Studying Cell–Surface Interactions In Vitro: A Survey of Experimental Approaches and Techniques - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 39

regulate various cellular processes. In outside-in signaling ligand binding is
transmitted into the cell interior by conformational changes of the receptor
carrying information that mediates cell growth, differentiation, proliferation,
migration, morphology and survival [ 1 , 13 , 17 - 20 ]. In addition, integrins transfer
signals from the cells to the ECM, a process termed inside-out signaling. This
process is mainly involved in regulation of integrin conformation, ligand-binding
affinity and ECM remodeling [ 19 ].
4 Experimental Techniques for Studying Cell-Surface
Interactions
The constant improvement in biomaterial synthesis and surface modifications has
provided various different synthetic materials that should be tested and screened
for their ability to promote cell adhesion and to support or even induce cellular
functionalities. A set of experimental approaches has been used in the past to
evaluate the cyto-compatibility of a given surface. These established techniques
cover a significant range of technical sophistication comprising low and high tech.
On the low-tech side, for instance, the number of cells that has adhered to a surface
under study within a given time is quantified by simple cell counting upon
microscopic examination. For cell proliferation studies, this measurement is
repeated at regular intervals. Apart from counting, the amount of cells adhering to
a surface under study can be determined by photometry after intracellular uptake
of membrane-permeable dyes or other methods of cell staining or biochemical
assays. The colorimetric MTT assay is an established in vitro assay for evaluating
the cyto-compatibility of biomaterials by measuring the metabolic activity of the
cells in contact with the surface. It is based on the intracellular reduction of a
colorless tetrazolium salt to colored formazan, which only occurs in metabolically
active cells with sufficient supplies of reducing agents (FADH 2 , NADH). The
amount of the colored formazan is proportional to the number of vital cells and can
easily be quantified by photometry.
More on the high-tech side are high-resolution microscopic techniques that are
capable of imaging the morphology of cells in contact with a surface under study,
such as scanning force microscopy (SFM) or scanning electron microscopy
(SEM). Both approaches provide detailed images of the upper cell surface with a
high spatial resolution. Other microscopic approaches can also be used to image
the cells on the surface at different resolutions and contrast.
All the methods mentioned above do not have direct access to the interface
between the lower cell membrane and the substrate that the cell is adhered to. We
refer to this interface as the cell-surface junction (cf. Fig. 2 ). However, most of the
processes important for adhesion, spreading or migration of cells are localized
at this particular interface between cell and surface. Thus, all experimental tech-
niques capable of reporting from this hidden area should be very useful for