Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 452

buffer or from cultures, can be assessed by a scanning force microscope associated
with a phase-contrast microscope [ 843 ].
In addition to endothelial cells that cover blood and lymph vessels, mature
endothelial cells (circulating endothelial cells [CEC]) as well as endothelial pro-
genitor cells (circulating endothelial progenitor cells [CEPC]; Sect. 9.3 ) circulate in
the blood flow at a very low concentration. Resident endothelial cells may detach
during the normal turnover after apoptosis, with clearance by the reticuloendothelial
system, in the absence of endothelial damage. Circulating endothelial progenitor
cells are able to form patches at sites of endothelial discontinuity to ensure the
integrity of the vessel wall [ 844 ]. 2
Flattened endothelial cells (thickness 1-2 m, width 10-15 m, length in the

streamwise direction 60-100 m) have rest and stretched (flow-adapted) configura-

tions. The height variation due to projective nucleus has been measured along the
endothelium with a maximum of 750 nm.
Observed plasmalemmal granules and ring-like structures of various sizes have
been assumed to be associated to the cytoplasmic layer of the plasma membrane
rather than the outer one. Moreover, fibers can be seen, likely associated with actin
filaments, that are bound to the cytoplasmic face of the plasma membrane.
The cell membrane is covered by a thin glycocalyx. The between-cell space
width ranges from 10 to 20 nm with tight and gap junctions.
Endothelial cells contain Weibel-Palade bodies, long rod-shaped storage or-
ganelles (length 1-5 200 nm). These storage and secretory granules

are filled with von Willebrand factor. 3 The von Willebrand factor recruits platelets
to the site of injury. 4 The architecture of the Weibel-Palade body and tubular
folding of von Willebrand factor requires a low pH [ 846 ]. The tubules must
not be disassembled prior to exocytosis. Once released, von Willebrand factor
unfolds rapidly and efficiently at neutral pH to trap circulating platelets, as it forms
platelet-catching filaments (length

m, caliber ∼

∼ 100 m). Weibel-Palade bodies also contain

P-selectins.

2 Circulating endothelial progenitor cells migrate from the bone marrow into the blood circulation.
They can differentiate into mature endothelial cells. They operate not only in re-endothelialization
and vascular healing, but also in vasculogenesis and angiogenesis. PTPRc low (low CD45 expression
level), VEGFR2 + endothelial progenitors can be mobilized by growth factor such as erythropoi-
etin [ 845 ].
3 Multimeric von Willebrand factor is a glycoprotein (mass

> 20 MDa; length > 4 m). Precursor

of von Willebrand factor is synthesized by endothelial cells (mass 350 kDa). The von Willebrand

factor contains binding sites for factor-VIII, platelet membrane glycoprotein-1b, collagen-1, -3, and
-6, and

∼

α 2B β 3 integrin.
4 Large vWF polymers stored in Weibel-Palade bodies are released by Ca 2 + -calmodulin to bind
coagulation factor-VIII (hence avoiding its cleavage), platelet glycoprotein GP1b, and

α 2B β 3 -

integrin that associate to collagen, heparin, and vitronectin.