The role of ultrastructural abnormalities of the blood-brain barrier in the development of brain glioblastoma radioresistance

Abstract

Background: Glioblastoma (GB) is the most commonly diagnosed brain tumor. Its management involves adjuvant therapies, such as radiation. The cause of high probability of GB local relapse is its radioresistance related to hypoxia arising from abnormal blood-brain barrier permeability in GB vessels and in the peritumoral zone (PZ).Aim: To study pathophysiology of hypoxia in the residual GB based on the abnormalities of the morphological elements of the capillary walls building up the blood-brain barrier in GB and PZ capillaries.Materials and methods: Samples for morphological evaluation were taken during surgery for GB in 5 patients. The samples were prepared for transmission electron microscopy according to the standard technique with fixation in 2% glutaraldehyde in phosphate buffer, post-fixation with osmium tetroxide, embedding in the epon-araldite mixture, and contrast staining of ultrathin sections with uranylacetate and lead citrate. Abnormalities of the capillary cells (mitochondrial vacuolization and vacuolization of endoplasmic reticulum in endothelial cells, pericytes and astrocytes), as well as of the acellular element of the capillary wall, i.e. basement membrane, were assessed in two groups of capillaries – those of GB (n = 38) and those of PZ (n = 32).Results: Abnormalities characteristic for apoptosis and oncosis were found in the cells of the GB and PZ capillaries of the blood-brain barrier, such as endothelial cells and pericytes. However, in the GB capillaries these abnormalities were signifcantly more frequent (р < 0.001). Only half (52.6%) of the GB capillaries had an edematous pericapillary astrocyte layer. In all other capillaries, astrocyte sprouts either were visualized as separate morphological elements (13.2%) or were not visualized at all (34.2%). All PZ capillaries had the astrocyte layer, being edematous in 68.8% of the capillaries and totally edematous only in 25%. Thickened basement membrane was found in the vast majority (89.5%) of the GB capillaries and only in 25% of the PZ capillaries (р < 0.001).Conclusion: Findings of abnormal cell elements in the GB capillaries leading to peritumoral edema and consequent hypoxia are highly likely to be the cause of the remnant GB radioresistance.

Keywords

• glioblastoma
• capillary
• peritumoral zone
• electron microscopy
• endothelial cell
• pericyte
• astrocyte
• basement membrane
• blood-brain barrier
• re-irradiation