  In this paper we explore several structural and dynamical effects on the
projected velocity dispersion as possible causes of the fundamental plane (FP)
tilt of elliptical galaxies. Specifically, we determine the size of the
systematic trend along the FP in the orbital radial anisotropy, in the dark
matter (DM) content and distribution relative to the bright matter, and in the
shape of the light profile that would be needed to produce the tilt, under the
assumption of a constant stellar mass to light ratio. Spherical, non rotating,
two--components models are constructed, where the light profiles resemble the
$R^{1/4}$ law. For the investigated models anisotropy cannot play a major role
in causing the tilt, while a systematic increase in the DM content and/or
concentration may formally produce it. Also a suitable variation of the shape
of the light profile can produce the desired effect, and there may be some
observational hints supporting this possibility. However, fine tuning is always
required in order to produce the tilt, while preserving the {\it tightness} of
the galaxies distribution about the FP.
