Zebrafish spinal cord repair is accompanied by transient tissue stiffening

Abstract

1 Severe injury to the mammalian spinal cord results in permanent loss of function due 2 to the formation of a glial-fibrotic scar. Both the chemical composition and the 3 mechanical properties of the scar tissue have been implicated to inhibit neuronal 4 regrowth and functional recovery. By contrast, adult zebrafish are able to repair 5 spinal cord tissue and restore motor function after complete spinal cord transection 6 owing to a complex cellular response that includes neurogenesis and axon regrowth.7 The mechanical mechanisms contributing to successful spinal cord repair in adult 8 zebrafish are, however, currently unknown. Here, we employ AFM-enabled nano-9 indentation to determine the spatial distributions of apparent elastic moduli of living 10 spinal cord tissue sections obtained from uninjured zebrafish and at distinct time 11 points after complete spinal cord transection. In uninjured specimens, spinal gray 12 matter regions were stiffer than white matter regions. During regeneration after 13 transection, the spinal cord tissues displayed a significant increase of the respective 14 apparent elastic moduli that transiently obliterated the mechanical difference 15 between the two types of matter, before returning to baseline values after completion 16 of repair. Tissue stiffness correlated variably with cell number density, 17 oligodendrocyte interconnectivity, axonal orientation, and vascularization. The 18 presented work constitutes the first quantitative mapping of the spatio-temporal 19 changes of spinal cord tissue stiffness in regenerating adult zebrafish and provides 20 the tissue mechanical basis for future studies into the role of mechanosensing in 21 spinal cord repair. 22 42 recovery in adult zebrafish within 6-8 weeks post-injury [8]. 43Morphological changes, proliferation, migration and differentiation also constitute 44 responses that mechanosensitive neurons and glia exhibit when exposed to distinct 45 mechanical environments [14][15][16][17]. In vitro studies of neural cells reported an 46 increased branching of neurons on compliant, but directed axonal growth on stiff 47 substrates [14, 18]. Astrocytes and microglia display morphological characteristics of 48 an activated phenotype and upregulate inflammatory genes and proteins when 49 exposed to a mechanical stimulus that deviates from their physiological mechanical 50 4 environment both in vitro and in vivo [17]. Oligodendrocyte precursor cells increase 51 their expression of myelin basic protein and display an elaborated myelin membrane 52 on stiffer substrates as compared to more compliant substrates indicating a preferred 53 mechanical environment for differentiation [15]. 54 In vivo, this mechanical environment is formed by the surrounding nervous tissue 55 whose mechanical properties are determined by factors such as the combined 56 material properties of neighboring cells, cell density, myelin content, collagen 57 content, extra cellular matrix composition and cell interconnectivity [19, 20]. As these 58 may change during develop...

Microscopic multifrequency MR elastography for mapping viscoelasticity in zebrafish

“…55,56 However, because this method is based on the deflecting responses off transparent beads, the invasive injection of foreign material is necessary, which might also change the elastic properties of the tissue. Another powerful method is scanning atomic force microscopy, 57 which, however, needs thin tissue slices and careful surface preparation, hindering its direct comparison with bulk parameters measured by MRE. Additionally, many methods exist to measure specific mechanical properties of individual parts of the fish, such as caudal fins, 58 heart sections, 8 or neuromasts, 59 but these methods cannot easily be applied to heterogeneous tissues within the fish body as investigated in our study.…”

Pressure Drives Rapid Burst‐Like Coordinated Cellular Motion from 3D Cancer Aggregates

“…[ 1,2 ] While migrating independently or within solid tissues, cells constantly experience shear forces, compression, tension, and hydrostatic as well as osmotic pressures. [ 3–7 ] Because mechanical homeostasis ensures a complete force balance in tissues, single cells are not often observed to move on their own. However, this balance is broken when a migrating cell or a group of migrating cells need to generate well‐orchestrated forces.…”

Softening of the chronic hemi-section spinal cord injury scar parallels dysregulation of cellular and extracellular matrix content

“…Naïve white and grey matter have unique material properties and can be analyzed separately. It has previously been reported in fish and mammalian models that white matter has a lower Young's modulus than grey matter in bulk [23,[67][68][69]. Most agree that the variance is due in part to differences in morphometry and composition of white and grey matter.…”

“…We expect such decreased Young's modulus could impede neurite extension at chronic time points and hinders CNS ability for regeneration, and that the underlying cause of decreased Young's modulus should be identified. Indeed, a recent zebra fish study supports this hypothesis revealing that transient stiffening of the SCI does not impede neurite extension or prevent eventual complete regeneration [67].…”