Biomedical Engineering Reference
In-Depth Information
demonstrated a striking difference, with more than double the extent of bone formation com-
pared to control without extended MSCs. With extensive bone formation between the host
bone and implant, there was complete bridging of the gap, including periosteal callus develop-
ment. Furthermore, complete vascularization of the new tissue demonstrated the abundant
neovascularization that paralleled bone regeneration.
Confident that rat MSC-HA/TCP implants were effective in regenerating bone in this
critically-sized model, subsequent studies were performed focusing on the use of human MSCs
in an equivalent model. 36 To investigate human cells, Harlan Nude (Hsd:Rh-rnu) rats were
subjected to the femoral gap described above. Osteogenesis was apparent at 4 weeks following
implantation in the human MSC-loaded implants, but was absent in the cell-free implants. At
8 weeks, the MSC-loaded implants were well integrated with the host as evidenced in the
radiodense bridging between the host and the MSC-implants. The bridging increased in
radioopacity by 12 weeks and was concurrent with callus formation spanning the defect.
Similar studies have been performed in large animal models, to mimic human situations.
Bone marrow samples were drawn from the iliac crest of large mongrel hounds; MSCs were
then isolated and expanded from each donor. 37 The osteogenic potential of each donor was
verified through subcutaneous implantation of MSCs loaded onto HA/TCP. The canine femo-
ral gap defect was then used to demonstrate MSC-mediated bone formation in a large animal
model. MSCs were loaded onto 60/40 HA/TCP porous scaffolds (MBCP ® Biomatlante France)
custom milled into hollow cylinders to closely approximate the defect dimensions. The im-
plants were then shipped to the surgical facility for implantation. A 21mm osteoperiosteal
defect was created unilaterally in the mongrel hounds. Implants were implanted in an autolo-
gous manner, with cell-free HA/TCP implants and autograft bone segments serving as con-
trols. This study confirmed the feasibility of bone marrow harvesting, MSC isolation and bone
formation for autologous MSC implantation. However limitations of the HA/TCP scaffold
were (1) the lack of remodeling/resorption of the ceramic, which in turn limited the rate of
bony integration, (2) its brittle mechanical properties. Others experiments were designed to
investigate fully resorbable CaP scaffolds by varying the ratio of HA to TCP to increase the
solubility of the BCP. The rate of degradation or resorption of HA/TCP ceramics in vivo can
be accelerated by increasing the amount of the more soluble phase, TCP. 24 In order to design a
scaffold more efficient for bone tissue engineering in combination with MSC a specific opti-
mum balance between the more stable phase (HA) and the more soluble phase (TCP) must be
achieved. The goal of this study was to determine the optimal HA/TCP ratio seeded with
MSCs that promoted rapid and uniform bone formation in vivo. To this end, human MSCs
were evaluated on various HA/TCP compositions in an ectopic model in immunocompromised
mice. 38
Six calcium phosphate ceramic compositions were examined: 100% Hydroxyapatite (100
HA), 100% β -tricalcium phosphate (100 TCP) and four formulations of HA/TCP (76/24,
63/37, 56/44 and 20/80, ratio of wt%HA/wt%TCP, Biomatlante, France). All compositions
were manufactured with a porosity of 60%-70% and a pore size range of 300 to 600 µ m, and
3mm cubic shaping. Bone marrow was collected from human donors and MSCs were isolated,
culture expanded and cryopreserved. MSC-loaded implants and cell-free implants were im-
planted subcutaneously in the back of SCID mice. Implants were harvested at 6 and 12 weeks
and processed for routine decalcified histology. More bone formed within the pores of
MSC-loaded 20/80 HA/TCP than all other implants at 6 weeks. By 12 weeks, MSC-loaded
56/44 and 20/80 HA/TCP demonstrated the greatest amount of bone and were equivalent to
each other. The least amount of bone formed within the pores of 100 TCP and 100 HA
throughout the study. These results demonstrated the potential of using faster resorbing ceram-
ics, 56/44 and 20/80 HA/TCP, in combination with MSCs for bone regeneration. 38,39 Future
studies will test these results in more clinically relevant models. Use of this therapy in the clinic
has begun in a Phase I clinical trial applying autologous MSCs to a 60/40 HA/TCP scaffold for
 
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