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Encyclopedia of Physical Science and Technology EN016J-783 August 1, 2001 10:58

830 Tissue Engineering

elucidated by using measurements of extracellular muscle myoblasts (muscle progenitor cells) into contrac-
products alone. For example, flux distribution at split tile myotubes containing parallel myofibers aligned in the
points that converge at another point of the network direction of the applied force.
cannot be resolved. Another limitation is that only net
fluxes are determinable, while isotopic methods can
sometimes resolve the rates of the forward and the back- C. Transport Phenomena in
ward reactions. This methodology may be particularly Tissue Engineering
useful when used in combination with stable isotopes
1. Cell Migration
to provide fluxes that cannot be directly determined by
the isotopomer analysis. Metabolic flux analysis, once Cell migration is often a critically important step in many
validated for the particular case under study, is potentially applications of tissue engineering. For example, bioma-
very useful as it is noninvasive and cost effective. terial implants used for nerve regeneration, cartilage, and
Another important aspect of the metabolic network that skin wound healing require that the host’s cells migrate
can be investigated by metabolic engineering techniques is into the matrix implant. Cell migration speed depends on
the “rate-controlling” enzymes of the pathway (i.e., the en- a complex balance between cell tractional forces and the
zymes governing flux in the metabolic network). Over the stickiness of the matrix to the cell. The highest cell speeds
past 30 years, several theoretical frameworks for this type are obtained at intermediate attachment strengths, which
of analysis have been developed. Of these, one of the most allow the leading edge of the cell to anchor itself to the
widely used is metabolic control analysis. Metabolic con- surface while the receding edge comes off the surface. A
trol analysis aims at quantifying the control that individual substrate with low adhesiveness does not allow the cell to
or groups of enzymes exert on the flux through a particular form any anchors to the surface that can resist cell trac-
pathway by studying the response of the system to changes tional forces, which results in poor migration. Similarly,
in nutrient levels and other factors that alter the activity of cells “glued” to a surface that is too sticky are not able to
specific enzymes in the network. This analysis is generally move forward because cell–substrate bonds at the reced-
quite difficult to perform experimentally and is often based ing edge of the cell cannot be broken.
on many assumptions; however, it provides valuable in- The determination of cell-migration parameters is im-
sight into the mechanisms governing metabolic adaptation portant in order to predict the speed at which cells can
to changes in the environment and a rational basis for ge- invade a tissue construct. The prediction of cell migra-
netically engineering cells to perform specific functions. tion behavior based on the knowledge of cell mechanics,
interaction of cell receptors with appropriate ligands on
the extracellular matrix, and function of the cytoskeleton
4. Effects of Mechanical Forces
is possible, but complicated, and involves difficult mea-
on Cells and Tissues
surements. On the other hand, phenomenological cell-
The environment in which cells are cultured has tradition- migration parameters can be determined via analysis of
ally been defined by the presence of soluble factors such as single cell trajectories or cell concentration profiles of cell
hormones and growth factors, and the chemical properties populations in specific devices which allow for the visu-
of the surface on which they adhere and grow. In addition, alization of cells during the migration process. The most
certain physical forces may influence cellular function and simple model to analyze single cell trajectories is the per-
may be used as tools to induce specific phenotypes in sistent random-walk model (Fig. 10). This model assumes
cells. For example, it is believed that mechanical loading that cells are not restricted in their range of movement
plays an important role in the synthesis and deposition of (within the duration of the experiment), they can move in
extracellular matrix by cells in load-bearing tissues such any direction with equal probability, and, once they move
as cartilage and bone in vivo. Thus, incorporating me- in a certain direction, they exhibit a characteristic persis-
chanical loading schemes in the culture environment such tence time before they change direction. The parameters of
as cyclical compression may be beneficial. Furthermore, the model, persistence time (P) and cell speed (S), can be
the mechanical loading apparatus can be coupled to sen- used to calculate an equivalent diffusivity coefficient (also
sors to provide a continuous assessment of the mechanical called random motility coefficient), which is a measure of
parameters (compressive strength and module of elastic- the propensity of a cell population to spread:
ity) of the developing tissue. Another example is the effect 2
S P
of fluid shear stress and uniaxial stretch, both of which D = (5)
induce vascular endothelial cell elongation and align- n
ment, as well as the secretion of vasoactive compounds. where n is the number of dimensions (n = 2 for a surface,
Cyclic stretch has also been used to promote the fusion of n = 3 for a gel) where the migration occurs.