Designing the Biocompatibility of Biohybrids - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 289

A frequently investigated cell type in biohybrids for regenerative medicine is
the mesenchymal stem cell (MSC). MSCs are pluripotent cells that can dif-
ferentiate into a variety of cell types [ 4 ] and can be used in several therapies.
Because of the expression of MHC class I the MSCs will escape the lysis by
natural killer cells. Furthermore, MSCs inhibit immune cell proliferation and
activation [ 5 ]. In this respect the MSCs have an immune modulating potential
and are capable of producing an immunosuppressive environment expressing
cytokines [ 6 ]. Placing the MSCs into a three-dimensional construct (structural
compound) will allow for implantation at a specific location into the organism.
The choice of the scaffold material can affect the MSC differentiation and has
to be taken into account. As an example, the chondrogenic differentiation is
favoured by the presence of carboxyl- or hydroxyl-groups, whereas the osteo-
genic differentiation is benefited by the attendance of amino- and sulfhydryl-
groups in vitro [ 7 ]. The elasticity of the matrix influences the determination
into neuronal, muscle or bone lineage specification, as shown in cell culture
experiments [ 8 ].
Prior to seeding the scaffold material, the cells have to be harvested from
bone marrow, muscle, adipose tissue, umbilical cord or lung. Following har-
vesting, the cells have to be isolated and cultured. Contrary to the knowledge of
the unlimited potential of self-renewal of human MSCs, the cells cultured
in vitro underlay senescence and showed reduced differentiation potential [ 9 ].
Furthermore, Miura et al. demonstrated that the continuous passaging of murine
MSCs was leading to the formation of a fibrosarcoma after implantation in vivo
[ 10 ]. Moreover, the effect of the reduced supply of cells with oxygen and
nutrients in the center of any scaffolding material on the differentiation and
excretory activity of the cells needs to be considered in biohybrids for regen-
erative medicine [ 5 ].
As an investigative tool, non-destructive in vivo fluorescence imaging is a
valuable technique to enhance the design process of the structural component and
investigating the GvHR in the same animal at different time points [ 11 ]. Repetitive
measurements in the same animal provide an excellent option to follow the
biocompatibility and GvHR using targeted optical fluorescent imaging probes.
Moreover, the degradation pathway of fluorescent degradable scaffold components
can be followed by tagging the structural components of the scaffold.
4 Classification of Biohybrids
The successful use of biohybrids is reported multifold with great variation in
various biomedical applications. However, no systematic classification exists so
far. Here we suggest a simple classification of biohybrides according to their
functional unit and according to their possible therapeutical application.