122. The Primitive Circulatory Network

  1. The primitive circulatory network develops as a result of the almost simultaneous formation of the heart and the 3 vascular networks: intraembryonic, placental or umbilicoallantoic, and vitelline. These vascular embryonic networks form from vascular blood islands which arise in the mesenchyme. The islands hollow out and combine (coalesce) to form a capillary plexus in which certain branches are evident from an early stage of development. Only these persist. At a later stage, these primitive vessels derive their muscle and connective tissue layers from the neighboring mesenchyme
      1. Arteries: the ventral arteries, the first aortic arches, and the dorsal aortas are continuous. In the anterior region of the embryo, in each branchial arch, 5 pairs of aortic arches are formed successively and join the ventral to the dorsal aortas. The anterior arches eventually disappear, whereas the posterior ones are those that develop further
        1. The dorsal aortas extend from the cranial to the caudal region of the embryo and develop paired segmental arteries to the somites. The paired segmental arteries consist of a dorsal series, which vascularizes the neural tube, and a ventral series, which surrounds the primitive gut
        2. Some ventral arteries make a junction with the extraembryonic network
          1. The omphalomesenteric arteries continue as the vitelline arteries in the vascular system of the yolk sac
          2. The allantoic or umbilical arteries feed the placental network
      2. Veins: the paired anterior and posterior cardinal veins develop in the same way as do the aortas, but at a slightly later tim In the heart, they join to form the common cardinal veins, which open into the sinus venosus, close to the vitelline and umbilical veins
    2. THE PLACENTAL OR UMBILICOALLANTOIC VASCULAR NETWORK, which develops in the mesenchyme around the allantois, forms 4 large vessels
      1. The 2 allantoic or umbilical arteries are really 2 of the posterior segmental arteries of the aorta
      2. The 2 umbilical veins run into the sinus venosus. The 2 umbilical veins form a single trunk at the level of the umbilical cord
    3. THE VITELLINE VASCULAR NETWORK develops on the yolk sac surface, especially on its caudal half and forms 4 large vessels
      1. The 2 vitelline arteries, whose proximal parts are the omphalomesenteric arteries
      2. The 2 vitelline veins, which run ventrally in the embryo into the sinus venosus

the primitive circulatory network: image #1