  The structure and the evolution of Pulsar Wind Nebulae (PWNe) are studied by
means of two-dimensional axisymmetric relativistic magnetohydrodynamic (RMHD)
simulations. After the first imaging of the Crab Nebula with Chandra, a growing
number of objects has been found to show in the X-rays spatial features such as
rings and jets, that clearly cannot be accounted for within the standard
framework of one-dimensional semi-analytical models. The most promising
explanation suggested so far is based on the combined effects of the latitude
dependence of the pulsar wind energy flux, shaping the wind termination shock
and naturally providing a higher equatorial emission, and of the wind
magnetization, likely responsible for the jet collimation by hoop stresses
downstream of the shock. This scenario is investigated here by following the
evolution of a PWN interacting with the confining Supernova Remnant (SNR), from
the free expansion to the beginning of the reverberation phase. Our results
confirm the oblate shape of the wind termination shock and the formation of a
polar jet with supersonic velocities (v~0.5-0.7 c) for high enough values of
the equatorial wind magnetization parameter (sigma~0.01).
