  At present, there is no doubt that relativistic jets observed in active
galactic nuclei pass from highly magnetized to weakly magnetized stage, which
is observed as a break in the dependence on their width $d_{\rm jet}(z)$ on the
distance $z$ to the central engine. In this paper, we discuss the possibility
of observing another break, which should be located at shorter distances. The
position of this break can be associated with the region of formation of the
dense central core near the jet axis which was predicted both analytically and
numerically more than a decade ago, but has not yet received sufficient
attention. In this case, the observed width should be determined by the dense
core, and not by the total transverse size of the jet. The calculations carried
out in this paper, which took into account both the transverse electromagnetic
structure of the jet and the change in the spectrum of emitting particles along
its axis, indeed showed such behaviour. We also found the evidence of the
predicted break in the jet expansion profile using stacked 15 GHz VLBA image of
M87 radio jet and constrain the light cylinder radius.
