Three dimensional electron microscopy reveals changing axonal and myelin morphology along normal and partially injured optic nerves

Abstract

Following injury to the central nervous system, axons and myelin distinct from the initial injury site undergo changes associated with compromised function. Quantifying such changes is important to understanding the pathophysiology of neurotrauma; however, most studies to date used 2 dimensional (D) electron microscopy to analyse single sections, thereby failing to capture changes along individual axons. We used serial block face scanning electron microscopy (SBF SEM) to undertake 3D reconstruction of axons and myelin, analysing optic nerves from normal uninjured female rats and following partial optic nerve transection. Measures of axon and myelin dimensions were generated by examining 2D images at 5 µm intervals along the 100 µm segments. In both normal and injured animals, changes in axonal diameter, myelin thickness, fiber diameter, G-ratio and percentage myelin decompaction were apparent along the lengths of axons to varying degrees. The range of values for axon diameter along individual reconstructed axons in 3D was similar to the range from 2D datasets, encompassing reported variation in axonal diameter attributed to retinal ganglion cell diversity. 3D electron microscopy analyses have provided the means to demonstrate substantial variability in ultrastructure along the length of individual axons and to improve understanding of the pathophysiology of neurotrauma.

Anatomy of the Human Optic Nerve: Structure and Function

“…Axon diameter along with myelin thickness [154], internode [155] and paranode gap [156] determines the functional properties of the nerves [157]. Axon diameter has been used to determine conduction velocity along various pathways.…”

Lycium barbarum (Wolfberry) Increases Retinal Ganglion Cell Survival and Affects both Microglia/Macrophage Polarization and Autophagy after Rat Partial Optic Nerve Transection

“…Many studies have used this model [4][5][6][7][8][9][10][11][12][13] . In these previous studies, PONT in rats was administrated from the superior side of optic nerves; however, in this study we cut the optic nerves from the temporal side and found that this operation was much easier to conduct than from the superior side due to the lack of obstruction from rectus superior and obliquus superior eye muscles ( Fig.…”

Radial contractility of Actomyosin-II rings facilitates cargo trafficking and maintains axonal structural stability following cargo-induced transient axonal expansion

“…The diameter of the long and thin axon has long been believed to be uniform for the same type of neurons. However, with the development of 3D EM reconstruction, diameter fluctuations have been detected along the length of axons in optical nerves (Giacci et al, 2018). Similarly, in live rat brains, axonal diameter fluctuations were revealed with super-resolution microscopy after the conduction of action potentials (Chereau et al, 2017).…”

“…Indeed, the expansion of axonal diameter surrounding large cargoes, i.e. autophagosomes 8 or mitochondria 9 , has been observed by super-resolution microscopy and 2D-electron microscopy (EM) in both normal and degenerating axons 10, 11 . Considering the spatial limitation exerted by the rigid axon membrane 12 , the trafficking of large cargoes is likely to be affected.…”

“…Indeed, the expansion of axonal diameter surrounding large cargoes, i.e. autophagosomes (Wang et al, 2015) or mitochondria (Yin et al, 2016), has been observed by super-resolution microscopy and 2D-electron microscopy (EM) in both normal and degenerating axons (Giacci et al, 2018; Maia et al, 2015). Considering the spatial constriction exerted by the rigid and stable circumventing actin MPS (membrane-associated periodic cytoskeletal structures) (Abouelezz et al, 2019a; Qu et al, 2017; Zhang et al, 2017; Zhong et al, 2014), the trafficking of large cargoes is likely to be affected.…”