  We propose a three dimensional mechanical model of embryonic tissue dynamics.
Mechanically coupled adherent cells are represented as particles interconnected
with elastic beams which can exert non-central forces and torques. Tissue
plasticity is modeled by a stochastic process consisting of a connectivity
change (addition or removal of a single link) followed by a complete relaxation
to mechanical equilibrium. In particular, we assume that (i) two non-connected,
but adjacent particles can form a new link; and (ii) the lifetime of links is
reduced by tensile forces. We demonstrate that the proposed model yields a
realistic macroscopic elasto-plastic behavior and we establish how microscopic
model parameters affect the material properties at the macroscopic scale. Based
on these results, microscopic parameter values can be inferred from tissue
thickness, macroscopic elastic modulus and the magnitude and dynamics of
intercellular adhesion forces. In addition to their mechanical role, model
particles can also act as active simulation agents and modulate their
connectivity according to specific rules. As an example, anisotropic link
insertion and removal probabilities can give rise to local cell intercalation
and large scale convergent extension movements. The proposed stochastic
simulation of cell activities yields fluctuating tissue movements which exhibit
the same autocorrelation properties as empirical data from avian embryos.
