  We present the evolution of the shell structure of nuclei in Hartree-Fock
calculations using Skyrme's density-dependent effective nucleon-nucleon
interaction. The role of the tensor part of the Skyrme interaction to the
Hartree-Fock spin-orbit splitting in spherical spin unsaturated nuclei is
reanalyzed. The contribution of a finite range tensor force to the spin-orbit
splitting in closed shell nuclei is calculated. It is found that the exact
matrix elements of a Gaussian and of a one-pion exchange tensor potential could
be written as a product Skyrme's short range expression times a suppression
factor which is almost constant for closed shell nuclei with mass number $A\geq
48$. The suppression factor is $\sim 0.15$ for the one-pion exchange potential.
