  Collective cell migration is a multicellular phenomenon that arises in
various biological contexts, including cancer and embryo development.
"Collectiveness" can be promoted by cell-cell interactions such as
co-attraction and contact inhibition of locomotion. These mechanisms act on
cell polarity, pivotal for directed cell motility, through influencing the
intracellular dynamics of small GTPases such as Rac1. To model these dynamics
we introduce a biased random walk model, where the bias depends on the internal
state of Rac1, and the Rac1 state is influenced by cell-cell interactions and
chemoattractive cues. In an extensive simulation study we demonstrate and
explain the scope and applicability of the introduced model in various
scenarios. The use of a biased random walk model allows for the derivation of a
corresponding partial differential equation for the cell density while still
maintaining a certain level of intracellular detail from the individual based
setting.
