Artificial Scaffolds and Mesenchymal Stem Cells for Hard Tissues - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 176

preparation [ 166 ]. Poly(carnitines) are not yet used in thin film technology;
however, these polycationic structures could be combined with polyanions

resulting in polyelectrolyte systems analogous to LbL films [ 163 ].
LB multilayers have been designed mainly to study surface processes such as
adhesion for sensor and membrane applications and integrated optics [ 167 ].
Lenhert and colleagues used LB lithography to pattern polystyrene surfaces and
investigate osteoblast alignment, elongation, and migration. This so-called
nanoimprinting enables the fabrication of nanostructured surface areas on a wide
spectrum of different biomaterials. For many biomaterial applications, relatively
large surface areas are required—they are beyond the limits of traditional
lithography. LB lithography, a recently developed method, was used to fabricate
regularly spaced grooves of different depths (50 and 150 nm) with a periodicity of
500 nm over several square centimeters on silicon surfaces. These topographies
were transferred onto polystyrene surfaces by means of nanoimprinting. Primary
osteoblasts were cultured on the patterned polymer surfaces, and were observed to
align, elongate, and migrate parallel to the grooves. Osteoblasts show a significant
anisotropic behavior on these surfaces, which can enhance cell settlement on the
surface or be used to direct tissue generation on the biomaterial interface [ 168 ].

4.2 MSCs on Artificial Surfaces

Artificial surfaces should be constructed to provide an environment mimicking the
ECM and the correct microenvironment for the SCs, similar to the postulated SC
niche [ 169 ], to construct an environment favorable for SC maintenance or dif-
ferentiation. To do so the first step is to understand this microenvironment, which
is not only very complex, but new data suggest that it seems to be different for
different kinds of SCs.
SCs have by definition the ability for self-renewal and differentiation towards
specific lineages. However the mechanism by which they regulate these two
characteristics is poorly understood. Two major hypotheses have been suggested.
In the asymmetrical system, the SC will divide and one daughter cell will remain a
SC, whereas the second cell will start to differentiate and leave the SC niche. The
question is, does this SC leave the niche and by doing so, due to the changed
microenvironment, start differentiating? Or does the SC start differentiation due to
the asymmetrical cleavage and then leave the niche? The second hypothesis is that
after an external trigger, presumably from within the SC niche, the SC will start
proliferating and thus lose the SC characteristics. Both daughter cells will leave the
SC niche which ultimately leads to a depletion of the SC pool. Which of these
hypotheses is true cannot yet be decided. There is data supporting both of them and
it is not unlikely that both systems exist, depending on the SC type. What is
already clear is that the microenvironment plays a key role in this event.