The development of successful therapeutic interventions for chronic spinal cord injury necessitates a greater understanding of the pathogenesis of the disease, including the delineation of its histopathological stages. Here, we describe and provide a temporal framework for several histopathological stages of SCI disease progression.
Spinal cord injured rats demonstrated an ascending and progressive dilation of the central canal lumen cranial to the lesion epicenter over time, which was associated with progressive degeneration of the ependymal and peri-ependymal regions. The central canal is an enigmatic structure of the central spinal cord, believed to function as a CSF pathway. Central canal dilation has been noted in autopsy studies of syringomyelic patients [33
] and an ultrastructural analysis of post-traumatic syrinxes revealed ependymal remnants lining portions of the cavity [34
]. Using serial MR imaging, Takamura and colleagues have documented a case of post-traumatic progressive central canal dilation leading to syrinx formation in a young adult [35
], but until now this feature has not been fully appreciated in an experimental model of the disease. In our studies, mild to moderate dilations of the central canal lumen were not associated with gross degeneration of the ependymal and peri-ependymal regions, whereas large aneurysmal dilations of the central canal lumen were consistently associated with a circumferential degeneration of the ependymal and peri-ependymal regions, indicating that gray matter degeneration in chronic SCI is preceded by changes in the ependymal region and/or intraluminal cerebrospinal fluid (CSF) dynamics. This pathogenic sequence is consistent with that seen in development of chronic hydrocephalus and suggests that distensile forces within fluid compartments resulting from obstruction may be common feature of the two diseases. Furthermore, the central canal dilation we observed was ascending and asymmetric from the point of injury suggesting a pathologic process beyond the mere loss of volume of surrounding tissue. In spinal cord injury, this ascending pathology may represent a tertiary form of injury arising from disturbances in CSF flow near the epicenter of injury arising from dural fibrosis. Moreover, the slow and progressive nature of the phenomenon may result from transient spikes in hydrostatic pressure, leading to a water hammer effect, and/or changes in intraluminal oncotic pressures that would precede ischemic changes in the cord. In humans, the cystic and cavitary lesions of syringomyelia are known to progress over time and thus the transmission of distensile pressures along the central canal may represent a mechanism by which these lesions can spread to adjacent, uninjured spinal cord segments. Our rodent data indicate that the progression occurred cranial to the lesion epicenter, whereas in humans progression occurs both cranial and caudal to the lesion epicenter. This difference may be influenced by the effects of gravity on the upright posture of humans.
Spinal cord injured rats demonstrated progressive ependymal cell ciliary loss cranial to the lesion epicenter, and a loss of ependymal cell cilia with large dilations of the central canal lumen. Cilia are specialized projections of ependymal cells that promote the flow of CSF within the central canal. Ependymal ciliary loss is known to predispose to hydrocephalus[36
] and is a feature of the disease [37
]. Therefore, this loss of ependymal cell cilia in the spinal cord may result in altered local CSF homeostasis, resulting in an accumulation of toxic metabolites and oncotic pressures, which could result in damage to the ependymal and periependymal regions. Notably, the ependymal cell layer disruption noted in our study is reminiscent of the ependymal denudation that proceeds the development of severe hydrocephalus in the hyh mouse [38
At late time points, large aneurysmal dilations of the central canal lumen were accompanied by denudation of the ependymal cell layer. Normally, ependymal cells form a pseudostratified monolayer of epithelium that regulates fluid and electrolyte balance between the CSF and neuropil [39
]. Disruption of the ependymal layer could therefore result in the loss of a protective epithelium. This loss of integrity and competence of the canal could result in exposure of the adjacent grey and white matter to hydrostatic and oncotic pressure gradients, leading to a dissection of stagnant CSF from within the canal into the gray and white matter of the cord, leading to structural and conductive deficits. Indeed, fluid from syringomyelia cysts is known to differ from normal CSF [40
], usually having a higher protein content [41
] and prolonged exposure of peri-ependymal tissues to this microenvironment may lead to its degeneration. In our studies, areas of ependymal denudation were consistently opposed to regions of peri-ependymal edema, gliosis, macrophage infiltration and loss of neuropil. Furthermore, local disruption of the blood brain barrier may contribute to the edema [40
] and subsequent ischemic injury, but may also represent a source of inflammatory molecules and plasma proteins that may adversely influence the microenvironment of nearby cells, including cells with stem/progenitor characteristics, thereby influencing their viability and patterns of differentiation.
It is intriguing to note that large, aneurysmal dilations of the central canal lumen were also associated with decreased proliferation of ependymal region cells. Cells of the ependymal region are vestiges of neuroepithelial cells that give rise to neurons and glia during mammalian development [42
] and are known to orchestrate the regenerative response in tailed amphibians [43
]. Ependymal region cells have been shown to proliferate [42
] and migrate [47
] following spinal cord injury. This finding has led some authors to speculate on their role in endogenous repair in humans [50
]. Indeed, the kinetics of ependymal region cell proliferation and differentiation have been correlated with the recovery of lower limb motor function in rats following contusion injuries [46
]. Of note, neural stem cells have been isolated from the CNS [47
], including regions near the central canal [53
]. Unlike the subventricular zone, the prototypical stem cell niche of the CNS (for a review, see [54
]), multipotent cells of the ependymal region appear restricted to glial lineages [46
]. Gliogenesis near the central canal includes the generation of ependymal cells [44
], reactive astrocytes [46
], oligodendrocyte precursors [55
] and microglia [56
]. Glia are supportive cells of the CNS and are critical for maintaining the structural and functional integrity of the spinal cord after injury [57
]. Even reactive astrocytes, long thought to be inhibitory to axonal regeneration, appear to play a role in repair of SCI lesions [46
]. Therefore, it stands to reason that disruption of the ependymal stromal epithelium, along with periependymal stem/progenitor cells, may represent a heretofore unrecognized pathogenic mechanism in spinal cord injury, which would hinder gliogenesis in the ependymal region and subsequently wound repair in the spinal cord. Indeed, the progressive disruption of this cell layer, through mechanical and cytotoxic means, could represent a disease mechanism that tips the balance between injury and repair in the spinal cord toward further cytoarchitectural destruction of lesions over time. In principle, this conceptualized disease process should be investigated in other multipotent niches as a basis for understanding related degenerative disorders and sequelae.