Cell proliferation and cytoarchitectural remodeling during spinal cord reconnection in the fresh-water turtle Trachemys dorbignyi

Abstract

In fresh-water turtles, the bridge connecting the proximal and caudal stumps of transected spinal cords consists of regenerating axons running through a glial cellular matrix. To understand the process leading to the generation of the scaffold bridging the lesion, we analyzed the mitotic activity triggered by spinal injury in animals maintained alive for 20–30 days after spinal cord transection. Flow cytometry and bromodeoxyuridine (BrdU)-labeling experiments revealed a significant increment of cycling cells around the lesion epicenter. BrdU-tagged cells maintained a close association with regenerating axons. Most dividing cells expressed the brain lipid-binding protein (BLBP). Cells with BrdU-positive nuclei expressed glial fibrillary acidic protein. As spinal cord regeneration involves dynamic cell rearrangements, we explored the ultra-structure of the bridge and found cells with the aspect of immature oligodendrocytes forming an embryonic-like microenvironment. These cells supported and ensheathed regenerating axons that were recognized by immunocytological and electron-microscopical procedures. Since functional recovery depends on proper impulse transmission, we examined the anatomical axon-glia relationships near the lesion epicenter. Computer-assisted three-dimensional models revealed helical axon-glial junctions in which the intercellular space appeared to be reduced (5–7 nm). Serial-sectioning analysis revealed that fibril-containing processes provided myelinating axon sheaths. Thus, disruption of the ependymal layer elicits mitotic activity predominantly in radial glia expressing BLBP on the lateral aspects of the ependyma. These cycling cells seem to migrate and contribute to the bridge providing the main support and sheaths for regenerating axons.

Endogenous Proliferation after Spinal Cord Injury in Animal Models

“…During embryonic development the transient expression of BLBP parallels neurogenesis, and there is evidence from transgenic mouse studies that nearly all neurons in the mouse brain are derived from BLBP + RG [53]. Additionally, BLBP + cells with RG morphology are localized to the central gelatinosa surrounding the central canal in the adult turtle [72, 75], are implicated in the reconnection of the turtle spinal cord after transection [68, 76] and are directly involved in the regeneration of the axolotl spinal cord after amputation of the tail [77]. In mammalian models of injury, BLBP + cells do not reconnect the spinal cord but undergo mitosis at an increased rate after injury [52].…”

Radial glial progenitors repair the zebrafish spinal cord following transection

“…By the end of embryogenesis most mammalian radial glia differentiate as astrocytes (Rakic, 2003). However in anamniotes radial glia persist widely in the CNS (García-Verdugo et al, 2002; Naujoks-Manteuffel and Roth, 1989; Zupanc and Clint, 2003), and their continued presence has been implicated in the striking ability of these animals to regenerate following injury (Chernoff et al, 2003; Hui et al, 2010; Rehermann et al, 2011). Thus radial glia have been suggested to represent an endogenous neural stem cell population.…”

Non-mammalian model systems for studying neuro-immune interactions after spinal cord injury

“…A few recent studies have also demonstrated axon regeneration after complete spinal cord transection in the turtle (Garcia et al, 2012; Rehermann et al, 2009; Rehermann et al, 2011). After injury, axon regeneration, primarily of propriospinal and sensory neurons, is followed by partial restoration of locomotion in ~50% of spinalized turtles (Rehermann et al, 2009; Rehermann et al, 2011).…”

“…After injury, axon regeneration, primarily of propriospinal and sensory neurons, is followed by partial restoration of locomotion in ~50% of spinalized turtles (Rehermann et al, 2009; Rehermann et al, 2011). In this model, histological assays demonstrated that a blood clot filled with red and white blood cells formed immediately after SCI.…”

“…At the same time, phagocytic macrophages were observed at the lesion (Rehermann et al, 2009). Studies are just beginning to explore cellular aspects of neurogenesis and molecular programs influencing regeneration in this organism (Garcia et al, 2012; Rehermann et al, 2011). For an illustration of turtle axons regenerating across the lesion site, see Figure 2D.…”