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An examination into how stem cells form bone in vivo
Birmingham E. 1 , Niebur G. 2 , McHugh P.E. 1 , McNamara L. 1
1 Department of Mechanical and Biomedical Engineering, and National Centre for Biomedical Engineering
Science, NUI Galway, Ireland. e.birmingham1@nuigalway.ie
2 Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN
46556, USA.
Abstract
Stem cells are capable of forming different types of
tissues such as bone, cartilage and fat when they
receive appropriate cues, such as biochemicals and
mechanical loading. However, the precise cues that
control the tissues that stem cells ultimately grow
remain unclear. The aim of this study is to examine
what are the natural signals stem cells receive from
their environment which stimulate them to form bone.
1. Introduction.
Mesenchymal stem cells (MSCs) within the marrow
of bone are subjected to a unique microenvironment
known as the stem cell niche. Here MSCs can
experience not only biochemical signalling from
surrounding support cells (osteocytes, osteoblasts,
fibroblasts, adipocytes etc.) but also a mechanical
loading influenced by the trabecular bone structure and
marrow composition (1). While both biochemical
signalling from support cells and mechanical loading
have been shown to direct stem cell differentiation in
vitro, their influence within the niche remains largely
unknown.
Our objectives are (a) to examine the role of support
cells on the differentiation of MSCs and (b) to develop
an in vitro model which allows us to apply
physiological loads to MSCs as they reside in the niche.
2. Materials and Methods.
We examined the influence of mechanical loading
on MSCs residing in their natural home of the bone
marrow through the use of a custom built bioreactor
(Figure 1).
Media output
Bone sample
Actuating bar
Media input
Figure 1: Custom built bioreactor used to biomechanically
stimulate explanted ovine bone tissue samples containing the
trabecular structure and marrow.
The function of support cells on MSCs within the stem
cell niche was also examined using conditioned media,
collected from separately cultured osteoblasts or
osteocytes and co-culture studies.
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These set-ups allow us to delineate the importance of
biochemical signalling between bone’s regulatory cells,
osteoblasts and osteocytes, and MSCs.
3. Results and Discussion.
The support cell study showed a greater expression
of alkaline phosphatase (osteogenic marker) in MSCs
which had been co-cultured with osteocytes rather than
osteoblasts highlighting the role that osteocytes play in
regulating MSC differentiation (Figure 2) (2) and the
importance of examining MSCs in their natural
environment.
Figure 2: ALP activity for MSCs co-cultured with either
osteocytes or osteoblasts.
With our bioreactor we retain this niche environment
and also expose bone samples to physiological loads.
Our preliminary observations show that a dynamic
mechanical loading environment enhances the survival
of the bone marrow in vitro (Figure 3), whereas
unloaded static samples degraded in the first two weeks.
Figure 3: Histological section of trabecular bone and marrow
stained with H&E.
Future results from different loading regimes could
provide valuable information on the role of
mechanotransduction on bone development in vivo.
5. References
1. Kuhn et al., J Cell Physiol, 222, pp. 268-277, 2010
2. Heino et al., Exp Cell Res, 294, pp. 458-468, 2004.