  During embryonic morphogenesis, tissues undergo dramatic deformations in
order to form functional organs. Similarly, in adult animals, living cells and
tissues are continually subjected to forces and deformations. Therefore, the
success of embryonic development and the proper maintenance of physiological
functions rely on the ability of cells to withstand mechanical stresses as well
as their ability to flow in a collective manner. During these events,
mechanical perturbations can originate from active processes at the single cell
level, competing with external stresses exerted by surrounding tissues and
organs. However, the study of tissue mechanics has been somewhat limited to
either the response to external forces or to intrinsic ones. In this work, we
use an active vertex model of a 2D confluent tissue to study the interplay of
external deformations that are applied globally to a tissue with internal
active stresses that arise locally at the cellular level due to cell motility.
We elucidate in particular the way in which this interplay between globally
external and locally internal active driving determines the emergent mechanical
properties of the tissue as a whole. For a tissue in the vicinity of a
solid-fluid jamming/unjamming transition, we uncover a host of fascinating
rheological phenomena, including yielding, shear thinning, continuous shear
thickening (CST) and discontinuous shear thickening (DST). These model
predictions provide a framework for understanding the recently observed
nonlinear rheological behaviors {\it in vivo}.
