What is the origin of the endothelial cells?

Endothelial cells (ECs) have different origins depending on the stage of development.

Embryonic Origins of Endothelial Cells

During the embryonic stage, vascular endothelial cells originate from the mesoderm, which is one of the three primary germ layers in early embryos. This origin takes place in specific extraembryonic and embryonic regions. The process through which these cells arise is known as vasculogenesis.

Key points about embryonic EC origin:

• Mesodermal Origin: The mesoderm gives rise to various tissues, including the vasculature.
• Vasculogenesis: De novo formation of blood vessels from precursor cells, which leads to the generation of the initial endothelial cell network.
• Location Specific: ECs don't arise from any region of the mesoderm but specific areas that will become the early vascular structures.

Adult Origins of Endothelial Cells

In adults, the renewal and replacement of endothelial cells primarily depend on two sources:

1. Local Resident ECs: These are the endothelial cells that are already present in the blood vessel walls. They divide and proliferate to repair the endothelium.
2. Endothelial Progenitor Cells (EPCs): These are cells found in the bone marrow and the blood, which are capable of differentiating into mature endothelial cells, and then incorporated into the vasculature.

Key points about adult EC origin:

• Renewal vs. De Novo formation: In adults, ECs primarily arise from local cell proliferation and not from vasculogenesis.
• Endothelial Progenitor Cells: EPCs contribute to vascular repair and angiogenesis (new blood vessel formation) in adults, but they aren't the primary source of ECs in adults like they are in embryos.

Therefore, endothelial cells have a complex developmental and maintenance pattern, involving distinct origins and mechanisms, according to the stage of development.