Endothelial cells can be found throughout the whole vascular system and they also play a vital role in the movement of white blood cells[2] ECs were first thought to arise from extraembryonic tissues because blood vessels were observed in the avian and mammalian embryos.

This can be distinguished from angiogenesis, which is the creation of new capillaries from vessels that already exist through the process of splitting or sprouting.

[4] It is important to talk more about vasculogenesis because this is what makes ECs different from other types of cells that are found in the body.

During vasculogenesis, the heart and vascular plexus form while the organism is still an embryo, compared to anigiogenesis which is essentially the extension of this.

Not only is VEGF critical for mesoderm cells to become an EC, but also for EPCs to differentiate into mature endothelium.

As stated before, proper functioning of the self-renewal system is essential for the organism to live a long healthy life.

[8] When vasculogenesis occurs the cells transform into different versions throughout the process to eventually become the earliest blood vessels.

However, the study found that Tal1 is actually required for vascular remodeling of the capillary network, rather than early endothelial development itself.

[11] Fetal liver kinase-1 (Flk-1) is a cell surface receptor protein that is commonly used as a marker for ESCs and EPCs.

[7] The two lineages arising from the EPC and the hematopoietic progenitor cell (HPC) form the blood circulatory system.

[12] A study found that in the beginning stages of mouse embryogenesis, commencing at embryonic day 7.5, HPCs are produced close to the emerging vascular system.

In the yolk sac's blood islands, HPCs and EC lineages emerge from the extraembryonic mesoderm in near unison.

This creates a formation in which early erythrocytes are enveloped by angioblasts, and together they give rise to mature ECs.

Some researchers have found that cells with hemangioblast properties have been located in the posterior end of the primitive streak during gastrulation.

In this experiment, it was found that yolk-sac progenitors only contributed on a small amount to hematopoiesis compared to the embryo.

[16] This experiment also showed that blood cells that were made by the yolk sac were not present when the bird hatched.

The fact that this expression was seen in both EC and HPC lineages led researchers to propose a common origin.

Angioblasts can travel during the formation of the circulatory system to configure the branches to allow for directional blood flow.

Pericytes and smooth muscle cells encircle ECs when they are differentiating into arterial or venous arrangements.

Surrounding the ECs creates a brace to help stabilize the vessels known as the pericellular basal lamina.

Studies have shown that when vascular trauma occurs, EPCs and circulating endothelial progenitors (CEPs) are attracted to the site due to the release of specific chemokines.

These cell types accelerate the healing process and prevent further complications such as hypoxia by gathering the cellular materials to reconstruct the endothelium.

These cancerous cells do this by secreting factors such as VEGF and by reducing the amount of PGK, an anti-VEGF enzyme.

ESCs play an incredibly important role in establishing the vascular network that is vital for a functional circulatory system.

However, zebrafish and mouse embryos have widespread use for easily observed development of vascular systems, and the recognition of key parts of molecular regulation when ECs differentiate.