Tetraploid cycle in ageing solid tumours.

When the mouse mammary adenocarcinoma 755 (Ca-755) reaches the plateau phase of growth, non-cycling cells with a G2-DNA content can be observed. They may belong to the diploid cell cycle but they could also be blocked in G0 or G1 of a tetraploid cycle. This hypothesis was tested in three ways: (1) non-cycling G2 nuclei were stained with a combination of Feulgen and naphthol yellow which revealed two populations, one with a low protein content and the other with a high protein content--the latter may represent nuclei ready to begin a new phase of DNA synthesis; (2) Feulgen staining and autoradiography were performed after tritiated thymidine had been administered to mice continuously: this showed that there were cells synthesizing DNA with a DNA index above 2; and (3) cells having 80 chromosomes, corresponding to the tetraploid cycle, were found almost exclusively in the plateau phase tumours. On the other hand, the use of texture and DNA parameters of the Feulgen stained nuclei showed that they were concentrated in a diploid cycle for tumours in the exponential phase of growth and were divided between a diploid and tetraploid cycle for 'plateau' cells. Neither the cause for, nor the role played by, polyploid cells is known.