  During embryonic development tissue morphogenesis and signaling are tightly
coupled. It is therefore important to simulate both tissue morphogenesis and
signaling simultaneously in in silico models of developmental processes. The
resolution of the processes depends on the questions of interest. As part of
this chapter we will introduce different descriptions of tissue morphogenesis.
In the most simple approximation tissue is a continuous domain and tissue
expansion is described according to a pre-defined function of time (and
possibly space). In a slightly more advanced version the expansion speed and
direction of the tissue may depend on a signaling variable that evolves on the
domain. Both versions will be referred to as 'prescribed growth'. Alternatively
tissue can be regarded as incompressible fluid and can be described with
Navier-Stokes equations. Local cell expansion, proliferation, and death are
then incorporated by a source term. In other applications the cell boundaries
may be important and cell-based models must be introduced. Finally, cells may
move within the tissue, a process best described by agent-based models.
