Introduction
Our body has two different vessel networks:
- the blood vessels that form a circulation system for the blood
- the lymph system, the vessels of which transport the lymph in only one direction
The blood vessels bring oxygen, nutrients and a wide variety of immune cells to the individual organs and transport CO2 and catabolites away.
The lymph system takes up fluids and immune competent cells that have left the blood vessels and conveys them back into the blood circulation system by passing them through various lymph nodes.
The first signs of vessel formation are found in the region of the umbilical vesicle (extraembryonic) at stage 8. Mesodermal cell masses are seen there that are also known as hemangioblasts. Within these mesodermal cell masses, those in the center become rounded and develop into the precursors of the blood cells (hemocytoblasts) while the peripheral cells come together as delimiting endothelial cells (angioblasts).
Both the formation of the vessels as well as that of the cardiac tube have thus a close relationship to the endoderm that appears to have an inductive influence. Already before stage 9 intraembryonic vessels also form from angioblasts that have differentiated within the splanchnopleurae. Angioblast cells migrate away and settle in the various organs, attracted by angiogenetic factors that stimulate vessel formation.
The molecular basis of blood vessel development
The actual vasculogenesis, the formation of tubes by the angioblasts, takes place in that neighboring cells come to lie beside each other and thereby delimit a vessel lumen. The angioblasts further differentiate themselves into pericytes and into fibroblasts and smooth muscle cells. Ultimately, the forming of the vessel stems from a plexus of capillaries is also determined by hemodynamic factors.
However, not all of the vessels arise from a capillary network; the dorsal branches of the aorta are strongly determined by the metamerism of the somites.
Schematic diagram of the maturation and stabilization of blood vessels. The blue arrows show that various growth factors (VEGF, Ang1, EphrinB2) and the corresponding receptors (VEGFR1/2 Tie1/2) influence vessel formation.
The interactive diagram indicates the factors responsible for the differentiation of embryonic vessels either to lymph or to venous vessels.
Recently various growth factors have been found that are responsible for the formation and further development of vessels. It is also known that these factors must work together in concert so that functional vessels can form. In the interactive diagram the most important growth factors are shown that determine whether vessels differentiate further or atrophy in the course of the development.