The role of the subependymal plate in glial tumorigenesis

Glial cells in the mammalian olfactory bulb

The mammalian olfactory bulb (OB), an essential part of the olfactory system, plays a critical role in odor detection and neural processing. Historically, research has predominantly focused on the neuronal components of the OB, often overlooking the vital contributions of glial cells. Recent advancements, however, underscore the significant roles that glial cells play within this intricate neural structure. This review discus the diverse functions and dynamics of glial cells in the mammalian OB, mainly focused on astrocytes, microglia, oligodendrocytes, olfactory ensheathing cells, and radial glia cells. Each type of glial contributes uniquely to the OB's functionality, influencing everything from synaptic modulation and neuronal survival to immune defense and axonal guidance. The review features their roles in maintaining neural health, their involvement in neurodegenerative diseases, and their potential in therapeutic applications for neuroregeneration. By providing a comprehensive overview of glial cell types, their mechanisms, and interactions within the OB, this article aims to enhance our understanding of the olfactory system's complexity and the pivotal roles glial cells play in both health and disease.

Vitamin C deficient reduces proliferation in a human periventricular tumor stem cell-derived glioblastoma model

Glioblastoma multiforme (GBM) is the most common and aggressive brain tumor with a median survival of 14.6 months. GBM is highly resistant to radio- and chemotherapy, and remains without a cure; hence, new treatment strategies are constantly sought. Vitamin C, an essential micronutrient and antioxidant, was initially described as an antitumor molecule; however, several studies have shown that it can promote tumor progression and angiogenesis. Thus, considering the high concentrations of vitamin C present in the brain, our aim was to study the effect of vitamin C deficiency on the progression of GBM using a GBM model generated by the stereotactic injection of human GBM cells (U87-MG or HSVT-C3 cells) in the subventricular zone of guinea pig brain. Initial characterization of U87-MG and HSVT-C3 cells showed that HSVT-C3 are highly proliferative, overexpress p53, and are resistant to ferroptosis. To induce intraperiventricular tumors, animals received control or a vitamin C-deficient diet for 3 weeks, after which histopathological and confocal microscopy analyses were performed. We demonstrated that the vitamin C-deficient condition reduced the glomeruloid vasculature and microglia/macrophage infiltration in U87-MG tumors. Furthermore, tumor size, proliferation, glomeruloid vasculature, microglia/macrophage infiltration, and invasion were reduced in C3 tumors carried by vitamin C-deficient guinea pigs. In conclusion, the effect of the vitamin C deficiency was dependent on the tumor cell used for GBM induction. HSVT-C3 cells, a cell line with stem cell features isolated from a human subventricular GBM, showed higher sensitivity to the deficient condition; however, vitamin C deficiency displayed an antitumor effect in both GBM models analyzed.

Influence of glioblastoma contact with the subventricular zone on survival and recurrence patterns

BACKGROUND

There is growing evidence that the subventricular zone (SVZ) may be involved in both the initiation and progression of glioblastoma (GB). We aimed to assess tumor proximity to the SVZ as a potential prognostic factor in GB.

METHOD

Retrospective study of 133 patients diagnosed with primary GB who underwent surgery followed by temozolomide-based chemoradiation between 2010 and 2016. All lesions were classified according to their anatomic relation with the SVZ. We determined the effect of tumor contact with the SVZ on progression-free survival (PFS), overall survival (OS), type, and patterns of recurrence.

RESULTS

At a median follow-up of 18.6 months (95% CI 15.9-21.2), PFS and OS were 7.5 (95% CI 6.7-8.3) and 13.9 (95% CI 10.9-16.9) months, respectively. On the univariate analyses, initial contact with the SVZ was a factor for poor prognosis for both PFS (6.1 vs. 8.7 months; p = 0.006) and OS (10.6 vs. 17.9 months; p = 0.037). On the multivariate analysis, tumor contact with the SVZ remained statistically significant for PFS, but not OS. Patients with SVZ-contacting tumors presented a higher rate of aggressive clinical progression (30.9% vs. 11.3%; p = 0.007) and contralateral relapse patterns (23.4% vs. 9.1%; p = 0.048).

CONCLUSIONS

Our results suggest that glioblastoma contact with the SVZ appears to be an independent prognostic factor for poor PFS. The presence of an SVZ-contacting tumor was associated with more aggressive recurrences and a higher rate of contralateral relapses. These findings suggest that this variable may be a new prognostic factor in glioblastoma.

Use of Preoperative Ependymal Enhancement on Magnetic Resonance Imaging Brain as a Marker of Grade of Glioma

Objectives:

Neural stem cells within the subventricular zone (SVZ) are thought to be responsible for the origin and the heterogeneous nature of the gliomas. The relationship of the gliomas to the SVZ can be appreciated as ependymal enhancement on contrast magnetic resonance imaging (MRI). This study evaluates the rate of ependymal enhancement and its association with the histopathological grade of gliomas.

Patients and Methods:

Seventy-five patients with radiological features of glioma were recruited. Preoperative MRI was evaluated for the presence of ependymal enhancement and fluid-attenuated inversion recovery (FLAIR) signal proximity of tumor to ependyma, and the association to grade was investigated.

Results:

Seventy-five patients studied showed a male predominance (62.66%) with a mean age of 44.91 ± 13.64 years. Evidence of ependymal enhancement was positive in 24% (n = 18), 46.67% (n = 35) showed no evidence, and in 29.33% (n = 22), assessment was inconclusive. According to WHO grading, 76% (n = 57) were high-grade gliomas (HGGs) including Grade III (n = 17) and Grade IV (n = 40) and 24% (n = 18) were low-grade gliomas (LGGs) Grade II. HGGs were significantly associated with ependymal enhancement (P < 0.01) and FLAIR signal proximity to the ependyma (P < 0.001). Among HGGs, rate of ependymal enhancement and FLAIR signal proximity was more in Grade IV than Grade III but was not statistically significant (P > 0.05).

Conclusion:

SVZ is associated with HGGs. These MRI features can be helpful in predicting the tumor grade preoperatively and by including LGGs, the role of SVZ in the heterogeneous disease process of glioma can be studied as a whole, not only in the glioblastoma (GBM).

High-dose Neural Stem Cell Radiation May Not Improve Survival in Glioblastoma

AIMS

To evaluate the effect of radiotherapy dose-volume parameters of neural stem cell (NSC) compartment on progression-free survival (PFS) and overall survival after post-resection chemoradiation in newly diagnosed glioblastoma.

MATERIALS AND METHODS

Sixty-one patients with unifocal glioblastoma were included. Ipsilateral (NSC_Ipsi), contralateral (NSC_Contra) and combined NSC (NSC_Combined) were contoured on radiotherapy planning computerised tomography datasets. NSC dose-volume parameters were correlated with PFS and overall survival. Serial magnetic resonance imaging scans were assessed to understand the frequency of pre- and post-treatment involvement of the NSC by contrast enhancing lesions (CELs).

RESULTS

Baseline involvement of NSC with CELs was seen in 67.2% and 95.9% had CELs and FLAIR abnormalities at progression. With a median follow-up of 14.1 months (interquartile range 9.4-20.6 months), median PFS and overall survival were 14.5 (95% confidence interval 11.6-17.5) and 16.2 (95% confidence interval 13.3-19.2) months, respectively. Poor Eastern Cooperative Oncology Group performance score, advanced recursive partitioning analysis class, unmethylated O6-methylguanine methyltransferase (MGMT) status, higher than median of mean NSC_Ipsi dose were associated with significantly inferior PFS and overall survival on univariate analysis. On multivariate analysis, unmethylated MGMT status, higher than median of mean doses to NSC_Ipsi and poor compliance to adjuvant temozolomide were independent predictors of inferior survival.

CONCLUSIONS

In this cohort, 67.2% of newly diagnosed glioblastoma patients had NSC involved with CELs at presentation and 95.9% at progression. This might be an imaging surrogate of the current notion of gliomagenesis and progression from NSC rests. A high radiation dose to NSC_Ipsi was significantly associated with inferior survival. This could be a function of larger tumours and planning target volumes in those with pre-treatment NSC involvement. Methylated MGMT and good compliance to adjuvant temozolomide were independent predictors of better survival. Until further evidence brings hope for glioblastoma, elective, partial NSC irradiation remains experimental.

Traumatic brain injury and subsequent glioblastoma development: Review of the literature and case reports

BACKGROUND

Previous reports have proposed an association between traumatic brain injury (TBI) and subsequent glioblastoma (GBM) formation.

METHODS

We used literature searches and radiographic evidence from two patients to assess the possibility of a link between TBI and GBM.

RESULTS

Epidemiological studies are equivocal on a possible link between brain trauma and increased risk of malignant glioma formation. We present two case reports of patients with GBM arising at the site of prior brain injury.

CONCLUSION

The hypothesis that TBI may predispose to gliomagenesis is disputed by several large-scale epidemiological studies, but supported by some. Radiographic evidence from two cases presented here suggest that GBM formed at the site of brain injury. We propose a putative pathogenesis model that connects post-traumatic inflammation, stem and progenitor cell transformation, and gliomagenesis.

Anatomical Involvement of the Subventricular Zone Predicts Poor Survival Outcome in Low-Grade Astrocytomas

The subventricular zone (SVZ) has been implicated in the origination, development, and biological behavior of gliomas. Tumor-SVZ contact is also postulated to be a poor prognostic factor in glioblastomas. We aimed to evaluate the prognostic consequence of the anatomical involvement of low-grade gliomas with the SVZ. To that end, we reviewed 143 patients with diffuse astrocytomas, and tumor lesions were manually delineated on magnetic resonance images. We initially investigated the prognostic role of SVZ contact in all patients. Additionally, we investigated the influence of the anatomical proximity of the tumor lesion centroids to the SVZ in the SVZ-involved patient cohorts, as well as location within the SVZ. We found SVZ contact with tumors to be a significant prognostic factor of overall survival in all patients with diffuse astrocytomas (p = 0.027). In the SVZ-involved cohort, a shorter distance from the tumor centroid to the SVZ (≤30 mm) correlated with shorter overall survival (p = 0.022) on univariate analysis. However, there was no significant difference in overall survival with respect to the SVZ region involved with the tumor (p = 0.930). Multivariate analysis showed that a shorter distance between the tumor centroid and the SVZ (p = 0.039) was significantly associated with poor overall survival in SVZ-involved patients. Hence, this study helps establish the prognostic role of the anatomical interaction of tumors with the SVZ in low-grade astrocytomas.

Glioma Stem Cells: Signaling, Microenvironment, and Therapy

Glioblastoma remains the most common and devastating primary brain tumor despite maximal therapy with surgery, chemotherapy, and radiation. The glioma stem cell (GSC) subpopulation has been identified in glioblastoma and likely plays a key role in resistance of these tumors to conventional therapies as well as recurrent disease. GSCs are capable of self-renewal and differentiation; glioblastoma-derived GSCs are capable of de novo tumor formation when implanted in xenograft models. Further, GSCs possess unique surface markers, modulate characteristic signaling pathways to promote tumorigenesis, and play key roles in glioma vascular formation. These features, in addition to microenvironmental factors, present possible targets for specifically directing therapy against the GSC population within glioblastoma. In this review, the authors summarize the current knowledge of GSC biology and function and the role of GSCs in new vascular formation within glioblastoma and discuss potential therapeutic approaches to target GSCs.

Differentiation between Solitary Cerebral Metastasis and Astrocytoma on the Basis of Subventricular Zone Involvement on Magnetic Resonance Imaging

Purpose

To determine the relationship between the subventricular zone (SVZ) and astrocytoma based on magnetic resonance imaging (MRI) and whether SVZ involvement can be used to distinguish solitary cerebral metastases (SCMs) from astrocytomas.

Methods

This retrospective study involved 154 patients with solitary low-grade astrocytoma (LGA), high-grade astrocytoma (HGA), and SCM, who underwent T1-weighted imaging (T1WI), Gd-DTPA–enhanced T1WI, and T2-weighted imaging (T2WI) or fluid-attenuated inversion recovery (FLAIR) T2WI. The spatial relationship between the tumor and SVZ was classified as “involvement” or “segregation” on contrast-enhanced T1WI for enhanced tumors and T2WI/FLAIR T2WI for non-enhanced tumors. Patient-based SVZ-contact rates were compared between the LGA, HGA, and SCM groups. The frequencies of involvement of various lateral ventricle regions by astrocytoma were compared. The correlation between SVZ involvement and tumor necrosis was analyzed.

Results

Patient-based SVZ-contact rates in SCM, LGA, and HGA were 24.1%, 68.8%, and 85.4%, respectively. Univariate analysis showed that the SVZ-contact rate was significantly different between SCM and astrocytoma (24.1% vs. 75.2% P < 0.001), also between LGA and HGA (68.1% vs. 85.4% P=0.037). After the tumor volume was adjusted as a covariate, SVZ-contact rates still differed between SCMs and astrocytomas (Odds ratio [OR]: 4.58, 95% Confidence interval [CI]: 1.65 to 12.8, P=0.004). Tumor volume differed between LGA and HGA (P< 0.001), and influenced the association between SVZ involvement and astrocytoma grade (P = 0.05). Among the lateral ventricle regions, the frontal horn was the most frequently involved by astrocytomas. SVZ-contact rates were higher in necrosis group compared with non-necrosis groups (83.9% vs. 50.0%, P < 0.001) among astrocytoma patients. Necrosis positively correlated with SVZ involvement in astrocytomas (r_s = 0.342, P < 0.001), but did not correlate with SVZ involvement in SCMs (P = 0.193).

Conclusions

Compared to SCMs, solitary cerebral astrocytomas exhibited spatial proximity to the SVZ, which might distinguish the supratentorial astrocytomas from SCMs.

The autotaxin-lysophosphatidic acid-lysophosphatidic acid receptor cascade: proposal of a novel potential therapeutic target for treating glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most malignant tumor of the central nervous system (CNS). Its prognosis is one of the worst among all cancer types, and it is considered a fatal malignancy, incurable with conventional therapeutic strategies. As the bioactive multifunctional lipid mediator lysophosphatidic acid (LPA) is well recognized to be involved in the tumorigenesis of cancers by acting on G-protein-coupled receptors, LPA receptor (LPAR) antagonists and LPA synthesis inhibitors have been proposed as promising drugs for cancer treatment. Six LPARs, named LPA1-6, are currently recognized. Among them, LPA1 is the dominant LPAR in the CNS and is highly expressed in GBM in combination with the overexpression of autotaxin (ATX), the enzyme (a phosphodiesterase, which is a potent cell motility-stimulating factor) that produces LPA.Invasion is a defining hallmark of GBM. LPA is significantly related to cell adhesion, cell motility, and invasion through the Rho family GTPases Rho and Rac. LPA1 is responsible for LPA-driven cell motility, which is attenuated by LPA4. GBM is among the most vascular human tumors. Although anti-angiogenic therapy (through the inhibition of vascular endothelial growth factor (VEGF)) was established, sufficient results have not been obtained because of the increased invasiveness triggered by anti-angiogenesis. As both ATX and LPA play a significant role in angiogenesis, similar to VEGF, inhibition of the ATX/LPA axis may be beneficial as a two-pronged therapy that includes anti-angiogenic and anti-invasion therapy. Conventional approaches to GBM are predominantly directed at cell proliferation. Recurrent tumors regrow from cells that have invaded brain tissues and are less proliferative, and are thus quite resistant to conventional drugs and radiation, which preferentially kill rapidly proliferating cells. A novel approach that targets this invasive subpopulation of GBM cells may improve the prognosis of GBM. Patients with GBM that contacts the subventricular zone (SVZ) have decreased survival. A putative source of GBM cells is the SVZ, the largest area of neurogenesis in the adult human brain. GBM stem cells in the SVZ that are positive for the neural stem cell surface antigen CD133 are highly tumorigenic and enriched in recurrent GBM. LPA1 expression appears to be increased in these cells. Here, the author reviews research on the ATX/LPAR axis, focusing on GBM and an ATX/LPAR-targeted approach.

Variations of ITSS-Morphology and their Relationship to Location and Tumor Volume in Patients with Glioblastoma

BACKGROUND

Susceptibility weighted imaging and assessment of intratumoral susceptibility signal (ITSS) morphology is used to identify high-grade glioma (HGG) in patients with suspected brain neoplasm.

PURPOSE

The aim of this study was to outline variations in ITSS-morphology and their relationship to location as well as volume of the lesion in patients with glioblastoma (GB).

MATERIALS AND METHODS

Contrast-enhanced SWI (CE-SWI) images of 40 patients with histologically confirmed GB were analyzed retrospectively with particular attention to ITSS-morphology dividing all lesions into two groups. Considering the location of the lesion within brain parenchyma, lesions with and without involvement of the subventricular zone (SVZ+/SVZ-) were discerned. Additionally, the contrast-enhancing tumor volume was evaluated. Statistical analysis was based on a classification analysis resulting in a classification rule (tree) as well as Mann-Whitney-U test.

RESULTS

The distribution of ITSS-scores showed differences between the SVZ+ and SVZ- groups. While SVZ-GB showed only fine-linear or dot-like ITSS, in SVZ+ GB the ITSS-morphology changed with the tumor volume, that is, in larger tumors dense and conglomerated ITSS were the predominant finding.

CONCLUSION

Our findings indicate that ITSS-morphology is not a random phenomenon. Location of GB, as well as tumor volume, appear to be factors contributing to ITSS morphology.

Radial glial cells: key organisers in CNS development

Radial glia are elongated bipolar cells present in the CNS during development. Our understanding of the unique roles these cells play has significantly expanded in the last decade. Historically, radial glial cells were primarily thought to provide an architectural framework for neuronal migration. Recent research reveals that radial glia play a more dynamic and integrated role in the development of the brain and spinal cord. They represent a major progenitor pool during early development and can give rise to a small population of multipotent cells in neurogenic niches of the adult CNS. Radial glial cells are a heterogeneous population, with divergent and often poorly understood roles across different brain and spinal cord regions during development; this heterogeneity extends to specialised adult subtypes, such as tanycytes, Müller glial cells and Bergman glial cells which possess morphological similarities to radial glial but play distinct functional roles in the CNS.

Neuronal stem cells in the central nervous system and in human diseases

The process of cortical expansion in the central nervous system is a key step of mammalian brain development to ensure its physiological function. Radial glial (RG) cells are a glial cell type contributing to this progress as intermediate neural progenitor cells responsible for an increase in the number of cortical neurons. In this review, we discuss the current understanding of RG cells during neurogenesis and provide further information on the mechanisms of neurodevelopmental diseases and stem cell-related brain tumorigenesis. Knowledge of neuronal stem cell and relative diseases will bridge benchmark research through translational studies to clinical therapeutic treatments of these diseases.

Mouse models to interrogate the implications of the differentiation status in the ontogeny of gliomas

Glioblastoma multiforme (GBM) is the most common and lethal of human primary central nervous system (CNS) tumors, with a median survival of 14-16 months despite optimal surgery, radiation and chemotherapy. A reason for this dismal prognosis is insufficient understanding of the ontogeny of GBMs, which are highly heterogeneous at a pathological level. This pathological diversity, between and within GBMs as well as varying grades of gliomas, has not been fully explained solely on the grounds of oncogenic stimulus. Interaction with the tumor microenvironment is likely a source of this pathological heterogeneity, as well as the inherent characteristics of the tumor cell of origin. Currently, controversy exists on whether the initial transformed cell is a differentiated astrocyte, progenitor or neural stem cell. Putative cancer stem cells (CSCs), which have features of normal stem cell plus the ability to recapitulate the tumor phenotype in vivo in small numbers, have been identified from a variety of solid human cancers, including GBMs. Evidence suggesting that regions harboring normal stem cells in the adult CNS, such as the subventricular zone and the dentate gyrus, are more prone to viral and chemical oncogenesis, is supportive of the hypothesis that brain tumors arise from stem cells. However, it is still to be determined whether the appearance of brain tumor stem cells (BTSC) is the cause or consequence of tumor initiation and progression. This review discusses emerging evidence highlighting the relevance of the state of differentiation and regional heterogeneity in the ontogeny of GBM. This is an area of high interest in cancer in general, with potential significant therapeutic and prognostic implications.

Anatomic localization of O6-methylguanine DNA methyltransferase (MGMT) promoter methylated and unmethylated tumors: a radiographic study in 358 de novo human glioblastomas

Promoter methylation of O6-methylguanine DNA methyltransferase (MGMT) is associated with a favorable prognosis in glioblastoma multiforme (GBM) and has been hypothesized to occur early in tumor transformation of glial cells. Thus, a possible link exists between the site of malignant transformation and MGMT promoter methylation status. Using the Analysis of Differential Involvement (ADIFFI) statistical mapping technique in a total of 358 patients with GBM, we demonstrate that human de novo GBMs occur in a high frequency contiguous with the posterior subventricular zone (SVZ); MGMT promoter methylated GBMs are lateralized to the left hemisphere, while MGMT unmethylated GBMs are lateralized to the right hemisphere; and tumors near the left temporal lobe have a significantly longer overall survival compared with tumors occurring elsewhere, independent of treatment or MGMT methylation status.

Epigenetic regulation of aging stem cells

The function of adult tissue-specific stem cells declines with age, which may contribute to the physiological decline in tissue homeostasis and the increased risk of neoplasm during aging. Old stem cells can be 'rejuvenated' by environmental stimuli in some cases, raising the possibility that a subset of age-dependent stem cell changes is regulated by reversible mechanisms. Epigenetic regulators are good candidates for such mechanisms, as they provide a versatile checkpoint to mediate plastic changes in gene expression and have recently been found to control organismal longevity. Here, we review the importance of chromatin regulation in adult stem cell compartments. We particularly focus on the roles of chromatin-modifying complexes and transcription factors that directly impact chromatin in aging stem cells. Understanding the regulation of chromatin states in adult stem cells is likely to have important implications for identifying avenues to maintain the homeostatic balance between sustained function and neoplastic transformation of aging stem cells.

Brain tumor-initiating cells and cells of origin in glioblastoma

Glioblastoma Multiforme (GBM) is the most malignant and devastating primary brain tumour with a median survival of ∼12–16 months. Although recent large scale sequencing projects have shed considerable light into the complexity of the disease, there remains much to be elucidated in the hopes of generating effective therapeutic strategies. Although these studies investigate the mutations and expression of bulk tumour they have limits with respect to cell of origin and the concept of brain tumour initiating cells (BTIC). Current research has challenged the old paradigm of the stochastic model as recent evidence suggests that a subset of cancer cells within a tumor is responsible for tumor initiation, maintenance, and resistance to therapy. To gain a better understanding of the different compartment of cells that GBM comprise of require careful and elegant experiments. In addition to studying GBM, exploring the role of normal neural stem cells and progenitors cells is essential to partially explain whether these GBM BTIC behave similarly or differently then their non transformed counterparts. Here we discuss the recent literature between the two models, candidate regions of glioma genesis, candidate cells of origin for GBM, and possible therapeutic avenues to explore.

The nuclear receptor tailless induces long-term neural stem cell expansion and brain tumor initiation

Malignant gliomas are the most common primary brain tumors, and are associated with frequent resistance to therapy as well as poor prognosis. Here we demonstrate that the nuclear receptor tailless (Tlx), which in the adult is expressed exclusively in astrocyte-like B cells of the subventricular zone, acts as a key regulator of neural stem cell (NSC) expansion and brain tumor initiation from NSCs. Overexpression of Tlx antagonizes age-dependent exhaustion of NSCs in mice and leads to migration of stem/progenitor cells from their natural niche. The increase of NSCs persists with age, and leads to efficient production of newborn neurons in aged brain tissues. These cells initiate the development of glioma-like lesions and gliomas. Glioma development is accelerated upon loss of the tumor suppressor p53. Tlx-induced NSC expansion and gliomagenesis are associated with increased angiogenesis, which allows for the migration and maintenance of brain tumor stem cells in the perivascular niche. We also demonstrate that Tlx transcripts are overexpressed in human primary glioblastomas in which Tlx expression is restricted to a subpopulation of nestin-positive perivascular tumor cells. Our study clearly demonstrates how NSCs contribute to brain tumorgenesis driven by a stem cell-specific transcription factor, thus providing novel insights into the histogenesis and molecular pathogenesis of primary brain tumors.

New drugs for brain tumors? Insights from chemical probing of neural stem cells

The cancer stem cell hypothesis posits a direct relationship between normal neural stem cells (NSCs) and brain tumour stem cells (BTSCs). New insights into human brain tumour biology and treatment should thus emerge from the study of normal NSCs. These parallels have recently been exploited in a chemical genetic screen that identified a broad repertoire of neurotransmission modulators as inhibitors of both NSC and BTSC expansion in vitro. Prompted by these findings, we sought epidemiological support for effects of neuromodulation of brain tumours in vivo. We present observations from data collected from retrospective clinical studies suggesting that patients with a wide variety of neuropsychiatric disorders have decreased brain tumour incidence. We speculate that this reduction may derive from the use of drugs that collaterally affect the normal neural precursor compartment, and thereby limit a population that is suspected to give rise to brain tumours. Standard chronic neuropharmacological interventions in clinical neuropsychiatric care are thus candidates for redeployment as brain cancer therapeutics.

Stem cells and the origin of gliomas: A historical reappraisal with molecular advancements

The biology of both normal and tumor development clearly possesses overlapping and parallel features. Oncogenes and tumor suppressors are relevant not only in tumor biology, but also in physiological developmental regulators of growth and differentiation. Conversely, genes identified as regulators of developmental biology are relevant to tumor biology. This is particularly relevant in the context of brain tumors, where recent evidence is mounting that the origin of brain tumors, specifically gliomas, may represent dysfunctional developmental neurobiology. Neural stem cells are increasingly being investigated as the cell type that originally undergoes malignant transformation – the cell of origin – and the evidence for this is discussed.

Brain tumor stem cells

The dogma that solid tumors are composed of tumor cells that all share the same ability to produce proliferating daughter cells has been challenged in recent years. There is growing evidence that many adult tissues contain a set of tissue stem cells, which might undergo malignant transformation while retaining their stem cell characteristics. These include the ability of indefinite self-renewal and the capability to differentiate into daughter cells of tissue-specific lineages. Brain tumors such as medulloblastomas or glioblastomas often contain areas of divergent differentiation, which raises the intriguing question of whether these tumors could derive from neural stem cells (NSCs).This chapter reviews the current knowledge of NSCs and relates them to brain tumor pathology. Current therapy protocols for malignant brain tumors are targeted toward the reduction of bulk tumor mass. The concept of brain-tumor stem cells could provide new insights for future therapies, if the capacity for self-renewal of tumor cells and growth of the tumor mass would reside within a small subset of cancer cells.

Relationship of gliomas to the ventricular walls

The role of neural stem cells in gliomagenesis remains controversial. The aim of this study was to determine the anatomic relationship of human gliomas to the lining of the ventricular walls, known as the subventricular zone, an area replete with neural stem cells. We performed a retrospective radiographic analysis of 100 consecutive patients with gliomas and sought to determine the relationship of the lesions to the ventricular walls as seen on their MRI scans. Our results indicated that in 93% of cases the lesions contacted at least one region of the lateral ventricular wall. Contact with the ventricular wall was independent of the glioma size or mass effect. These findings were correlated to cytoarchitectural studies of the human subventricular zone. Our findings lend further support that there is an intimate association between gliomas and the subventricular zone.

[Radiation therapy for glial tumors: technical aspects and clinical indications]

Radiotherapy of glial tumors is rapidly evolving with the recent technical and therapeutic progress. About technical aspects, progress in technical imaging and development of non-coplanar conformal and IMRT techniques provide new possibilities for sparing healthy tissue while increasing dose in tumoral volume. Furthermore, functional and molecular imaging are helpful for delineation and for prediction of relapse. Even modest, the actual improvement of survival with radiochemotherapy leads now to new and important developments for clinical research according to clinical data (age, general status), biological data (MGMT promotor methylation and cytogenetic modifications) and technical data (quality of surgery and radiotherapy). Understanding of molecular mechanisms allows for rational targeting or specific pathways of repair, signaling angiogenesis associated with surgery and radiotherapy in a multidisciplinary approach.

Progress on potential strategies to target brain tumor stem cells

The identification of brain tumor stem cells (BTSCs) leads to promising progress on brain tumor treatment. For some brain tumors, BTSCs are the driving force of tumor growth and the culprits that make tumor revive and resistant to radiotherapy and chemotherapy. Therefore, it is specifically significant to eliminate BTSCs for treatment of brain tumors. There are considerable similarities between BTSCs and normal neural stem cells (NSCs), and diverse aspects of BTSCs have been studied to find potential targets that can be manipulated to specifically eradicate BTSCs without damaging normal NSCs, including their surface makers, surrounding niche, and aberrant signaling pathways. Many strategies have been designed to kill BTSCs, and some of them have reached, or are approaching, effective therapeutic results. Here, we will focus on advantages in the issue of BTSCs and emphasize on potential therapeutic strategies targeting BTSCs.

Cancer stem cells and glioma

Despite continued advances in surgical and medical therapies, the outcomes for patients diagnosed with glioblastoma multiforme remain dismal. Recent data suggest that progression of these brain tumors is driven by a small subpopulation of tumor cells, which are termed cancer stem cells (CSCs) because of their capability to self-renew, proliferate and give rise to progeny of multiple neuroepithelial lineages. According to the CSC hypothesis, current therapies that are extremely cytotoxic to the bulk of highly proliferative tumor cells fail to obliterate the relatively quiescent and resistant CSC compartment, thereby allowing these cells to survive and drive tumor recurrence. This Review summarizes current knowledge regarding neural stem cells in the normal adult brain and CSCs in glial tumors and discusses the implications of the CSC hypothesis for the development of future therapies for brain tumors.

Brain tumor stem cells: bringing order to the chaos of brain cancer

Brain tumors are generally incurable cancers. Work from a number of laboratories strongly suggests that they are organized as a hierarchy based on a subset of cancer cells that have stem-cell properties. These cells have now been shown to be resistant to conventional therapy and responsive to differentiation therapy. New in vitro and in vivo models for interrogating brain tumor cells in stem-cell conditions have been developed that provide important new opportunities for elucidating the key pathways responsible for driving the proliferation of these cells. Continued application of the principles of stem-cell biology to the study of brain cancers is likely to continue to bring further important insight into these aggressive cancers, bringing new treatments and understanding of the origins.

Mechanisms of Disease: the role of stem cells in the biology and treatment of gliomas

The study of neural stem cell and progenitor cell biology has improved our understanding of the biology of brain tumors in a developmental context. Recent work has demonstrated that brain tumors may harbor small subpopulations of cells that share characteristics of neural stem cells. There is still an ongoing debate about the specific role of these stem-like cells in cancer initiation, development and progression. Nonetheless, the concept of cancer stem cells has offered a new paradigm to understand tumor biology and resistance to current treatment modalities. Molecular aberrations in these cancer stem cells might be crucial targets for therapeutic intervention, with the hope of achieving more durable clinical responses. Recent studies have demonstrated that endogenous and transplanted neural stem cells and progenitor cells show a marked tropism to brain tumors. Although the mechanisms that govern these processes are poorly understood, the use of neural stem cells and progenitor cells as delivery vehicles for molecules toxic to tumors offers a promising experimental treatment strategy. This Review summarizes recent advances in the basic understanding of neural stem cell and cancer stem cell biology and the progress towards translating these novel concepts into the clinic.

Brain tumour stem cells: the undercurrents of human brain cancer and their relationship to neural stem cells

Conceptual and technical advances in neural stem cell biology are being applied to the study of human brain tumours. These studies suggest that human brain tumours are organized as a hierarchy and are maintained by a small number of tumour cells that have stem cell properties. Most of the bulk population of human brain tumours comprise cells that have lost the ability to initiate and maintain tumour growth. Although the cell of origin for human brain tumours is uncertain, recent evidence points towards the brain's known proliferative zones. The identification of brain tumour stem cells has important implications for understanding brain tumour biology and these cells may be critical cellular targets for curative therapy.

Characterization of adult neural stem cells and their relation to brain tumors

The adult mammalian brain contains neural stem cells that are capable of generating new neurons and glia over the course of a lifetime. Neural stem cells reside in 2 germinal niches, the subventricular zone (SVZ) and the dentate gyrus subgranular zone. These primary progenitors have been identified in their niche in vivo; these cells have characteristics of astrocytes. Recent studies have shown that adult SVZ stem cells are derived from radial glia, the stem cells in the developing brain, which in turn are derived from the neuroepithelum, the earliest brain progenitors. Thus, SVZ stem cells are a continuum from neuroepithelium to radial glia to astrocytes, and are contained within what has been considered the lineage for astrocytes. However, it seems that only a small subset of the astrocytes present in the adult brain have stem cell properties. Recent findings have shown that SVZ stem cell astrocytes express a receptor for platelet-derived growth factor (PDGF), suggesting that the ability to respond to specific growth factor stimuli, such as PDGF, epidermal growth factor and others, may be unique to these stem cell astrocytes. Intriguingly, activation of these same signaling pathways is widely implicated in brain tumor formation. Since the adult brain has very few proliferating cells capable of accumulating the numerous mutations required for transformation, the adult neural stem and/or progenitor cells may be likely candidates for the brain tumor cell of origin. Indeed, activation of the PDGF or epidermal growth factor pathways in adult neural stem or progenitor cells confers tumor-like properties on these cells, lending support to this hypothesis.

Molecular pathogenesis of adult brain tumors and the role of stem cells

Primary brain tumors consist of neoplasms with varied molecular defects, morphologic phenotypes, and clinical outcomes. The genetic and signaling abnormalities involved in tumor initiation and progression of the most prevalent adult primary brain tumors, including gliomas, meningiomas, and medulloblastomas, are described in this article. The current understanding of the cell-of-origin of these neoplasms is reviewed, which suggests that the malignant phenotype is propelled by cells with stem-like qualities. A comprehensive understanding of the molecular basis of transformation and the cell-of-origin of these neoplasms will enable the formulation of more targeted treatment alternatives that could improve survival and quality of life.

Gliomagenesis and neural stem cells: Key role of hypoxia and concept of tumor “neo-niche”

Gliomas represent the most common primary brain tumors and the most devastating pathology of the central nervous system. Despite progress in conventional treatments, the prognosis remains dismal. Recent studies have suggested that a glioma brain tumor may arise from a "cancer stem cell". To understand this theory we summarize studies of the concepts of neural stem cell, and its specialized microenvironment, namely the niche which can regulate balanced self-renewal, differentiation and stem cell quiescence. We summarize the molecular mechanism known or postulated to be involved in the disregulation of normal stem cells features allowing them to undergo neoplasic transformation. We seek data pointing out the key role of hypoxia in normal homeostasis of stem cells and in the initiation, development and aggressiveness of gliomas. We develop the concept of tumor special microenvironment and we propose the new concept of neo-niche, surrounding the glioma, in which hypoxia could be a key factor to recruit and deregulate different stem cells for gliogenesis process. Substantial advances in treatment would come from obtaining better knowledge of molecular impairs of this disease.

Relationship of glioblastoma multiforme to neural stem cell regions predicts invasive and multifocal tumor phenotype

Neural stem cells with astrocyte-like characteristics exist in the human brain subventricular zone (SVZ), and these cells may give rise to glioblastoma multiforme (GBM). We therefore analyzed MRI features of GBMs in specific relation to the SVZ. We reviewed the preoperative and serial postoperative MR images of 53 patients with newly diagnosed GBM. The spatial relationship of the contrast-enhancing lesion (CEL) with the SVZ and cortex was determined preoperatively. Classification was as follows: group I, CEL contacting SVZ and infiltrating cortex; group II, CEL contacting SVZ but not involving cortex; group III, CEL not contacting SVZ but involving cortex; and group IV, CEL neither contacting SVZ nor infiltrating cortex. Patients with group I GBMs (n = 16) were most likely to have multifocal disease at diagnosis (9 patients, 56%, p = 0.001). In contrast, group IV GBMs (n = 14) were never multifocal. Group II (n = 14) and group III (n = 9) GBMs were multifocal in 11% and 29% of cases, respectively. Group I GBMs always had tumor recurrences noncontiguous with the initial lesion(s), while group IV GBM recurrences were always bordering the primary lesion. Group I GBMs may be most related to SVZ stem cells; these tumors were in intimate contact with the SVZ, were most likely to be multifocal at diagnosis, and recurred at great distances to the initial lesion(s). In contrast, group IV GBMs were always solitary lesions; these may arise from non-SVZ, white matter glial progenitors. Our MRI-based classification of GBMs may further our understanding of GBM histogenesis and help predict tumor recurrence pattern.

Brain tumor stem cells

The concept of brain tumor stem cells is gaining increased recognition in neuro-oncology. Until recently, the paradigm of a tumor-initiating stem cell was confined to hematopoietic malignancies where the hierarchical lineages of stem progenitor cells are well established. The demonstration of persistent stem cells and cycling progenitors in the adult brain, coupled with the expansion of the cancer stem cell concept to solid tumors, has led to the exploration of "stemness" within gliomas. Emerging data are highly suggestive of the subsistence of transformed multipotential cells within a glioma, with a subfraction of cells exhibiting increased efficiency at tumor initiation. However, data in support of true glioma stem cells are inconclusive to date, particularly with respect to functional characterization of these cells. Ongoing work aims at the identification of unique pathways governing self-renewal of these putative stem cells and at their validation as ultimate therapeutic targets.

Platelet-derived growth factor-mediated gliomagenesis and brain tumor recruitment

Platelet-derived growth factor (PDGF) is a growth factor family of ligands and receptors known to activate phosphatidylinositol 3-kinase, mitogen-activated protein kinase, Jak family kinase, Src family kinase, and phospholipase Cgamma signal transduction pathways, some of which have been causally linked to glioma formation. Extensive involvement of PDGF in development and its implication in a variety of pathologic conditions, including gliomagenesis, are mediated not only by autocrine effects but by paracrine effects. Many researchers view brain tumors as clonal entities derived from the cancer stem cell; however, recent documentation of the importance of the tumor microenvironment for glioma initiation and progression as well as the ability of neural stem or progenitor cells to migrate toward the sites of injury or tumor formation reveals additional complexities in brain tumorigenesis. Paracrine effects of PDGF in animal models of gliomagenesis, continued adult neurogenesis capable of increasing in response to brain injury, and the growth factor-rich environment of brain tumors suggest that recruitment may play a role in gliomagenesis. In this view, glioma formation involves recruitment of cells from the adjacent brain and possibly other sites.

The human subventricular zone: a source of new cells and a potential source of brain tumors

The mammalian brain has been perceived as a quiescent organ incapable of postnatal neurogenesis for many years. Most recently, several studies have demonstrated that the adult mammalian brain is indeed capable of neurogenesis and that the process is primarily confined to the subventricular zone (SVZ) of the forebrain and the subgranular zone (SGZ) of the hippocampus. Of these regions, the SVZ is the largest niche of neurogenesis in the adult mammalian brain. Within this niche resides a subpopulation of astrocytes with stem cell-like features of self-renewal and multipotentiality. Interestingly, there is also a subpopulation of cells within brain tumors that possess these same characteristics. Based on these findings, the emerging hypothesis is that brain tumor stem cells may be derived from neural stem cells and that both of these populations may originate from the SVZ. This possible connection stresses the importance of studying and understanding the role that the human SVZ plays in not only harboring neural and brain tumor stem cells, but how this microenvironment may support both neurogenesis and tumorigenesis. Furthermore, the obvious differences in the SVZ between humans and other animals make it important to understand the human model when studying human disease. Such an understanding may lead to novel therapeutic strategies for both neurodegenerative diseases and currently intractable brain tumors.

Brain tumour stem cells

The dogma that the genesis of new cells is a negligible event in the adult mammalian brain has long influenced our perception and understanding of the origin and development of CNS tumours. The discovery that new neurons and glia are produced throughout life from neural stem cells provides new possibilities for the candidate cells of origin of CNS neoplasias. The emerging hypothesis is that alterations in the cellular and genetic mechanisms that control adult neurogenesis might contribute to brain tumorigenesis, thereby allowing the identification of new therapeutic strategies.

[Analysis of target volumes for gliomas]

Gliomas are the most frequent tumors of the central nervous system of the adult. These intraparenchymal tumors are infiltrative and the most important criterion for definition of GTV and CTV is the extent of infiltration. Delineation of GTV and CTV for untreated and resected glioma remains a controversial and difficult issue because of the discrepancy between real tumor invasion and that estimated by CT or MRI. Is particularly helpful a joint analysis of the four different methods as histopathological correlations with CT and MRI, use of new modality imaging, pattern of relapses after treatment and interobserver studies. The presence of isolated tumor cells in intact brain, oedema or adjacent structures requires the definition of two different options for CTV: i) a geometrical option with GTV defined as the tumor mass revealed by the contrast-enhanced zone on CT or MRI and a CTV with an expanded margin of 2 or 3 cm; ii) an anatomic option including the entire zone of oedema or isolated tumor cell infiltration extending at least as far as the limits of the hyperintense zone on T2-weighted MRI. Inclusion of adjacent structures (such as white matter, corpus callosum, subarachnoid spaces) in the CTV mainly depends on the site of the tumor and size of the volume is generally enlarged.

Target Definition in the Thorax and Central Nervous System

It is the aim of conformal radiotherapy to restrict the high-dose region to the target volume as much as possible, thereby sparing the neighboring healthy tissues. However, to increase the therapeutic range, smaller margins tend to be used. This reduction of safety margins enhances the risk of unsuitable dosage because of mistaken target definition. Central nervous system (CNS) and lung cancers constitute sites that are particularly difficult to irradiate combining a large number of conceptual difficulties, allowing them to be considered as 2 particularly interesting study models. Imaging occupies an increasingly important place in these 2 types of tumors, especially with the development of new radiotherapy techniques. CNS and lung cancers represent an example of clinicopathological correlations. More specifically, CNS cancers represent an excellent model for estimation of new 3-dimensional navigational systems. For lung cancer, there is a combination of ballistic difficulties because of respiratory motion, the number and low tolerance of neighboring organs, and dosimetric difficulties because of the presence of inhomogeneities. This article reviews the main currently accepted criteria of choice justifying the size of gross tumor volume and clinical target volume margins for lung and CNS cancers.

Cancer stem cells in nervous system tumors

Most current research on human brain tumors is focused on the molecular and cellular analysis of the bulk tumor mass. However, evidence in leukemia and more recently in solid tumors such as breast cancer suggests that the tumor cell population is heterogeneous with respect to proliferation and differentiation. Recently, several groups have described the existence of a cancer stem cell population in human brain tumors of different phenotypes from both children and adults. The finding of brain tumor stem cells (BTSCs) has been made by applying the principles for cell culture and analysis of normal neural stem cells (NSCs) to brain tumor cell populations and by identification of cell surface markers that allow for isolation of distinct tumor cell populations that can then be studied in vitro and in vivo. A population of brain tumor cells can be enriched for BTSCs by cell sorting of dissociated suspensions of tumor cells for the NSC marker CD133. These CD133+ cells, which also expressed the NSC marker nestin, but not differentiated neural lineage markers, represent a minority fraction of the entire brain tumor cell population, and exclusively generate clonal tumor spheres in suspension culture and exhibit increased self-renewal capacity. BTSCs can be induced to differentiate in vitro into tumor cells that phenotypically resembled the tumor from the patient. Here, we discuss the evidence for and implications of the discovery of a cancer stem cell in human brain tumors. The identification of a BTSC provides a powerful tool to investigate the tumorigenic process in the central nervous system and to develop therapies targeted to the BTSC. Specific genetic and molecular analyses of the BTSC will further our understanding of the mechanisms of brain tumor growth, reinforcing parallels between normal neurogenesis and brain tumorigenesis.

Development of gliomas: potential role of asymmetrical cell division of neural stem cells

Asymmetrical cell division is a mechanism that gives rise to two daughter cells with different proliferative and differentiative fates. It occurs mainly during development and in adult stem cells. Accumulating evidence suggests that tumour cells arise from the transformation of normal stem cells. Here, we propose that the asymmetrical mitosis potential of stem cells is associated with the generation of migrating tumour progenitors. Application of this speculative model to glioma proposes that the sites where tumour-initiating stem cells reside are indolent and distinct from the tumour mass, and implies that the tumour mass is continuously replenished with new migrating tumour cells from these clinically silent regions. This hypothesis offers explanations for our inability to cure glioblastoma and points to asymmetrical division as a new potential therapeutic target.

Expression of oligodendrocyte progenitor cell antigens by gliomas: implications for the histogenesis of brain tumors

The early events in neoplastic transformation can be understood only by comparison of the neoplastic cell with its nontransformed counterpart. The most common central nervous system gliomas traditionally are thought to arise from mature astrocytes and oligodendrocytes. We examined the possibility that gliomas arise from a population of glia that has properties of oligodendrocyte progenitors. These glial cells express the NG2 chondroitin sulfate proteoglycan and the alpha receptor of platelet-derived growth factor in vivo. We identified NG2 and the alpha receptor of platelet-derived growth factor expression in tissue from seven of seven oligodendrogliomas, three of three pilocytic astrocytomas, and one of five glioblastoma multiforme. These data provide evidence that glial tumors arise from glial progenitor cells. Molecules expressed by these progenitor cells should be considered as targets for novel therapeutics.

Marrow-mindedness: a perspective on neuropoiesis

There are pluripotent stem cells in the adult brain that might not be very different from those found in bone marrow. Recent and profound advances in the field of neuropoiesis, which often rely on insights from studies of hematopoiesis and in some instances use cross-paradigms with this field, have already revealed that bone marrow has much in common with so-called 'brain marrow'. Proliferative primogenitors and developmentally regulated molecules are hallmarks of both neuropoiesis and hematopoiesis. This article will focus on recent advances in neuropoiesis within a central core of the mature brain that is referred to as brain marrow, discussing its pluripotency and proliferative capacity, in vitro and molecular assays used in its study, and markers of neuropoietic stem/progenitor cells. As hematopoiesis research has led to the discovery of numerous morphogenetic factors, it is anticipated that studies of neuropoiesis should also uncover many new factors and genes that affect the growth and differentiation of neural cells. Recent breakthroughs in the stem-cell field prompt an inclusion of rationale for the persistence of normal stem/progenitor cells even in the aged brain. By analogy with hematopoiesis research, a thorough investigation of brain marrow should provide basic insights into developmental and persistent neurogenesis while anticipating cell-transplant and gene therapies for debilitating neurological diseases.

A cerebral primitive neuroectodermal tumor in a squirrel monkey (Saimiri sciureus)

An adult squirrel monkey with a history of long-term exposure to microwave radiation was found at necropsy to have a malignant tumor of the right cerebral cortex. Gross examination revealed a mass with expanding borders in the right frontoparietal cortex with compression of the adjacent lateral ventricle. Microscopy revealed a tumor composed of sheets of moderate-sized cells, resembling an oligodendroglioma, with clear cytoplasm and central nuclei interrupted by delicate vasculature. Malignant features were present in the form of marked nuclear pleomorphism, frequent mitotic figures, and focal necrosis. A neuronal cell origin for this tumor was supported by immunohistochemical analysis, which revealed immunopositivity for neurofilament proteins and neuron-specific enolase. Staining for vimentin and glial fibrillary acid protein was negative, except in reactive astrocytes at the tumor margins and adjacent to intra-tumoral blood vessels. Antibody activity against Ki-67 antigen, a marker of rapidly proliferating tumor cells, and p53 oncoprotein was strongly positive, indicative of the aggressive and malignant nature of this tumor. The tumor was diagnosed as a cerebral primitive neuroectodermal tumor.

Region-specific DNA synthesis in brains of F344 rats following a six-day bromodeoxyuridine infusion

Prolonged exposure to certain alkylating chemicals induces glial and meningeal tumours in rats, probably resulting from DNA damage to dividing neural cells. The present work evaluated DNA synthesis in the brains of untreated, young adult male F344 rats in order to define a BrdUrd infusion protocol to more adequately assess proliferation in slowly dividing neural cell populations. BrdUrd (2.5 to 160 mg/ml) was administered for 6 days via subcutaneous osmotic pumps. Clinical toxicity was not observed at any dose. The labelling index (LI; % of cells per brain area that incorporated BrdUrd) and unit length labelling index (ULLI; % of cells per meningeal length that incorporated BrdUrd) were calculated for selected regions by counting labelled neural cells in defined areas of the right hemisphere in coronal brain sections. Intensely stained cells were numerous in the cerebral subependymal layer (LI = 35.8%); scattered in cerebral white matter tracts (e.g. corpus callosum and internal capsule; LI = 6.2%) as well as cerebral (ULLI = 4.2%) and cerebellar (ULLI = 3.6%) meninges; and rare in the hippocampus (LI > 0.1%). Mildy stained cells were dispersed in the pons (LI = 2.1%), deep cerebral (LI = 1.8%) and cerebellar (LI = 1.0%) grey matter, and thalamus (LI = 0.3%). Phenotypically, BrdUrd-positive cells in neuropil were glial cell precursors and their progeny, while those associated with meninges were usually located in the superficial subarachnoid space and appeared to be fibrocytes. Using BrdUrd infusion, LI for glial precursors at these sites ranged from two- to 10-fold higher than those reported previously after a brief parenteral pulse dose. These data indicate that continuous BrdUrd infusion for 6 days by subcutaneous osmotic pump is an efficient means of labelling neural cells throughout the brain.

Glioma invasion in the central nervous system

Invading glioma cells seem to follow distinct anatomic structures within the central nervous system. Tumor cell dissemination may occur along structures, such as the basement membranes of blood vessels or the glial limitans externa, that contain extracellular matrix (ECM) proteins. Frequently, invasive glioma cells are also found to migrate along myelinated fiber tracts of white matter. This behavior is most likely a consequence of using constitutive extracellular ligands expressed along the pathways of preferred dissemination. The extracellular space in anatomic structures, such as blood vessel basement membranes or between myelinated axons, is profoundly different, thus suggesting that glioma cells may be able to use a multiplicity of matrix ligands, possibly activating separate mechanisms for invasion. In addition, enzymatic modification of the extracellular space or deposition of ECM by the tumor cells may also create a more permissive environment for tumor spread into the adjacent brain. Tumor cell invasion is defined as translocation of neoplastic cells through host cellular and ECM barriers. This process has been studied in other cancers, in which a cascade of events has been described that involves receptor-mediated matrix adhesion, degradation of matrix by tumor-secreted metalloproteinases, and, subsequently, active cell locomotion into the newly created space. Although some of these mechanisms may play an important role in glioma invasion, there are some significant differences that are mainly the result of the profoundly different composition of the extracellular environment within the brain. This review focuses on the composition of central nervous system ECM and the recent evidence for the use by glioma cells of multiple invasion mechanisms in response to this unique environment.

Rhinencephalic glial cell nests and their possible role in glioma formation: morphometric studies do not reveal significant differences between brachycephalic and dolichocephalic dogs

Gliomas frequently occur in boxer dogs and are often located in the rhinencephalic allocortex. This brain region contains unusual glial cell nests (GCN). The presence of structural abnormalities in the GCN in the boxer dog might indicate that they are involved in the development of gliomas, which would explain the predisposition of this canine breed for glioma formation. Therefore, the brains of six brachycephalic (boxer dogs) and five dolichocephalic dogs were investigated morphometrically. The volumes of the whole brain, the allocortex, and the GCN were estimated following Cavalieri's principle. Unbiased estimates of the numerical density and total number of the two prevailing cell populations within the GCN were obtained using the optical disector method. There was no significant difference for the estimated parameters between brachycephalic and dolichocephalic dogs. The results of the present study did not show any evidence of boxer dog-specific features of the GCN, thus, failing to explain the striking glioma predisposition of boxer dogs.