The endothelium, first described over 100 years ago as an inert anatomical barrier between blood and the vessel wall, is now recognized as a dynamic organ with secretory, synthetic, metabolic, and immunologic functions. Forming a continuous lining to every blood vessel in the body, endothelial cells play an obligatory role in modulating vascular tone and permeability, angiogenesis, and in mediating haemostatic, inflammatory and reparative responses to local injury. To fulfil these roles the endothelium is highly dynamic, continuously responding to spatial and temporal changes in mechanical and biochemical stimuli. Such responsiveness is affected through receptors for growth factors, lipoproteins, platelet products and circulating hormones, which regulate changes in protein and mRNA expression, cell proliferation and migration or the release of vasoactive and inflammatory mediators.
All vascular endothelial cells have a common embryonic origin but show clear bed-specific heterogeneity in morphology, function, gene and protein expression, determined by both environmental stimuli and epigenetic features acquired during development. Thus, the endothelium should not be regarded as a homogenous tissue but rather a conglomerate of distinct populations of cells sharing many common functions but also adapted to meet regional demands.
The continuous endothelial cell layer provides an uninterrupted barrier between the blood and tissues in the majority of blood vessels and ensures tight control of permeability of the blood-brain barrier. In regions of increased trans-endothelial transport such as capillaries of endocrine glands and the kidney, the presence of fenestrae, transcellular pores approximately 70 nm in diameter with a thin fenestral diaphragm across their opening, facilitate the selective permeability required for efficient absorption, secretion, and filtering.