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In all types of blood vessels the endothelium of the tunica intima is highly specialised with endocrine, exocrine, cell adhesion, clotting and transport functions. (See Table 1) The endothelium is composed of flattened cells. In routine histological sections the cytoplasm of most endothelial cells is barely visible and only the small flattened nuclei are seen. Ultrastructurally, each cell can be seen to be anchored to an underlying basal lamina; individual cells are anchored together by adhesion junctions, including prominent tight junctions which prevent diffusion between cells. A prominent feature of endothelial cells is the presence of many pinocytotic vesicles which are involved in the process of transport of substances from one side of the cell to the other. In small blood vessels of the nervous system the endothelial cells express transport proteins which are responsible for active transport of all substances, for example glucose, into the brain. Endothelial cells are able to sense changes in blood pressure, oxygen tension and blood flow by as yet unknown mechanisms. In response to changes in local conditions they respond by secreting substances which have powerful effects on the tone of vascular smooth muscle. (See Table 2) Endothelial cells are also important in the control of blood coagulation. Under normal circumstances the endothelial surface prevents blood clotting and allows smooth flowing of the blood. The endothelium can adapt rapidly to changes in its environment. Under certain circumstances, especially in response to adverse stimuli such as wounds, infections or irritation (e.g. insect sting), the endothelium may become activated and change its function. The endothelium may become activated by cytokines and develop specialisation for emigration of lymphoid cells. The endothelial cells become cuboidal in shape and express surface adhesion molecules which facilitate lymphocyte adhesion and migration. This type of endothelium is normally seen in the specialised venules in the lymph node paracortex (high endothelial venules). Endothelium may become activated by cytokines and express cell adhesion molecules for neutrophils. This normally occurs after any form of tissue damage and allows neutrophils to migrate into local tissues in the process of acute inflammation. The substance P-selectin, a cell adhesion molecule, is stored in special vesicles (Weibel-Palade bodies) inside the endothelium. With appropriate stimulation, these vesicles dock with the endothelial cell membrane. P-selectin is then available on the cell surface for neutrophil adhesion. Endothelium is normally locally impermeable to substances in the blood. Under the effects of certain factors, for example histamine, endothelial cells lose attachment to each other and retract. This allows fluid and proteins to diffuse out into the local tissues causing tissue swelling termed oedema. This reorganisation of cell-cell junctions is rapid and reversible and takes place in the space of a few minutes.
Table 1. Properties of the endothelial cells of blood vessels
Table 2. Substances produced by endothelial cells [Human Biology Contents Page] [My Home Page] This page authored by John Ross. Please e-mail any comments or queries to johnross@cwcom.net or leave a message in the guest book. This page last updated on Wednesday, 30 June 1999 15:10 +0100 |
