dorsal skinfold chambers of mice
Alexander Schramma, Rolf Mu¨ lhauptc, Nils-Claudius Gellricha, Michael D. Mengerb
Biomaterials 27 (2006) 5027–5038
One of the most prominent issues yet to be resolved in tissue engineering is the move from 2D to 3D tissues. The major limiting factor, of course, is the limits of diffusion and the necessity of a vascular network to bring nutrients and remove wastes. When an engineering device is implanted, often a scaffold is involved. The scaffold needs certain mechanical properties to support the device, but it is also important that the scaffold encourages, or at the very least does not hinder, necessary biological processes such as angiogenesis. I chose this paper because of its rather direct approach to investigating this problem.
This study attacks this issue by testing some common tissue engineering scaffold materials and their effects on neovascularization/angiogenesis in vivo. They compared three types of implants --poly(L-lactide-co-glycolide) (PLGA), collagen–chitosan–hydroxyapatite hydrogel, and isogeneic calvarial bone blocks—as well as a sham control. The scaffolds were examined in vivo using a dorsal skin chamber in balb/c mice and intravital fluorescence microscopy that stained blood plasma and white blood cells. Computer technology assisted the researchers in making volumetric blood flow and cell counting analysis. At the end of the experiment, histology and immunohistochemistry experiments were used to study the scaffolds ex vivo.
The quick and dirty results showed that the PLGA scaffold was similar to the isogeneic bone implant immunological response and neovascularization. Although, the isogeneic implant has a larger and more complex vascular networks which may be due inherent growth factors in bone matrix. The hydragel, meanwhile, showed little vascularization and high levels of apoptotic cells near the border of the implant. The hydragel also incited a high inflammatory response. End of story—the researchers believe that the hydragel may be toxic.
One quality issue stressed by these researchers was the effect of pore size of the scaffold. Different pore sizes can effect angiogenesis (see paper for references) and the researchers were concerned that normal scaffold preparation techniques resulted in a large distribution of pore sizes. In this study the researchers used a special technique (rapid prototyping/3D printing) to make sure the scaffolds had a homogenous pore size.
6 comments:
The hydrogel is being used as the scaffold for tissue engineering in a lot of research. When you said that researchers believe the hydrogel may be toxic...is it just the collagen-chitosan-hydroxyapatite hydrogel specifically?
Nothing in the experimental design technically tested toxicity. There was a high level of apoptotic cells near the border of the implant. The layer of muscle under the hydrogel implant had a lower density and showed signs of damage. This is the evidence they used to suggest it may be toxic. It is not proof. They definitely can't say anything about other types of hydrogels.
The PLGA scaffold doesn't have the growth factors that contributed to the more complex vascular networks in the isogeneic implant. So what do you think made its response so similar to that of the isogeneic implant? Was it a combination of pore size and/or some chemical/material factor that makes PLGA a better material for promoting vascularization?
The isogeneic implant, I'm assuming, was used because of the reduced immune response. did the group mention the level of inflammation of the hydrogel in comparison to an allogeneic implant? Perhaps done in a previous study?
You mention material and pore size as major factors in the success of a scaffold type. What about the mechanical properties of each scaffold?
While toxicity of the hydrogel is plausible, there could be other factors that could have contributed to the high levels of inflammatory responses in the hydrogels. One possibility is the breaking off of submicron particles during degradation, which would cause frustrated macrophage response and lead to a chronic inflammatory response. Was there any evidence of degradation in the hydrogel?
I also am wondering about the statement "researchers used a special technique (rapid prototyping/3D printing) to make sure the scaffolds had a homogenous pore size." Does this mean that all scaffolds had the same pore sizes or that each pore size was the same within each scaffold?
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