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.
characteristic coat on
the plasma membrane
very many pinocytotic
vesicles which are called caveolae.
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
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.
Continuous endothelium which usually have endothelial cells which
maintain contact with each other,
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
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
Pericytes are undifferentiated cellular elements found in
the vascular tree. They are usually associated with endothelial cells
in the walls of vascular organs.