first organ system to reach a functional state
      3. Blood circulation usually is started by the end of week 3

week 3 of development:  cardiovascular system development: image #1