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Folia Neuropathologica
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vol. 53
Review paper

Biopathology of astrocytes in human traumatic and complicated brain injuries. Review and hypothesis

Orlando José Castejón

Folia Neuropathol 2015; 53 (3): 173-192
Online publish date: 2015/09/29
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Astrocyte changes in traumatic brain injuries

Astrocyte reactions to traumatic brain injuries have been the topic of many studies and of much debate for almost a century. The current consent is that glial cells, mainly astrocyte cells, have a pivotal role in degeneration and regeneration of the gray matter, and that astrocyte changes are one of the hallmarks of the response of the central nervous system (CNS) to injury [1,8,12,15,17,44,58,65,68-70,74,76,81,86,88,94-96,98,99,102,109,129,138]. The astro­cyte cell changes in brain injuries have been widely studied at light and electron microscope levels. Such earlier studies in human and experimental animals reported a wide spectrum of pathological changes, such as astrocyte swelling, division and proliferation (gliosis), gemistocytic astrocytes, a type of round to oval hypertrophic astrocyte cell with abundant cytoplasm containing glial filaments and an eccentric nucleus or two nuclei, clasmatodendrosis or astrocytes with breaking up of astrocytic protoplasmic expansions, active and reactive changes, accumulation of glial fibres and increased synthesis and expression of glial fibrillary acidic protein (GFAP), vimentin and glutamine synthetase, phagocytic properties, glycogen accumulation, intermediate filament gene expression, secretion of neurotrophic factors, migration, and down regulation of astrocyte glutamate transporter. An increased immunochemically detectable GFAP level was one of the earliest responses to specifically characterize CNS injuries [18,21,51,52,54,60,65,68,77,81,125].
Castejón previously described astrocyte subtypes, morphological astrocytic changes and human neurobehavioral disorders in traumatic human oedematous cerebral cortex [27-32]. Ragaisis [120] found ischaemic swelling of astrocytes in brain contusion after increasing concentrations of potassium ion. Derugin et al. [45] and Fotheringham et al. [55] reported reactive astrocytes after transient middle cerebral artery occlusion. Del Biggio et al. [44] described astrocytic swelling and eosinophilia in human post-mortem brain indicative of plasma extravasation and brain oedema. According to Rai­vich et al. [119], damage to the CNS leads to cellular changes not only in the affected neurons but also in adjacent glial cells and endothelia, and frequently to recruitment of cells of the immune system. These cellular changes form a graded response which is a consistent feature in almost all forms of brain pathology. These authors provide evidence about the biological function of the neuroglial activation in the injured brain. Vajtr et al. [138] described in human cortical biopsies cytotoxic astrocyte oedema occurring during blood brain barrier damage after traumatic brain injury. According to Eng [52], GFA protein or GFAP is the major protein constituent of glial intermediate filaments in differentiated fibrous and protoplasmic astrocytes of the CNS. A putative function ascribed to glial filaments as a component of cytoskeletal substructures is defining and maintaining the shape of the astrocyte.
In the present review, I shall present the view supported by our light and electron microscopic laboratory data based on examinations of more than 60 human cortical biopsies immediately taken in the surgical room, and optimally fixed during the neurosurgical treatment. These studies have provided some insight into the subsets of astrocyte cells generated after severe and complicated traumatic brain injuries. A widely accepted view is that adult human CNS is inherently different from experimental animal models, and that astrocyte reactivity changes according to the aetiology or primary insult of human neuropathological conditions. Therefore, we characterize astrocyte pathological and heterogeneous populations in different cortical regions of patients with complicated and severe brain trauma associated with subdural, extradural haematoma or hygroma (a subdural body of cerebrospinal fluid (CSF), without blood, believed to be derived from chronic subdural haematomas). Additionally, we have compared the electron microscopic features of astrocyte subtypes in complicated human traumatic brain injuries with those reported in a large variety of nervous and mental diseases, and in vivo and in vitro experimental conditions, in an attempt to establish a link between basic and clinical neuroscience, and to gain a better insight into the real significance of heterogeneity of astrocytes that occur at the immediate site of severe and complicated human brain injuries. This comparative extrapolation of findings will allow us to explore new and common pathogenetic mechanisms, and to study new therapeutic strategies.

Light and electron microscopy fixation procedures for human cortical biopsies

Two to five mm thick cortical biopsies from the perifocal area of a traumatic lesion of the frontal and parietal region were taken by the neurosurgeon, and immediately fixed in the neurosurgical room in 4% glutaraldehyde-0.1 M phosphate or cacodylate buffer, pH 7.4, at 4C, in order to avoid delay fixation. After 2 hours of the glutaraldehyde fixation period, the cortical biopsies were divided in our laboratory into approximately 1 mm slides, and observed under a stereoscopic microscope to check the quality of fixation of the sample, the glutaraldehyde diffusion rate, and the brownish coloration of the surface and deeper cortical regions, indicative of good glutaraldehyde fixation by the immersion technique. Immersion in fresh glutaraldehyde solution of 1 mm slices was secondarily done for 2 hours after eliminating the remaining blood from the cortical biopsy by washing in similar 0.1 M phosphate or cacodylate buffer, pH 7.4, to avoid oxidation of the primary fixative solution. Secondary fixation in 1% osmium tetroxide 0.1 M phosphate buffer, pH 7.4, was carried out for 1-2 hours at 4C. Black staining of the cortical slices also was observed under a stereoscopic microscope to determine the osmium tetroxide diffusion rate and quality of secondary fixation. The samples were then rinsed for 5 to 10 minutes in phosphate or cacodylate buffer of similar composition to that used in the fixative solution, dehydrated in increasing concentrations of ethanol, and embedded in Araldite or Epon. For proper orientation during the electron microscope study and observation of cortical layers, approximately 0.1 to 1 m thick sections were stained with toluidine blue and examined with a Zeiss photomicroscope. Light microscope study of neurons, glial cells, and blood-brain barriers was performed. Ultrathin sections, obtained with Porter-Blum and LKB ultramicrotomes, were stained with uranyl acetate and lead citrate, and observed in a JEOL 100B transmission electron microscope (TEM) at magnifications ranging from 20,000 to 90,000×. Approximately one hundred electron micrographs were taken of each cortical biopsy. Those cortical biopsies with delayed and poor fixation, due to inactivation of the primary fixative for the presence of tissue blood, were discarded. Since poor or delayed fixation of brain parenchyma exhibits features quite similar to those observed in an oedematous tissue, in each case we critically differentiate between true pathology and fixation artefacts. The different submicroscopic features that characterize moderate and severe brain oedema allowed us to discard fixation artefacts, mainly when analysing substructures such as the multilamellar arrangement of myelin layer [23]. To characterize a subpopulation of astrocytes we assemble a montage of serial electron micrographs to analyse the entire cell body and processes. A clear distinction between astrocytes and oligodendrocytes was made in each case examined.

Heterogeneity of astrocyte population after severe and complicated brain injuries

The following astrocyte populations are found at the light and electron microscopic levels in the oedematous human cerebral cortex associated with complicated brain trauma: reactive clear and dense swollen astrocytes, reactive hypertrophic astrocytes, lipofuscin-rich astrocytes, glycogen-rich and glycogen-depleted astrocytes, and phagocytic astrocytes [28,30,32].

Clear oedematous astrocytes in light and electron microscopy

Semithin plastic sections stained with toluidine blue show the presence of clear and dense notably swollen astrocytes at the perineuronal, interfascicular, and perivascular localizations. They appear surrounded by notably enlarged extracellular spaces (Fig. 1A), which feature the status spongiosus of brain parenchyma. Perineuronal astrocytes appear intimately applied to shrunken and ischaemic neuronal bodies (Fig. 1B). Binucleated and multinucleated astrocytes, indicating astrocyte proliferation, are also observed (Fig. 1C). Electron micrographs show the clear astrocytes characterized by an electron lucid cytoplasm matrix, widened rough endoplasmic reticulum, clear and swollen mitochondria with dislocated or fragmented cristae, lysosomes, phagolysosomes, and glycogen granules [24,26] (Fig. 1D).
Very severely oedematous clear astrocytes show lacunar or vacuolated enlargement of rough endoplasmic reticulum, an increased amount of glial filaments, fragmentation of Golgi apparatus, and numerous lysosomes (Fig. 2A).
Oedematous astrocyte were also reported by Ito et al. [67] in focal cortical infarction, and by Kuchiwaki et al. [79] in cytotoxic brain oedema.
These astrocytes are characterized by robust processes containing bundles of glial filaments (Fig. 2B).
Similar cable-like bundles of glial filaments were reported by Graeber and Kreutzberg in facial nerve axotomy [60].
Figures 2C and 2D show “control” electron micrographs of the apparently normal astrocyte cell body and processes from a patient with anomaly of the anterior cerebral artery. It is important to keep in mind that brain oedema is produced as soon as the brain is exposed to the air during neurosurgery following opening of the meningeal covering. The neighbouring neuropil shows the best ultrastructural preservation obtained with the immediate glutaraldehyde-osmium fixation procedure of cortical biopsies in the neurosurgical room. Note the integrity of subcellular structures of apparently normal nerve cell bodies and processes, the continuity of limiting plasma membrane profiles, glial filaments, and the membrane-to-membrane, 20 nm in width, non-dilated extracellular space separating nerve cell processes in the neuropil [24].

Brain trauma induces dense swollen reactive astrocytes

Dense, swollen astrocytes are easily identified by the higher electron density of their cytoplasmic matrix. They exhibit vacuolization of rough endoplasmic reticulum, detachment of associated ribosomes, plasma membrane disruption, clear and swollen mitochondria with cristae fragmentation and dissolution, and bundles of glial filaments extended toward the vacuolated astrocyte processes [28,29] (Figs. 3A and 3B).
Clear and dense astrocyte swelling also have been widely reported by Allen et al. [5] after high velocity penetrating head injuries, Bullock et al. [15] following human cerebral contusion, Dietrich et al. [48] after fluid percussion brain injury, Fujisawa et al. [56] using a rat model of subdural haematoma, Mathew et al. [94] in an experimental model of contusion and focal cortical injury, Kuroiwa et al. [80] during early ischaemia, and by non-traumatic neuropathological conditions, such as Matyja et al. [95] in amyotrophic lateral sclerosis (ALS), a model of slow glutamate excitotoxicity. Similar findings have also been observed in vitro in a variety of experimental conditions, such as swelling-induced release of glutamate, aspartate, and taurine from astrocyte cultures, hypoosmotic media, high K+, high glutamate, ethanol, free fatty acids, lacto-acidosis and acid-base exchange [7,73-75], and the role of Na+, K+ and Cl– cotransporters in brain ischaemia [39].

Interfascicular and swollen perisynaptic astrocyte processes

The clear and swollen astrocytic processes containing beta type-glycogen granules can be traced in the neuropil in moderate oedema far from the traumatic perifocal region, where they appear surrounding the synaptic contacts, swollen and bead-shaped dendritic profiles, and degenerated myelinated axons. These swollen astrocyte processes compress and indent the degenerated myelinated axons. At the level of this compression zone, the degenerated myelinated axons appear constricted with distorted and vacuolated myelin sheath [23] (Figs. 4A and 4B).
The presence of notably swollen astrocytes and degenerated myelinated axons has been described previously by Graeber and Kreutzberg [60] following facial nerve axotomy, by Gilmore et al. [59] after sciatic axotomy, and by Murray et al. [103] in dorsal root or peripheral nerve lesions.
In those patients with very severe brain oedema, located at the perifocal area, the haematogenous oedema fluid rejects the astrocyte perisynaptic cytoplasm, and the pre- and postsynaptic endings and the synaptic cleft appear in direct contact with the extracellular space [29,31] (Fig. 4C).
These findings indicate that astrocytes lost both the prime location to receive synaptic information from released neurotransmitters and the neurotransmitter receptor expression in response to injury [104]. Additionally, the diffusion of nerve impulse transmission to the extracellular space indicates loss of astrocyte modulation of extracellular space [6]. If we consider that the perisynaptic astrocyte should be viewed as an integral modulatory element of the tripartite synapse [6], we are really dealing with dissociated tripartite synapses.

Hypothesis on the biopathological significance of absent perisynaptic astrocyte ensheathment

Our electron microscopic studies suggest that in the perisynaptic glial-synaptic contact dissociation process, the specific functions of astrocytes in synaptic transmission, such as the ability to take up and metabolize neurotransmitters, are disrupted – for example, glutamate uptake and glutamate release at the synapses that they envelop, to protect these synapses against inappropriate activation by excess transmitter released or regulate synaptic transmission [6], and also inactivate neurotransmitters [92]. Such modulatory regulation or inactivation is disrupted by traumatic brain oedema. The alteration of these specific functions would help to explain the contribution of pathological astrocytes to development of neurobehavioral disorders, such as loss of consciousness, neurological deficits, and seizures [27,31]. In addition, Uranova et al. [137] have described ultrastructural alterations of both synaptic contacts and astrocytes in the postmortem caudate nucleus of schizophrenic patients, suggesting that perisynaptic ensheathment dissociation is not a specific finding of human traumatic brain injuries.

The glio-basal dissociation process in severe traumatic brain injuries

At the light microscopy level, clear and dense perivascular astrocytes appear mostly attached to the outer surface of the capillary basement membrane mainly in areas of moderate, and some regions of severe oedema located far from the traumatic perifocal region [25,37] (Fig. 5A and 5B).
In severe and complicated brain injuries, a glio- basal dissociation process occurs and a separation of astrocyte end-foot limiting plasma membrane from the capillary basement membrane is observed (Fig. 5C). Additionally, the astrocyte perivascular end-feet are fragmented and remnants of their limiting plasma membranes are observed. Extensive areas of the capillary basement membrane devoid of astrocytic end-feet appear in direct contact with the enlarged extracellular space containing electron lucid or proteinaceous oedema. These findings demonstrate the absence of the glial cell component of the blood-brain barrier, and indicate the glial barrier dysfunction induced by the traumatic brain oedema [37], and interruption of the route between capillaries, astrocytes and the neuronal membranes by which metabolic substrates reach the neurons [25,37,139]. Subsequently, vasogenic and cytotoxic oedema are superimposed, and degeneration and death of pyramidal and non-pyramidal nerve cells occur [25,37]. Similar findings were reported by Ito et al. [67] in experimental ischaemic brain oedema in gerbils.

Astrocytes’ supply of substrates of glycogen metabolism to ischaemic neurons and oligodendroglial cells

Glycogen-rich and glycogen-depleted perivascular astrocyte end-feet are found in severe traumatic brain injuries [30]. The glycogen-rich perivascular astrocyte end-feet show enlarged vesicular and vacuolar profiles of endoplasmic reticulum, and beta- and alpha-type glycogen granules. In contrast, glycogen-depleted astrocyte end-feet are distinguished by a scarce amount or absent isolated beta- and gamma-type glycogen granules, characterized according to Drochman’ s classification [49]. This classification allowed us to distinguish among beta or monogranular, alpha or in rosette clusters, and gamma or mini type, according to glycogen granular size and degree of aggregation (Fig. 6A and 6B).
In physiological conditions there is no evidence for transfer of glycogen substrate energy from glial cell stores to neurons [42]. Our electron microscopic findings suggest that glycogen accumulation and breakdown are induced by the severity of traumatic brain injuries [30]. The fact that both glycogen-rich and glycogen-depleted astrocyte perivascular processes are simultaneously found strongly suggests the hypothesis that in human brain trauma astrocytes exert a neuroprotective action supplying substrates of glycogen metabolism for the survival of ischaemic neurons and oligodendroglial cells. In support of our hypothesis is the fact that glycogen accumulation after an initial decrease is a distinctive feature of astrocytes responding to injury [64]. According to Magistreti et al. [89], astrocytes may help neurons with their energy requirements during periods of activity.

Enlargement, fusion and disruption of interastrocytary gap junctions induced by the traumatic agent

At the level of interastrocytary gap junctions (gap junctions localized between two neighbouring astrocyte end-feet belonging to the same or to different clear or dark perivascular astrocytes), the severe traumatic injury induces widening of confronted astrocyte end-feet limiting membranes and enlargement of extracellular space between the astrocyte end-feet, or they appear either fused or disrupted by the notably swollen astrocyte end-feet [22,26] (Fig. 6A and 6B).
There is a relationship between swollen capillary endothelial cells and swollen perivascular astrocytes. The open endothelial junctions [35] and the increased transendothelial vacuolar and vesicular transport [32] discharge the brain oedema fluid into the thickened basement membrane, and into the astrocyte perivascular end feet [37].
These findings indicate blood brain barrier breakdown and the genesis of vasogenic brain oedema in severe human brain trauma. These findings also tend to demonstrate that in human traumatic brain oedema, the connecting hemichannels of each confronting astrocytic end-foot process forming the gap junctions [46,89,104,121] are set apart or damaged with subsequent disruption of connexin proteins. In normal physiological conditions, astrocytes in vitro modulate the blood-brain barrier permeability, but there is no correlation with alterations of tight junction protein from the cellular contacts. However, as previously demonstrated by Hossain et al. [66], astrocytes respond to an ischaemic insult reorganizing their gap junctions. Thierault et al. [133] described alteration of connexin 43 and astrocytic gap junctions after acute compression injury. Nerve cell injury causes large intracellular increase in intracellular K+ and Ca2+, which could lead to astrocytic uncoupling [108]. In addition, intracellular acidification mediated by lactate also results in irreversible astrocytic uncoupling [3]. Additionally, Li et al. [84] reported the phosphorylation status of gap junctions and connexin 43 in rat brain after cerebral focal ischaemia.
The above-mentioned findings on damaged astro­cytic gap junctions suggest a whole alteration of the pan-glial syncytium postulated by Rash et al. [122] and Li et al. [84], or an interruption of the gap junction wiring described by Dermietzel et al. [46]. In addition, it would mean an alteration of the coordinating role of gap junctions in motor behaviour [71].

Dense and reactive hypertrophic astrocytes

Hypertrophic astrocytes are observed at the inter­fascicular level at the neuropil and in perivascular localization. Their perivascular end-feet can be observed attached to the capillary basement membrane. They are characterized by increased cytoplasmic matrix density, distended rough endoplasmic reticulum profiles, swollen mitochondria, fragmentation of Golgi apparatus, and presence of alpha- and beta-glycogen particles. Close examination of the cytoplasm at higher magnification shows increased numbers of glial filaments, microtubules, and numerous round and lobulated lysosomes. They exhibit robust and extended processes in the neuropil containing compact bundles of glial filaments [26,28,29] (Fig. 7A).
These types of astrocytes have also been found by Kaur et al. [70] after a non-penetrative blast. However, hypertrophic astrocytes seem to be an un­specific population present in traumatic brain injuries since they have been described in a large variety of distinct human nerve pathological entities, and in experimental animal models, such as those reported by Ludowyk et al. [87] in chronic experimental autoimmune encephalitis in aged rats, Kurgel and Ivy [72] in kindling-induced seizures, Krusulovie et al. [78] in nerve cells of taiep rats, and by Harsan et al. [63] in dysmyelinated jimpy mouse brain. The mouse jimpy mutation of the X-linked proteolipid protein (Plp) gene causes dysmyelination and premature death of the mice. The established phenotype is characterized by severe hypomyelination, increased numbers of dead oligodendrocytes and astrocytosis. This astrocytosis exhibits similar features as in the case of the astrocytes described above. Therefore, dense and reactive hypertrophic astrocytes are not a subpopulation of astrocytes specifically induced by severe and complicated human brain injuries.
Dark astrocytes, which should be considered as a variety of hypertrophic astrocytes, have been found by Gallyias et al. [57], and by Tóth et al. [136] after compressive or concussive head injuries. These reactive astrocytes with dense processes are GFAP immunoreactive [51-53], and have been co-labelled by antibodies to GFAP and vimentin [54]. For further details the reader is referred to the elegant review of Malhotra et al. [90] on reactive astrocytes.

Lipofuscin-rich astrocytes favour the aging process and neuronal degeneration in traumatic brain injuries

In traumatic brain injuries, accumulation of lipofuscin granules has been found in the astrocyte body and processes [32] (Fig. 7B, 7C and 7D).
This lipofuscin accumulation has been widely reported in normal aging subjects [100]. Although age is considered an important factor reducing astrocyte reactivity, in aging patients with severe brain trauma, increased reactivity of astrocytes was observed characterized by a large amount of lipofuscin-rich astrocytes. The significance of lipofuscin accumulation for astrocyte biopathology is a subject that remains relatively poorly defined in the world literature. Our studies have demonstrated that lysosomes coexisting with an increased amount of lipofuscin granules are observed in young and adult patients with brain trauma, tumours and vascular anomalies, implicating a dysfunction of the endolysosomal system in astrocytes [32]. We have also reported lipofuscin granules in neonate and infant patients with congenital hydrocephalus [32], suggesting that lipofuscin formation is a life span process in immature and mature astrocytes, and not only due to the aging process, as it has been classically conceptualized. The neurodegeneration associated with lipofuscin accumulation may be caused by that accumulation, and induces astrocyte activation. Lipofuscin and ceroid are usually held responsible for impaired cellular performance, via oxidative damage and the irreversible accumulation of fluorescent products of lipid peroxidation. The participating evidence of free radical oxidative interactions in promoting astrocyte injury in such conditions as brain trauma, ischaemia, toxicity, and in neurodegenerative diseases such as Parkinson’ s disease, Alzheimer’ s dementia, multiple sclerosis, and lipofuscinosis, is growing and accumulating, and favouring the idea of an overall spectrum of an altered endosomal/lysosomal system. Accumulation of heterogeneous non-degraded macromolecules in dysfunctional lysosomes and autoly­sosomes ultimately leads to early-onset apoptotic death with subsequent activation of astrocytes [111]. The general hypothesis, as developed from chronic neurodegenerative diseases [14,16], is that astrocyte activation occurs in juvenile neuronal ceroid lipofuscinosis (JNLC) and Batten disease. Additionally, the role of peroxy radicals or their products formed by lipoperoxidation of polyunsaturated fatty acids has also been implicated [13].
In spite of recent advances in immunohistochemical identification of biochemical markers, the ultrastructural identification of lipofuscinic pigments remains the gold standard to identify neuronal ceroid lipofuscinosis (NCL), together with the clinical aspects and respective gene defects [14]. Neuronal cell cultures offer a good model to study systematically lipofuscin’ s impact on astrocyte biopathology.

Traumatically reactive astrocytes phagocytose synaptic contacts, myelinated axons and undifferentiated nerve cell debris

Clear and swollen dense perineuronal phagocytic astrocytes can be seen engulfing remnants of degenerated myelin sheath, myelin ovoids, degenerated myelinated axons, and entire degenerated axo-dendritic contacts, and fragments of nerve cell debris.
These two types of phagocytic astrocytes are frequently found in traumatic brain oedema: the clear type characterized by lamellar processes containing osmiophilic vesicles, as previously described by Gonatas et al. [62] (Fig. 8A), and a second type: the dark phagocytic astrocytes [26,28]. Figure 8B shows a clear phagocytic astrocyte cytoplasm with osmiophilic vesicles in the neuropil engulfing a presynaptic ending. These findings indicate the participation of phagocytic astrocytes in the removal of degenerated nerve cell substructures in the traumatic oedematous human cerebral cortex [28,29].
Table I below summarizes the features of astrocyte changes in moderate and severe oedema. The physical stress elicited by the intensity of the traumatic agent induces rupture of the astrocyte plasma membrane, nuclear membranes, cytomembranes and Golgi apparatus, and perivascular astrocyte end-feet exhibit membrane defects induced by decreased Na+-K+-ATPase activity exerted by the trauma [41], and changes in the extracellular ion Na+ and K+ concentrations [101]. Increased membrane tension could open stretch-activated ions channels in astrocyte cultures [73], and astrocyte amino acids could diffuse through these channels (swelling-induced release of amino acids from astrocytes). Mechanical injury alters the volume of activated ion channels [47]. As pointed out by Levi and Gallo [82], it is possible that the establishment of a vicious circle will cause self-potentiation of the pathological process. For example, the accumulation of extracellular glutamate and K+ following the traumatic and ischaemic insults could induce astrocyte swelling with further glutamate and K+ efflux, and consequent potentiation of swelling and excitotoxicity.

Astrocyte cell death types in severe and complicated traumatic brain injuries

In severe brain trauma complicated by subdural or epidural haematoma we found astrocyte cells exhibiting oncotic cell death type characterized by electron lucent and swollen cytoplasm and nucleoplasm, lobulated nucleus, presence of intranuclear inclusions, apparently intact and disrupted nuclear pore complex, numerous clear vacuoles, lysosomes, and numerous small dense bodies [33] (Fig. 9A).
The presence of typical apoptotic astrocyte cell death is characterized by chromatin condensation, a disrupted perinuclear cistern and the finding of apoptotic bodies in the nucleus and cytoplasm (Fig. 9B).
Other astrocytes display coexisting oncotic and apoptotic cell death, showing chromatin condensation, empty electron lucent and disrupted cytoplasm, and swollen degenerated mitochondria. Some of these astrocytes show postmortem morphological features of necrotic cell death, characterized by disorganized nuclear chromatin, clear or dense fibrillar and granular euchromatin, clear or dense nucleoplasm, remnants of cytomembranes, small dense vesicles, massive vacuolar degeneration, plasma membrane frag­mentation and degeneration, and dispersed, degenerated mitochondria at cytoplasmic and extracellular localizations [33] (Fig. 9C).
Ischaemic neuronal death influenced by astrocytes has been described by Swanson et al. [132]. Experimental brain injury induces regional distinct apoptosis during the acute and delayed post-traumatic period [43].
A continuum of increased oncosis, apoptosis and necrosis appeared to involve astrocytes and oligodendrocytes in traumatic human brain oedema [33], and in animals following blast exposure [139]. In patients with higher brain dysfunction after mild traumatic brain injury, diagnostic imaging showed cortical neuron loss in the frontal lobes, using single-photon emission computed tomography (SPECT) with I-iomazenil, as a radioligand for the central benzodiazepine receptor [105]. Swanson and Kauppinen [132] have recently examined the influences of astrocytes on ischaemic neuronal death.

The key role of aquaporin in traumatic astrocyte swelling

Aquaporins (AQPs) play pivotal roles in cerebral water movement as essential mediators during oedema and fluid accumulation. Aquaporin 4 (AQP4), a water channel protein located at the blood-brain barrier, might facilitate the removal of this excess of water from the parenchyma into the blood. Aquaporin 4 was first located on astrocyte end-feet but later on the whole membrane of astrocytes that became hypertrophic in the most severe traumatic brain injuries [130]. The key role of aquaporin in astrocyte swelling and traumatic brain oedema has been emphasized by several investigators [61,130,131]. More recent studies indicate a key role of aquaporin in astrocyte swelling, ischaemia and brain oedema [2,85,106,107,113,141]. According to some of these studies, AQP4 expression follows an adaptive profile to the severity of traumatic brain oedema, which is probably a protective response mechanism. A very important avenue for future work in traumatic brain injuries and brain oedema is to study aquaporin as a target for pharmaceutical treatment.

The dissociated astrocyte-neuronal unit in human traumatic brain injuries

As mentioned above, normal neuronal-astrocyte cooperation is important for signalling, energy meta­bolism, extracellular ion homeostasis, volume regulation, and neuroprotection [10]. The findings described above in traumatic and complicated human brain injuries in the oedematous cerebral cortex reveal a structural alteration of the astrocyte-neuronal unit. Swollen and ischaemic neurons and swollen perineuronal astrocytes should be considered an abnormal metabolic cooperation of the astrocyte-neuronal unit. This dissociation would mean an altered astrocyte response to chemical signals from damaged neurons, which would send back other messages in the form of neuroactive and neurotrophic substances. These alterations would include not only those classical neurotransmitters and neuromodulators, but also growth factors, cytokines and prostanoids [50,97].
Swollen astrocytes could be responsible for the degeneration of damaged neurons, not only for the exerted outer surface physical stress due to the notably swollen astrocyte soma, but in addition, for the establishment of high extracellular levels of glutamate released during severe traumatic brain injuries (glutamate excitotoxicity) [28,29,73-75].
These findings have led to the prevalent hypothesis that excitatory amino acid efflux is a major contributor to the development of neuronal damage subsequent to traumatic injury. Obrenovitch et al. [110] ask an interesting question related to the role of glutamate excitotoxicity in traumatic brain injuries.

Hypothesis of astrocyte responsibility for production of seizures

Astrocyte mitochondrial matrix swelling and loss of cristae [34], and early inactivation of cytochrome oxidase, have been reported after injury [41]. Since astrocyte glutamine synthesis requires ATP, it is rational to argue that in human brain trauma due to mitochondrial swelling, astrocytes would not be able to convert accumulated glutamate to glutamine. If we consider that glutamine is also a quantitatively important precursor of GABA [9], we could infer that a disturbance of the GABA-glutamine cycle and glutamate excitotoxicity could explain the seizures observed in some patients studied in our laboratory [27]. An impairment of glutamate-inactivating ability of astrocytes in human brain trauma [92] could be important in increasing the excitotoxic effect of glutamate on neurons. This hypothesis should be considered in explaining the degeneration of pyramidal and non-pyramidal nerve cells leading to nerve cell death.
Table II summarizes the abnormal astrocyte-neuronal unit in traumatic moderate and severe oedema.

Concluding remarks

Severe and complicated traumatic human brain injuries induce notably swollen astrocytes and numerous subpopulations of astrocyte subtypes, such as clear and dense oedematous astrocytes, hypertrophic reactive astrocytes, glycogen-rich and glycogen-depleted astrocytes, lipofuscin-rich astrocyte, and phagocytic astrocytes. Swollen clear and dense perineuronal astrocytes appear compressing and indenting dark, ischaemic nerve cells, degenerated myelinated axons, and synaptic contacts. At the level of the neuropil, in areas of moderate brain oedema, the perisynaptic astrocyte cytoplasmic layer appears covering axodendritic synaptic contacts, dendrites and myelinated axons. In severe oedematous regions the perisynaptic astrocytic glial ensheathment is rejected and lost by the brain oedema fluid deposited in the enlarged extracellular space. The synaptic contacts appear in direct continuity with the extracellular space impairing neurotransmission.
Glycogen-rich and glycogen-depleted perivascular astrocyte end-feet are found applied or dissociated from the capillary basement membrane. The interastrocytary gap junctions appear separated, fused and fragmented. Reactive hypertrophic astrocytes exhibit dense cytoplasmic matrix, increased amounts of dilated smooth and rough endoplasmic reticulum, microtubules, gliofilaments, and vacuolization and fragmentation of Golgi apparatus. Lipofuscin-rich astrocytes and phagocytic astrocytes are also frequently observed, induced by lipid peroxidation, and an overall spectrum of altered endosomal/lysosomal system. Some hypotheses are raised related to the absence of the perisynaptic layer, the neurobiological significance of glycogen-rich and glycogen-depleted astrocytes, the abnormal astrocyte-neuron unit, and the astrocyte responsibility for production of seizures. The results are compared with those described in experimental brain trauma, related neuropathological conditions, ischaemic processes, and in vivo and in vitro experimental conditions, in an attempt to establish a link between basic and clinical neuroscience, and to design new therapeutic strategies.


The logistic support of the Foundation for Academic Development of Zulia University is deeply appreciated. Our thanks to Lic. Orlando Castejón De Pablos and Netty Ojeda for the digital work of electron micrographs. This study has been carried out with the help of a subvention obtained from Castejón Foundation.


Authors report no conflict of interest.


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