Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 359

astrocyte

NaK−ATPase

BL

EC

LAT

GLUT

TJ

NaKCl BL

TJ

ABC

pericyte

Na/K−ATPase EAAT

K

Aqp

EAAT

Fig. 7.5 Schematic drawing of the blood-brain barrier, with endothelial cells (EC) of the cerebral
capillary, surrounded by basal lamina (BL), perivascular extensions of astrocytes and pericytes
(Source: [ 667 ]). Endothelial cells are joined by tight junctions (TJ). Endothelium contains carriers
for glucose (glucose transporter [GLUT]), amino acids (L system for large neutral amino acids
[LAT]), glutamate (excitatory amino acid transporters [EAAT]), ions (Na/K-ATPase), and ABC
transporters. Astrocyte extensions facing the capillary wall have patches with aquaporins (Aqp4)
and K + channels (K). They contain also Na + -K + AT P a s e , N a + -K + -Cl − cotransporters, and
EAATs.

The restrictive blood-brain barrier with its enzymes and transporters develops
during embryogenesis. Endothelial cells characterized by a reduced vesicular
transport form nascent vessels in the central nervous system to which pericytes
are recruited, before astrocytes are generated [ 678 ]. Pericytes regulate the function
and proper structure of the blood-brain barrier, in particular the formation of
tight junctions and vesicle transfer in endothelial cells of the central nervous
system [ 678 ]. They inhibit the expression of molecules that increase vascular
permeability and immunocyte infiltration. Pericytes reduces the permeability of the
blood-brain barrier to water and low- and high-molecular-mass molecules [ 671 ]. In
addition, pericytes may mediate the attachment of astrocyte end-feet to endothelial
cells [ 671 ]. The PDGFb-PDGFR

β

complex is required in pericyte recruitment

during angiogenesis.

7.4.1 BBB Restricted Permeability

The brain endothelium has a much lower capacity for endo- and transcytosis
than that of peripheral endothelia. Furthermore, intra- and extracellular enzymes
yield a metabolic barrier. Ectoenzymes, such as peptidases and nucleotidases, are