Low incidence of tumourigenicity in agarose colonies from spontaneous murine mammary tumours

In these experiments individual colonies growing in agarose seeded with monocellular suspensions from freshly disaggregated naturally-occurring mouse mammary tumours, induced by the murine mammary tumour virus (MMTV), were reimplanted into mammary fat pads of virus-free mice. It was found that only a small proportion of these colonies generated tumours and that the implantation of multiple colonies in each site did not result in disproportionate, synergistic, increase in tumour takes. It was also observed that the proportion of colonies which were tumourigenic on reimplantation differed for each donor tumour and represented only a small fraction of the total cell population (0.001%-0.1%). However, this value was significantly higher in tumours which produced large numbers of deposits in lung colony assays following i.v. injections, than in tumours of low pulmonary colonisation potential. A point of particular interest was that tumours derived from agarose colonies of spontaneously metastatic donor tumours were substantially more spontaneously metastatic themselves than those from nonmetastatic donors, indicating that this property is heritable through numerous cell divisions, manipulations in vitro and transplantation procedures. From these results it is concluded that measurement of clonogenicity in agar is useful as an index of the capability of a tumour to propagate itself and to colonise new sites, but that individual agarose colonies are not all the progeny of potentially immortal stem cells.