HIF-1? And VEGF are also involved in osteogenesis, so the influence of these growth factors on the differentiation choices being made by the seeded stem cells is unknown (Polzer 7). The impact of prolonged hypoxic conditions on the seeded cells is another. Although Polzer and colleagues examined the timing of cell seeding relative to prevascularization, they discovered that the artificial scaffold rapidly filled with connective tissue. This process effectively clogged the matrix and prevented efficient seeding.

By comparison, researchers conducting spinal cord injury research into the efficacy of regenerative medicine techniques have been producing promising results (Sykova et al. 1113-1114). Hydrogels seeded with mesenchymal stem cells or bone marrow stem cells have produced positive results in both animal models and in phase I/II clinical trials. In rodents, the lesions were smaller 35 days post surgery and were functionally advanced compared to controls. White matter in the spinal cord was more intact and axons had begun to invade the scaffold from both ends of the transection. While most of seeded cells produced connective tissue, as the scaffold was being degraded by macrophages it was being replaced by additional connective and vascular tissue, astrocytic processes, and NF-160 positive axons. Functionally, patients who undertook the regenerative medicine intervention appeared to suffer no ill effects and may be doing better than they would have without the surgical intervention.

The research conducted by Polzer et al. And Sykova et al. explains why the use of an artificial scaffold remains a controversial regenerative medicine...

There are a number of things that can go wrong, either at the beginning or long-term. Of the five previous windpipe patients treated by Dr. Macchiarini, four are doing fine and a fifth has died (Sifferlin). When the Time article describing Hannah's surgery first appeared, she had begun to partially breathe on her own and respond to the doctors and nurses. These improvements emerged within the relatively short span of a month. As Dr. Alan Trounson, president of the California Institute for Regenerative Medicine, warns, there will be a physiological reaction of some kind to the surgical intervention (Park). Given all the different variables that could have an impact on the outcome of a surgical intervention like Hannah's, predicting the reaction or reactions remains more an exercise in educated guessing than science.
Works Cited

Park, Alice. "Cancer Patient Received a Man-Made Windpipe." Time.com, 12 Jan. 2012, Online. Internet. 1 Jul. 2013. Available http://healthland.time.com/2012/01/13/cancer-patient-receives-a-man-made-windpipe/.

Polzer, Hans et al. "Comparison of Different Strategies for in Vivo Seeding of Prevascularized Scaffolds." Tissue Engineering: Part C, published online May 21 ahead of print. Online.

Sifferlin, Alexandra. "Toddler gets New Windpipe from Her Own Stem Cells." Time.com, 1 May 2013, Online. CNN.com. Internet. 1 Jul. 2013. Available http://www.cnn.com/2013/05/01/health/toddler-stem-cells-windpipe.

Sykova, Eva et al. "Bone Marrow Stem Cells and Polymer Hydrogels -- Two Strategies for Spinal Cord Injury Repair." Cellular and Molecular Neurobiology 26.7-8 (2006): 1113-1129.