Experimental Animal Models for Tissue-Engineered Bone Regeneration - Engineered Bone: Tissue Engineering Intelligence Unit - page 91

bearing bones decreases osteoblast number, bone formation rate, bone mass, bone maturation
and mechanical strength. 11
Biomechanical Features
Variations observed in the mechanical properties of bone arise from differences in
cross-sectional geometry, relative proportions of trabecular and cortical bone, amount of
mineralisation, degree of porosity and Haversian remodelling. Specific biomechanical behaviour
therefore correlates more with the particular shape and function of the bone, the size of the
animal and its lifestyle than with its taxonomic position. Specific bone loading conditions
must be taken into consideration when choosing an experimental design. As emphasised by
Van der Meulen, 12 conclusions regarding bone mechanical function based solely on geometry
or bone mineral content are inappropriate: when performing biomechanical assays one must
remember that there is no alternative to testing whole bone strength.
Immunological Status
An assessment of allografts or the osteogenic potential of various allogenic or xenogenic
cellular components may benefit from implantation in immunocompromised animals. For
these reasons, immunocompromised nude or SCID mice are currently used.
Physiological Characteristics
Bone physiology is partly conditioned by oestrogenic metabolism in females. Augmenting
bone healing procedures are therefore usually performed in males. Ovariectomy-dependent
bone loss models developed to mimic human osteoporosis are exceptions to that rule. Mice,
rats and NHP 13 are currently used for this purpose. Preliminary studies have indicated that
ovariectomised ferret 14 or sheep 6 could also be interesting models for human osteoporosis. The
degree of osteopenia obtained is species dependent: osteopenia in ovariectomised monkeys is
mild compared with the profound cancellous osteopenia observed in ovariectomized rats. NHP
models do mimic early post-menopausal changes in skeletal biology but not the disease of
post-menopausal osteoporosis.
Genetic Status
Unless specifically required, genetic variations are not usually taken into consideration and
experimental trials are currently performed in pure breed animals (i.e., undefined genetic ho-
mozygosity) or “mongrels” (i.e., unknown genotype). Genetic variations in bone regenerative
capacity nevertheless do exist as demonstrated by Li. 15
Genetic uniformity is required for cell or tissue transplantation trials: hybrid rodents, rab-
bits (New-Zealand White rabbit), micropigs (Yucatan Micropig) or immunocompromised in-
bred mice (SCID or nude mice) are used for this purpose. Genetic selection of laboratory
animals in bone engineering can also be performed to develop specific bone diseases (i.e., SAM
mice developing osteoporosis).
Model Objectivity and Reproducibility
Bone regeneration must not only be evaluated in animals treated with the innovating tech-
nique but also in sham-operated animals (negative controls) and in animals treated with au-
tologous corticocancellous bone grafts (positive controls) which remains the gold standard
material in bone tissue engineering. Each group should include at least 5 animals to allow
analysis of statistical significance.
Accurate follow up and assessment of bone regeneration both through conventional radio-
graphic, microradiographic, histologic and ideally biomechanical data must be achieved. Re-
cent availability of imaging techniques such as micro-computed tomography opens up new
perspectives. 16 Quantitative methods allow objective comparison of bone healing between groups
and are preferred over semi-quantative techniques. Quantitative assessment of bone healing is
possible provided that animals are of similar sizes and breeds, that bone sizes and curvatures as