cells, and the testicular localization of the intermediate filament
protein nestin, known to be expressed in neural stem cells, by our group
was the first step to define mural cells (pericytes and vascular smooth
muscle cells) of the testis microvascu- ture as the stem/progenitor
cells of the adult Leydig cells. In summary, we were able to demonstrate
specific proliferation of vascular progenitors and their subsequent
transdifferentiation into steroidogenic Leydig cells, which - in
addition - rapidly acquire neuronal and glial properties. Since both
newly developed fetal and adult Leydig cell populations show the same
features, a common origin of both popu- tions seems likely. Pericytes
are distributed throughout the body, and there is convincing evidence
for their stem/progenitor cell properties in diverse organs. Under
appropriate (locally defined) conditions these pericytes, which reside
in the vascular stem cell niche as dormant stem cells, become activated,
proliferate, migrate and differentiate towards different somatic cell
types of the body. Since most mesenchymal stem/ progenitor cell types
exhibit essential similarity to pericytes and certain mesenchymal stem
cells represent pericyte descendants, we propose that mesenchymal stem
cells in the perivascular niche are daughter cells of pericytes. Thus,
pericytes are promising candidates for ancestor cells of all adult stem
cells in the organism. There is strong evidence that early stem cells
(cells arising during embryogenesis), such as the pericytes, exhibit
both mesodermal and neural progeny, which might explain the
neuroendocrine properties of the Leydig cells.
