Name: toluidine blue staining of resin-embedded sections for evaluation of peripheral nerve morphology
Uploaded: 2018-07-03T15:00:00
Description: Watch this Scientific Journal Video about Toluidine Blue Staining of Resin-Embedded Sections for Evaluation of Peripheral Nerve Morphology at JoVE.com

The authors would like to thankthe Jenkins Microscopy Facility at the University of Wyoming for their help, and the members of the Bushman lab, Kelly Roballo, Hayden True, Wupu Osimanjiang and Subash Dhunghana, for assistance in animal care. This publication was made possible by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under Grant # 2P20GM103432.

Adult Sprague Dawley rats were used in this project and all procedures were approved by the University of Wyoming Institutional Animal Care and Use Committee.
1. Surgery and In Vivo Nerve Fixation
NOTE: Vendor information for all materials and equipment used in this protocol are listed in the Table of Materials.
NOTE: In vivo nerve fixation is used to preserve the tissue and reduce structural degradation t...

<ol>
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Examinations of the morphological structures of peripheral nerve injury and regeneration are frequent subjects of study13. In this protocol, we describe the steps to obtain high quality images for histological data quantifications using rat sciatic nerve tissue embedded in resin blocks and stained with toluidine blue. This technique provides an image of nerve morphology in which the nerve regeneration can be quantified by measuring the number of axons, degree of myelination, presence of infiltrati...

Peripheral nerves extend throughout the body, innervating target tissues with motor or sensory axons1. Peripheral nerve defects caused by medical disorders and trauma represent a major public health concern and have large economic impacts2,3. Despite the advances in assessing the outcomes of peripheral nerve injuries and understanding nerve regeneration, traditional methods such as nerve histology and staining techniques are essential tools to qualitatively and quantitatively assess nerve health as a terminal outcome measurement in animal models or excised human tissue. This is often paired with electrophysiological measurements of peripheral nerve function, where morphometry can reveal why functional nerve regeneration did or did not occur.
Toluidine blue staining of resin embedded semi-thin peripheral nerve sections is a specialized method for imaging myelinated nerve fibers, providing high quality and clear detailed images of nerve structures4,5,6. Toluidine blue is an acidophilic metachromatic stain, discovered by William Henry Perkin in 18567, and has been used in several medical applications8. Toluidine blue-stained peripheral nerve sections obtained from resin-embedded nerve segments allows for clear visualization of nerve structures. Visualization of myelin sheath structure can be enhanced by the use of osmium tetroxide post-fixation4,9. Osmium tetroxide is a toxic oxidant and lipid fixative agent that interacts with the double bonds in lipids, resulting in starkly defined lipid-rich myelin sheaths10. However, osmium tetroxide is toxic, expensive, requires a longer incubation of nerve segments, and is not always used.
Alternative methods of processing and staining have been developed for visualization of peripheral nerve morphology; Paraffin, cryogenic sectioning, and epoxy resin-embedded nerve sectioning followed by staining with toluidine blue or phenylenediamine solution has been used to quantify morphological changes of peripheral nerve regeneration 11,12. These methods each have their advantages and yield essential data on the number of axons, myelin thickness, axon diameter, and axon diameter to myelinated fiber diameter (g-ratio) 11,13,14,15.
The primary distinction of the resin-embedding in this protocol is that it facilitates obtaining 1-2 &#956;m thickness cross-sections due to the hardness of the resin while maintaining the histological qualities of the nerve. These thin sections, as opposed to the 4-5 &#956;m thickness sections obtained from paraffin embedding, provide peripheral nerve sections with higher resolution, allowing for a more accurate quantification of axon myelination, such as the g-ratio, that cannot be obtained from thicker sections16. While cryogenic sectioning can be used to obtain 1-2 &#181;m sections, it has been our experience that it is more difficult to obtain sections without numerous large cracks. Such cracked sections can cause inaccurate counting of the number of axons and aspects of myelination.
In addition to toluidine blue staining17, a silver staining method18 and Masson&#39;s trichrome staining4 can also be used to show nerve axons. However, using resin embedding of rat median nerve sections stained with either hematoxylin and eosin or Masson&#39;s trichrome showed faint myelin sheaths and unrecognized structures, whereas toluidine blue staining showed clear myelin sheath image and easily can be quantified4. Despite some limitations, toluidine blue staining of resin embedded peripheral nerves is a valuable technique that can be used when high resolution images of nerve morphology are required.
The primary disadvantage for resin embedding is that it is time-consuming and does not allow for immunostaining of the same tissue due to the difficulty of antigen retrieval when compared to paraffin and frozen embedded sections techniques. Thus, it is not generally possible to utilize the same tissue for immunostaining that is processed via resin-embedding for toluidine blue staining. Although not used here, if immunohistochemistry is desired in resin embedded sections, the use of glycol methacrylate embedding resins allows for immunohistochemistry to be performed on tissue sections, but it is relatively expensive19. This can be somewhat mitigated by cutting the peripheral nerve into separate segments, some for resin-embedding and others for immunostaining directly after fixation.
The process of toluidine blue staining of resin embedded peripheral nerves, as with most histopathological analysis, can be broken up into five stages, including fixation, dehydration, embedding, sectioning, and staining20. We aim here to provide a protocol and practical guideline for using resin embedded rat sciatic nerve sections stained with toluidine blue to acquire high quality images.

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toluidine blue staining of resin-embedded sections for evaluation of peripheral nerve morphology

Peripheral nerves extend throughout the body, innervating target tissues with motor or sensory axons. Due to widespread distribution, peripheral nerves are frequently damaged because of trauma or disease. As methods and strategies have been developed to assess peripheral nerve injury in animal models, function and regeneration, analyzing the morphometry of the peripheral nerve has become an essential terminal outcome measurement. Toluidine blue staining of nerve cross sections obtained from resin embedded nerve sections is a reproducible method for qualitative and quantitative assessments of peripheral nerves, enabling visualization of morphology number of axons and degree of myelination. This technique, as with many other histological methods, can be difficult to learn and master using standard written protocols. The intent of this publication is therefore to accentuate written protocols for toluidine blue staining of peripheral nerves with videography of the method, using sciatic nerves harvested from rats. In this protocol, we describe in vivo peripheral nerve fixation and collection of the tissue, and post-fixation with 2% osmium tetroxide, embedding of nerves in epoxy resin, and ultramicrotome sectioning of nerves to 1-2&#956;m thickness. Nerve sections then transferred to a glass slide and stained with toluidine blue, after which they are quantitatively and qualitatively assessed. Examples of the most common problems are shown, as well as steps for mitigating these issues.

Resin embedded peripheral nerve sections stained with toluidine blue allow for accurate histological data quantifications. An overview of the procedure is shown in (Figure 2). Sciatic nerves sections embedded in resin medium and stained with toluidine blue showed clear images with optimal resolution (Figures 3). Nerve damage can cause many changes in nerve morphological structures, for example, changes in nerve fiber, axon diameter, and myeli...

Watch this Scientific Journal Video about Toluidine Blue Staining of Resin-Embedded Sections for Evaluation of Peripheral Nerve Morphology at JoVE.com

Toluidine Blue Staining of Resin-Embedded Sections for Evaluation of Peripheral Nerve Morphology

Here we present a protocol to visualize fine structures of peripheral nerves by obtaining and staining 1-2 &#181;m sections with toluidine blue

Peripheral nerves extend throughout the body, innervating target tissues with motor or sensory axons. Due to widespread distribution, peripheral nerves ...

This method could help answer key questions in the fields of peripheral nerve injury and regeneration such as what is the morphometry of a peripheral nerve, how many axons are present, what is the state of myelination, and is there fibrotic tissue present? The main advantage of this technique is that it allows for high-resolution visualization of nerve features. Begin by placing an anesthetized rat in the prone position on the top of a dissection mat.Place a nose cone to maintain anesthesia with 1-2%isoflurane. To ensure adequate depth of anesthesia test the animals for pedal withdrawal and palpebral reflexes of the hind feet. Once an appropriate depth of anesthesia is achieved, shave the hair of the hind limbs and sterilize the shaved areas with 70%ethanol.Palpate the femur to identify the proper location for the incision. The femur is located just proximal to the most accessible segment of the sciatic nerve. After making a two to three centimeter incision in the skin along the hind limb from the knee up to the greater trochanter locate the plane between the biceps femoris muscle and the gluteus maximus muscle.Then use microdissection scissors to separate the underlying fascia and expose two to three centimeters of the underlying sciatic nerve. Use a retractor to widen the gap between the two muscles. With fine forceps and iris scissors carefully separate the nerve from the surrounding connective tissue, taking great care not to compress or cut the nerve.Next, cover the exposed nerve with Trump's Fixative and let sit for 10 minutes. If necessary, place gauze under the hind leg to improve the angle so that more fixative can be added into the cavity. Remove the fixative after 10 minutes and repeat this step two more times.Under the dissecting microscope cut the sciatic nerve from both sides using fine dissection scissors, making sure not to stretch or pinch the nerve. Immediately put the nerve sections in 15 mL tubes containing Trump's Fixative and fix at four degrees Celsius for one week, changing the fixative every 48 hours. Post-fixation, carefully remove any remaining fat and connective tissues from the nerve, and use a sharp scalpel to cut the nerve into segments approximately five millimeters in length.Nerve segments can be separated into different labeled tubes. Immerse nerve segments in freshly prepared 2%osmium tetroxide for two hours. Using an epoxy embedding kit, prepare the final embedding mixture.Mix the epoxy embedding medium with DDSA solution and mix the epoxy embedding medium with NMA solution. Mix both solutions for at least 20 minutes with a magnetic stirrer. Immediately before use, combine the accelerator DPM to solution B and then combine this with solution A such that the accelerator's final proportion is between one point five and 2%of the total volume.Transfer the nerve segments to one point five milliliter tubes and after washing with PBS, start the dehydration process by the replacing the PBS with increasing concentrations of acetone and distilled water for 10 minutes each then in 100%acetone three times for 10 minutes each. One critical step is the tissue dehydration. Water and resin will not interact, so any water remaining in the tissue will not be infiltrated by the resin leaving holes in the section devoid of histological qualities.For resin infiltration, first place the nerve segments in a one-to-one mix of embedding mixture and 100%acetone for 30 minutes. After 30 minutes, transfer the segments to a two-to-one mix of embedding mixture and 100%acetone for 30 minutes. Place the nerve segements in silicone rubber embedding molds and gently add the resin on top of nerves, making sure to cover the whole nerve segment while avoiding air bubbles.Leave the resin to polymerize at 60 degrees Celsius overnight. Place a resin block with embedded nerve into the ultramicrotome holder with the trapezoidal side facing up. While viewing through the ultramicrotome scope, use a single-edged blade to trim excess resin surrounding the nerve tissue.Do not penetrate the longitudinal surface of the nerve segment which is recognizable by its dark staining by osmium tetroxide. With a plain glass knife on the ultramicrotome make multiple cross sections to expose a uniform cross-section surface of the nerve. Once this is done, switch to a glass knife with the boat filled with distilled water at room temperature and adjust the ultramicrotome to one to two microns to cut thin sections.Use a metal loop to transfer floating thin sections from the glass knife boat to a drop of deionized water on a glass slide. Another critical step is during the transfer of thin section from the glass knife to slide. These thin sections are very fragile and can break or fold.After sectioning, dry the sections on the slides by passing a slide over a flame several times, making sure not to overheat the sections. Slides can be stored at room temperature for several days if not immediately staining with toluidine blue. To stain the nerve sections, use a plastic pipette or micropipetter to add a drop of toluidine blue solution on the top of the nerve sections.After 20 to 30 seconds, rinse off all toluidine blue solution excess by gently dipping the slides into a jar of deionized water, repeating three to four times until sections are clear. Dry the slides for at least 15 minutes at 60 degrees Celsius or overnight at room temperature. After drying, add regular mounting medium and cover the sections with a coverslip.Finally, examine the mounted sections under a light microscope. A 100X oil immersion lens is recommended for detailed images to calculate g-ratios. Sciatic nerve sections embedded in resin medium and stained with toluidine blue showed clear images with optimal resolution at increasing magnification from 10X to 20X, 40X, and 60X.This method preserves nerve structure and allows for high-resolution, high-magnification imaging which facilitates the measurement of several parameters such as the g-ratio, which is the ratio of axon diameter, indicated by the arrow labeled B, to the total fiber diameter, labeled A.A variety of factors can lead to less than optimal peripheral nerve sections, including the presence of cracks in the section due to improper handling of the nerve. Holes can also occur in the section due to insufficient dehydration. Another possible error in the procedure is folding of the sections, which can be remedied by using inoculating loops for section transfer onto the glass slide.Following this procedure other methods like axon quantification can be performed to answer additional questions like what's the degree of regeneration in a regenerative nerve? After watching this video you should have a good understanding of how to obtain one to two micron peripheral nerve sections to assess nerve morphometry.

Research

JoVE Journal

Neuroscience

Organotypic Slice Culture of GFP-expressing Mouse Embryos for Real-time Imaging of Peripheral Nerve Outgrowth

For many purposes, the cultivation of mouse embryos ex vivo as organotypic slices is desirable. For example, we employ a transgenic mouse line (tauGFP) in ...

Immunohistochemical Analysis in the Rat Central Nervous System and Peripheral Lymph Node Tissue Sections

Immunohistochemistry (IHC) provides highly specific, reliable and attractive protein visualization. Correct performance and interpretation of an IHC-based ...

Transplantation of Olfactory Ensheathing Cells to Evaluate Functional Recovery after Peripheral Nerve Injury

Olfactory ensheathing cells (OECs) are neural crest cells which allow growth and regrowth of the primary olfactory neurons. Indeed, the primary olfactory ...

Visualizing the Effects of a Positive Early Experience, Tactile Stimulation, on Dendritic Morphology and Synaptic Connectivity with Golgi-Cox Staining

To generate longer-term changes in behavior, experiences must be producing stable changes in neuronal morphology and synaptic connectivity. Tactile ...

The Indirect Neuron-astrocyte Coculture Assay: An In Vitro Set-up for the Detailed Investigation of Neuron-glia Interactions

The Indirect Neuron-astrocyte Coculture Assay: An In Vitro Set-up for the Detailed Investigation of Neuron-glia Interactions

Proper neuronal development and function is the prerequisite of the developing and the adult brain. However, the mechanisms underlying the highly ...

Analyzing Dendritic Morphology in Columns and Layers

In many regions of the central nervous systems, such as the fly optic lobes and the vertebrate cortex, synaptic circuits are organized in layers and ...

Targeting Alpha Synuclein Aggregates in Cutaneous Peripheral Nerve Fibers by Free-floating Immunofluorescence Assay

To date, for most neurodegenerative diseases only a post-mortem histopathological definitive diagnosis is available. For Parkinson&#39;s disease (PD), the ...

Preparation of Mouse Retinal Cryo-sections for Immunohistochemistry

Preparation of high-quality mouse eye sections for immunohistochemistry (IHC) is critical for assessing the retinal structure and function and for ...

3D Kinematic Analysis for the Functional Evaluation in the Rat Model of Sciatic Nerve Crush Injury

Compared to the Sciatic Functional Index (SFI), kinematic analysis is a more reliable and sensitive method for performing functional evaluations of ...

Preparation of Peripheral Nerve Stimulation Electrodes for Chronic Implantation in Rats

Peripheral nerve cuff electrodes have long been used in the neurosciences and related fields for stimulation of, for example, vagus or sciatic nerves. ...