  We study the doping evolution of the electronic structure in the pseudogap
state of high-Tc cuprate superconductors, by means of a cluster extension of
the dynamical mean-field theory applied to the two-dimensional Hubbard model.
The calculated single-particle excitation spectra in the strongly underdoped
regime show a marked electron-hole asymmetry and reveal a "s-wave" pseudogap,
which display a finite amplitude in all the directions in the momentum space
but not always at the Fermi level: The energy location of the gap strongly
depends on momentum, and in particular in the nodal region, it is above the
Fermi level. With increasing hole doping, the pseudogap disappears everywhere
in the momentum space. We show that the origin and the "s-wave" structure of
the pseudogap can be ascribed to the emergence of a strong-scattering surface,
which appears in the energy-momentum space close to the Mott insulator.
