Dynamic mechanical stimulation for bone tissue engineering.

Mechanical loading is an important regulatory factor in bone homeostasis, and
plays an essential role in maintaining the structure and mass of bone throughout a
lifetime. Although the exact mechanism is unknown the data presented in this thesis
supports the concept that substrate signals influence MSC growth and differentiation. A
better understanding of the cellular and molecular responses of bone cells to mechanical
stimuli is the key to further improvements to therapeutic approaches in orthopaedics,
orthodontics, periodontics, bone repair, bone regeneration, implantology and tissue
engineering. However, the mechanisms by which cells transduce mechanical signals are
poorly understood. There has also been an increased awareness of the need for
improvement and development of 3-D in vitro models of mechanotransduction to mimic
the 3-D environment, as found in intact bone tissue and to validate 2-D in vitro results.
The aims of the project were (i) to optimize a model system by which bone cells
can survive in 3-D static culture and their responses to mechanical stimuli can be
examined in vitro, (ii) to test the effects of intermittent mechanical compressive loading
on cell growth, matrix maturation and mineralization by osteoblastic cells, (iii) to examine
the role of the primary cilia, (iv) to assess the effect of dynamic compressive loading on
human mesenchymal stem cells in the 3-D environment.
The optimized model system has the potential to be used in in vitro studies of
bone in 3-D environments including a better understanding of the mechanically
controlled tissue differentiation process and matrix maturation in the engineered bone
constructs. It has less complicated equipment and techniques compared to dynamic
seeding and culture systems making it easy to use in the laboratory. In addition, cells are
not pre stimulated by any mechanical stimuli during seeding and culture which enables
the researcher to study selected mechanical stimuli and mechanotransduction in bone
tissue constructs. The model can mimic the bone environment providing a better
physiological model than cells cultured in 2-D monolayer.
Using our 3-D system, several loading regimens were compared and it was
shown that intermittent short periods of compressive loading can improve cell growth
and/or matrix production by MLO-A5 osteoblastic cells during 3-D static culture. This
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suggests that the cells are responding to the mechanical compression stimulus either by
directly sensing the substrate strain or the fluid shear stress induced by flow through the
porous scaffold. We also demonstrated that our mechanical loading system has the
potential to induce osteogenic differentiation and bone matrix production by human
MSCs in the same way as treatment with dexamethasone. Although the exact
mechanism is unknown the data presented supports the concept that the dynamic
compressive loading influence MSC growth, differentiation and production.
In further experiments, we used the optimized 3-D model system to study the
effects of mechanical loading on primary cilia, which have recently been shown to be
potential mechanosensors in bone. We demonstrated that mature cells lacking a cilium
were less responsive, less able to upregulate matrix protein gene expression and did not
increase matrix production in response to mechanical stimulation suggesting that the
primary cilia are sensors for mechanical forces such as fluid flow and/or strain induced
shear stress.