DID YOU KNOW...

…that not all endothelial cells are the same? Endothelial cells were first described in 1845 by Todd and Bowman as an independent diaphanous wall of cell-nuclei lining large and small vessels. Todd and Bowman studied the anatomy of many species (including man) and observed cellular linings in numerous vascular systems including the kidney, the work for which Bowman is best known. At that time, thin non-muscle and non-fibrous cellular membranes were called epithelial cells or epithelial particles.¹ Not until 1872 did an American gynecologist, Edmund Peaslee, coin the word endothelium (from the Greek word endon, within) to distinguish the blood vessel lining from the integument. For most of its history, endothelium was considered a homogeneous continuum of inert cells described in classic literature as a simple membrane of nucleated cellophane. This perception  of endothelium persisted until the 1950’s when electron microscopy revealed that endothelium, from various organs and different vascular beds within the same organ, was physically distinct.² Isolation and culture of endothelial cells advanced the idea that heterogeneous endothelium co-exist within the same vascular bed. In the lung, pulmonary artery endothelial cells, the first lung endothelium isolated, were considered representative of the pulmonary vasculature. However, subsequent studies utilizing cells isolated from pulmonary capillaries (microvasculature) revealed that capillary endothelial cells form a tighter barrier that is less permeable to fluid flux than pulmonary artery endothelial cells. Moreover, inflammatory mediators cause endothelial dysfunction in cells isolated from one region of the pulmonary vascular bed but not in endothelial cells isolated from a different vascular segment.³ Considerable progress has been made in endothelial biology since it was first perceived as an inert layer of cells.  However, the full range of physiological and functional heterogeneity exhibited by endothelium is not known and remains an area of extensive study.

1. Todd RB, Bowman W. The physiological anatomy and physiology of man. Philadelphia: Blanchard and Lea, 1857: xxxi, [25]-926 p.
2. Aird WC. Endothelial cells in health and disease. Boca Raton: Taylor & Francis, 2005: xviii, 486 p.
3. Stevens T, Rosenberg R, Aird W, Quertermous T, Johnson FL, Garcia JG, Hebbel RP, Tuder RM, Garfinkel S. NHLBI workshop report: endothelial cell phenotypes in heart, lung, and blood diseases. Am J Physiol Cell Physiol 2001; 281:C1422-33.