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Bioprinting Tissue Engineered Intramembranous Bone Constructs
Darren M. Smith, MD1, Sameer Shakir, BS1, Sanjay Naran, MD1, Phil Campbell, PhD2, Joseph E. Losee, MD1, Greg Cooper, PhD1.
1University of Pittsburgh, Pittsburgh, PA, USA, 2Carnegie Mellon University, Pittsburgh, PA, USA.

PURPOSE
Pediatric craniofacial reconstruction is complicated by a limited supply of bone. Promising regenerative strategies have come in the form of BMP-2-generated endochondral bone. Most calvarial bone, however, is intramembranous in origin. We hypothesize that by leveraging bioprinting technologies to deposit precise doses of proteins known to play a key role in intramembranous ossification, we can favor intramembranous rather than endochondral osteogenesis while using lower (safer) doses of powerful morphogens.
METHODS
Five mm calvarial defects were trephinated in a mature murine model (N=8 per treatment) and filled with a bioprinted acellular dermal matrix (ADM) disc. Each half of each ADM disc was bio-printed with 1 of 4 combinations of varying doses of BMP-2 and TFG-B1. After 4 weeks, animals were sacrificed and the calvarial defects were subjected to radiographic analysis and histology.
RESULTS
The two treatment groups with less than 50 overprints of BMP-2 demonstrated less than 20% osteogenesis on radiographic evaluation. In contrast, both treatment groups with 50 overprints of BMP-2 demonstrated over 75% osteogenesis. Histology showed that the addition of TGF-B1 to BMP-2 yielded a more robust lamellar ossification pattern than that generated in response to BMP-2 alone.
CONCLUSIONS
BMP-2 confers a clear osteogenic advantage at 50 overprints. TGF-B1 seems to affect the quality of the bone generated: bone generated in response to 50 overprints of BMP-2 and 30 overprints of TGF-B1 appeared more compact, lamellar, and indeed more mature at 4 weeks than that generated in response to 50 overprints of BMP-2 and no TGF-B1. These findings may imply that TGF-B1 accelerates or enhances BMP-2-mediated osteogenesis. Specialized histology is underway to determine if the improved osteogenesis noted with TGF-B1 is associated with an intramembranous osteogenic pathway.


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