Biomedical Engineering Reference
In-Depth Information
organ fragments (Verschueren
et al.
, 1991).Typically, only cell movement in
a single direction (depth) is characterized in the gel invasion assay.
3.5
Individual cell motility
In contrast to the cell population assays described above, single cell motility
assays allow the analysis of cell migration independently from cell growth
(Table 3.2). By tracking single cells, the relative proportion of cells respond-
ing to a stimulus can be evaluated by analyzing the distributions of the
quantitative data measured on the population. Furthermore, these assays
provide detailed information on how individual cells move and identify the
important locomotive parameters such as speed and persistence.
Towards the understanding of cell locomotion, numerous
in vitro
assays
have been developed to more easily characterize
in vivo
cell motility under
controlled chemical conditions. These
in vitro
methods are generally used to
provide initial information because
in vivo
techniques are more diffi cult and
require more resources to perform and quantify. Despite the increasing evi-
dence that migration across planar substrates is very different from
in vivo
cell behavior, much has been learned from these convenient assays about
the mechanisms underlying the highly coordinated processes that result in
cell motility.
In vivo
assays are more challenging and resource demanding,
but they are ultimately necessary to fully characterize complex processes
such as tumor cell invasion and endothelial cell migration and organization
in angiogenesis.
3.5.1
In vitro
assays
Numerous
in vitro
assays have been developed to analyze cell motility and
the extracellular factors that direct cell movement. The four assays described
below have been used extensively to characterize locomotive processes rel-
evant to biomaterials applications including chemotaxis (movement along
a chemical gradient), haptotaxis (movement along an insoluble adhesion
gradient), and durotaxis (movement along a rigidity gradient).
Phagokinetic assay
Originally known as the colloidal gold migration assay, cell tracks were
visualized by the displacement or engulfment of gold or polymer particles
during locomotion (Albrecht-Buehler, 1977). More recently, quantum dots
have been employed in a similar manner so that fl uoresce detection may be
used in the measurement of metastatic potential of tumor cells (Gu
et al.
,
2007). In brief, cells are cultured on a glass substrate covered with particles.
As cells move across a layer of nanoparticles, they engulf and uptake the
