Endothelial cell

 

  

 

The endothelial cell is the predominant cell type in the cardiovascular system. The endothelial cell lines the inside of the whole of the vascular tree and it has the following features.

 

a)           very minimal intracytoplasmic organelles

b)           characteristic coat on the plasma membrane

c)           very many pinocytotic vesicles which are called caveolae.

d)           Microtubules

e)           Microfilaments

 

These features are present and adapted to the function of diffusion of substances from the intravascular compartment of the tissue especially in the capillaries. The pinocytotic vesicles are prominent in this function.

 

          But the way and manner in which endothelial cells maintain contact with each other varies from one tissue or organ to another. The following will serve as a guide to the disposition of endothelial cells in various organs of the cardiovascular tree.

 

a)      Capillaries have different types of endothelial lining depending on the function they wish to perform.

i.  Continuous endothelium which usually have endothelial cells which maintain contact with each other,

ii. Discontinuous endothelium in which endothelial cells are not in direct contact with each other. In the continuous endothelium type, cells are separated by junctional modifications.  These may be either zonula adherens which are found in capillaries existing in most muscle tissue and other tissues adapted for easy and quick diffusion of some substances from and into the blood via capillary networks, or zonula occludens found in capillaries exhibiting blood barriers (they may be found in the blood brain barrier of the brain, blood placental barrier, blood bladder barrier, blood testis barrier and other types of barriers yet unidentified.  Sometimes some capillaries exhibiting the continuous morphology have a mixture of gap junctions (zonula adherens) or tight junctions (zonula occludens).  It is obvious from the location of these capillary types, the function of the junctions. The gap junctions allow exchange of materials from the blood to tissue more rapidly than the tight junctions, which only allow limited exchange.

 

          Endothelial cell usually have thin basement membrane underlying it. In the discontinous capillary type, this basement membrane may be absent as is often the case in capillaries in the liver and spleen which allow maximum diffusion across their walls. In other types of discontinous endothelium, a thin plasma membrane may be present which is sometimes called diaphragm.

 

          The endothelial cells lining the inside of arteries exhibit prominent net work of rough endoplasmic reticulum. They have also been shown to contain inclusion bodies that might have some role in blood coagulation and also in the regulation of blood pressure. Those in the venules seem to contain more lysosomes than any other organ type in the vascular tree.  Endothelial cells seem to be active metabolically in the arteries in view of their contained rough endoplasmic reticulum. By virtue of their contained microfilaments, they are also capable of contraction in response to neurogenic or humoral (from the blood) stimuli. The activity of contraction and relaxation become important in pathological situations in which abnormal permeability of endothelial lining due to damage of the cell by toxic substances or absence of neurogenic contraction stimuli, occurs.  Also vitamin C deficiency may cause the widening of the gap junctions leading to increased permeability.

 

          Pericytes are undifferentiated cellular elements found in the vascular tree. They are usually associated with endothelial cells in the walls of vascular organs.
 


         

       

 

 

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