40.1:
Overview of the Vascular System
The innermost wall of all blood and lymph vessels comprises a single layer of endothelial cells that regulate the passage of molecules in and out of the fluid.
These cells are polarized in the direction of blood flow and separated from the outer layers by the basal lamina.
Endothelial cells and basal lamina form primary blood vessels that mature into arteries, capillaries, and veins.
Endothelial cells have distinct basal and apical surfaces. While the basal surface connects to the basal lamina, the apical surface is coated with glycocalyx. The Glycocalyx forms a selective, mesh-like barrier for the solutes and fluids passing across the endothelial lining.
Properties of endothelial cells, such as their shape, glycocalyx composition, and presence of intercellular junctions vary depending on their function.
For example, junction proteins connect adjacent blood endothelial cells without gaps to form zipper-like junctions and prevent interstitial fluid leakage.
In contrast, some lymphatic vessels may have a discontinuous endothelium, where the interjunctional gaps between the cells allow draining of lymph, immune cells, and macromolecules.
40.1:
Overview of the Vascular System
The vascular system comprises an extensive network of arteries, capillaries, and veins. The vascular system can be broadly divided into the blood and lymphatic systems. Typically, blood vessels can be categorized into three histological regions: tunica intima, tunica media, and tunica adventitia. The tunica intima consists of a single layer of endothelial cells attached to the basal lamina. Underlying the basal lamina is a connective tissue layer and an elastic lamina that gives stability and flexibility to the endothelial lining. Endothelial cells remain in close contact with the multipotent perivascular cells called pericytes. These pericytes help endothelial cells modulate blood flow by providing contractile strength. Below the tunica intima lies the tunica media composed of smooth muscle cells and elastin fibers. They mainly offer structural support to the endothelial lining of blood vessels. The outermost layer is formed by fibro-elastic connective tissue called the tunica adventitia.
The lymphatic system is composed of capillaries and large vessels called collecting vessels lined by endothelial cells. The lymphatic endothelial cells arise from blood endothelial cells during embryonic development. While the blood endothelial cells are mainly involved in transporting nutrients and oxygen, the lymphatic vascular system is essential for transporting immune cells, fat mobilization, and drainage of interstitial fluid. Although functionally distinct, the blood and the lymphatic vascular system interact closely at lymph nodes and regulate fluid homeostasis of the body.
Suggested Reading
- M.K. Pugsley et al., The vascular system An overview of structure and function. Journal of Pharmacological and Toxicological Methods 44 (2000) 333 -340.
- Florent Morfoisse et al., Lymphatic and blood systems: Identical or fraternal twins? International Journal of Biochemistry and Cell Biology 114 (2019) 105562.
- Krüger-Genge Anne, Anna Blocki, Ralf-Peter Franke, and Friedrich Jung. "Vascular endothelial cell biology: an update." International journal of molecular sciences 20, no. 18 (2019): 4411.