Beyond the initial axon segment of the spinal motor axon: fasciculated microtubules and polyribosomal clusters

Abstract

Dense undercoating, microtubular fascicles and scattered polyribosomal clusters have until now been considered to be the three structural features of the initial segment, and were thought not to extend beyond the initial segment into the myelinated parts of the axon. The aim of the present study was to make clear whether there is a sudden change in morphology between the unmyelinated and myelinated part. We followed spinal motor axons from the initial segment to the first internode by conventional electron microscopy and serial sectioning, and found that the microtubular fascicles and polyribosomal clusters do exist in the internodal axoplasm. The fasciculated microtubules were observed mainly in the first paranode. The polyribosomal clusters were found along the course of the first internode at a random distance, however, they occurred mainly in the proximal part of the first internode. The proportion of sections in which ribosomes were found, i.e. the incidence of ribosomes, in the first 30-µ m-long portion was 71 ± 24% (mean ± SD, n = 4), and significantly different from that in the second 30-µ m-long portion (3.2 ± 1.3%) (mean ± SD, n = 4) ( P < 0.005). The more distal part of the first internode was not investigated.

In situ hybridization and immunofluorescence on resin‐embedded tissue to identify the components of Nissl substance

“…The data were further consolidated by immunofluorescence, using an antibody to the ribosomal protein L26 that mirrored the signal from that observed in consecutive sections from ISH, thus confirming the presence of ribosomes in the axon hillock. The initial segment of the axon hillock region has several characteristic ultrastructural features as described by Palay et al (1968), including the presence of scattered clusters of ribo/polysomes, also confirmed by others (Li et al, 2005). This was in agreement with our immunofluorescence findings, that is, localizing the L26 antibody to this part of the axon hillock.…”

“…To date, only transmission electron microscopy methods have been used to characterize the contents of the axon hillock and the initial segment (Conradi, 1966;Palay et al, 1968). The major disadvantage of such an approach is the limited nature of tissue examination followed by compilation of micrographs to create full-size composite images of interest (Li et al, 2005). Additionally, single mRNA transcripts are difficult to distinguish by ultrastructure alone because they can be confused with spurious artifacts arising from heavy metal impregnation of the sample.…”

“…As ISH is more powerful for detecting single and poly-mRNA transcripts than the ultrastructural approach (Conradi, 1966;Li et al, 2005;Palay et al, 1968), we employed a combination of ISH and immunofluorescence approach to identify the components of Nissl substance in thick cryostat (NairRoberts et al, 2008) and thin resin sections (Singhrao et al, 2009).…”

Local translation in neuronal compartments: how local is local?

“…Upon sciatic nerve injury, Schwann cells (myelinating glial cells in the peripheral nervous system) deliver ribosomes including polyribosomes via membrane protrusions and multilamellar vesicles to the sciatic nerve axons, after which axonal ribosome numbers increase by orders of magnitude [155]. Consistent with this, an electron microscopy study suggested that spinal cord axons receive ribosomes from central nervous system glia [156]. The modes of delivery are not fully understood but broadly assumed to be from direct connections or exosome transfer [157].…”

Compartmentalization within neurites: Its mechanisms and implications

“…In mammalian neurons, cytoskeletal proteins, such as actin, ankyrin, and spectrin are concentrated at the AIS and play essential roles in molecular integrity and barrier function of the AIS (Rasband, 2010). AIS is also known to have ultrastructural features such as dense undercoating under the plasma membrane, microtubular fascicles, and scattered polyribosomal clusters (Palay et al, 1968) (Li et al, 2005). It remains to be shown whether these molecular and structural components are enriched at the barrier region of Drosophila neurons, and whether they play any role in the barrier function.…”