  Many real phenomena may be modelled as locally finite unions of
$d$-dimensional time dependent random closed sets in $\mathbb{R}^d$, described
by birth-and-growth stochastic processes, so that their mean volume and surface
densities, as well as the so called mean \emph{extended} volume and surface
densities, may be studied in terms of relevant quantities characterizing the
process. We extend here known results in the Poissonian case to a wider class
of birth-and-growth stochastic processes, proving in particular the absolute
continuity of the random time of capture of a point $x\in\R^d$ by processes of
this class.
