A Polymer Foam Conduit Seeded with Schwann Cells Promotes Guided Peripheral Nerve Regeneration
TESSA HADLOCK, M.D.,1,2 CATHRYN SUNDBACK, Sc.D.,2 DANIEL HUNTER, R.T.,3
MACK CHENEY, M.D.,1 and JOSEPH P. VACANTI, M.D.
Introduction
Synthetic conduits may be effective in regenerating peripheral nerve tissue by connecting the two damaged ends and allow for Schwann cell migration and proliferation. In this study, a fully biodegradable hollow conduit was fabricate for PLGA. PLGA was chosen because it is well-tolerated in vivo and its degradation rate can be controlled by varying the ratio of the two monomers. Schwann cells were seeded onto the conduits and implanted into mice, and the degree of axon regeneration was evaluated.
Materials and Methods
Conduits were fabricated by injecting an 85:15 PLGA solution into a steel mold and cooling down to freezing temperature. The sample was then freeze dried. Steel wires were inserted into the mold at various configurations to create different patterns of channels inside the conduit. The conduits were characterized with SEM, differential scanning calorimetry, and NMR.
Conduits were pre-wet with ethanl solution and coated with laminin. Schwann cells from Fisher rats were seeded. Cell adherence was assessed with MTT assay.
Conduits were implanted into rats with a portion of left sciatic nerve removed. Another group of animals was subject to autografts, which served as the control. Animals were sacrificed after 6 weeks and their nerve tissue was analyzed using histology.
Figure 1 Cross section patterns of various conduits
Results
Figure 2 SEM of cross section of 45-channel conduit at 130x and 400x
Figure 3 Histology section of toluidine blue stained neural regenerate A) One lumen of polymer conduit
B) Autograft
Table 1 Comparison of fiber width and percent neural tissue in PLGA conduit and autograft
Discussion
SEM showed the expected network of pores. The presence of Schwann cells aligning the channels
was verified using toluidine blue staining of the longitudinal sections of the conduits. For in vivo regeneration all conduits were intact upon harvest of the tissues, with no defects along the length of the conduit. Neural regenerate was present in each of the channels in all the experimental animals. All autografted animals demonstrated regeneration consistent with literature levels for similar gap lengths and survival times. At 6 weeks, the percentage of neural tissue per cross sectional area was similar for conduit and autograft. The mean axon diameter was much greater for the conduit than the autograft.
Critique
This paper presents a novel method for conduit design that facilitates neural regeneration. It provides a good summary of the motivations behind the study and justification for selecting PLGA as the fabrication material. The information provided in the materials and methods section is also complete and can be replicated.
The results and discussion section, however, was very brief and lacked adequate information. The images, especially the histological sections, were not labeled to show the location of the Schwann cells and regenerated nerve tissue. Also, only one image from each group of animals was shown, and it would have been more reliable if images from several different mice had been shown to confirm the results. The method for quantification of the percent nerve regeneration was not elucidated.
6 comments:
The idea is great, but the lack of pictures and results to discuss the wonderful implications of creating nerve guiding channels with novel geometry is discouraging.
I am particularly interested in how the authors used steel wires to create the pores. Specifically I wonder how they removed them to leave the pores behind and how many of those pores were then able to foster neural regeneration versus how many collapsed. Did the authors dissolve the steel or just pull it out somehow? Furthermore I wonder what size or arrangement of steel wire worked best and whether this pattern depended on location within the body. It seems to me larger pores would be required for regions that require the regeneration of larger diameter nerves.
Overall fascinating idea, but lacking in the display of results that would have really augmented the author's case.
Never regeneration using polymer foam conduit is a great example of in vivo tissue engineering that rely on the host's ability to regenerate the tissue with the help of biomaterial. But as mentioned by Ash and Angela, the picture showed in the result section are not sufficient to provide clear and convictive results. About the method section, I am curious why they want to use different patterns for the cross section of conduits. Did the author write anything about the different pattern and their application for different kind of nerve? Also, for the polymer foam, it said in the article that "its degradation rate can be controlled by varying the ratio of the two monomers." I am curious about if the degradation rate of the polymer foam (and hence the ratio of the two monomers) are patient specific and location specific. If it is, how do they determine how fast should the polymer degrade?
Ash - that's a good question about how they removed the steel wires. I don't think they can dissolve them because at such high temperatures, the polymer would probably become amorphous and change properties.
Zoey - The different patterns and diameters are probably for different nerves in the body. Ideally, the polymer should degrade the same rate as the nerve regenerates so that the new nerve tissue replaces the polymer.
Great summary. I think the paper should also mention the need for nerve conduits (ie: current gold standard is using autografts which takes nerves from other parts of the patient).
I am impressed the degradation rate of the PLGA can be controlled, which would probably allow for specificity of injure site -- some sites may require longer time to heal than others.
Schwann cells, as mentioned are critical to the conduit since they aid in the process of Wallergian Degeneration for nerve regeneration, but there are also several other functional cells that have been shown in literature to aid in nerve regeneration that can be looked into for future works.
Like everyone mentioned earlier, the idea of the paper is a hot topic (nerve regeneration), but the result provided is not really clear in showing the where and what is the Schwann cell in the SEM image. Also, nerve generation normally should take a long time, so would PLGA last long enough to actually allow nerve cell regeneration. Also, different nerves may have different regeneration time; does that mean they have to come up with a different PGA: PLA ratio conduit for each different case?
Really interesting concept! Like everyone said, I agree that there needs to be more pictures from a greater number of samples to validate the results. I am also curious as to how they determined to use 85:15 ratio used in this paper. Were studies performed to find an optimal ratio to be used?
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