  A gauge invariant metric fluctuations formalism from a non-compact
Kaluza-Klein (NKK) theory of gravity is presented in this talk notes. In this
analysis we recover the well-known result $\frac{delta \rho}{\rho}\simeq 2\Phi$
obtained typically in the standard 4D semiclassical approach to inflation and
also the spectrum of this fluctuations becomes dependent of the fifth
(space-like) coordinate. This fact allows to establish an interval of values
for the wave number associated with the fifth dimension.
