Abstract
The proliferation dynamics of chicken embryo cells after transformation by Rous sarcoma virus, Schmidt‐Ruppin strain (RSV‐SR) has been investigated in vitro. The optimal schedule for serial cultivation for normal as well as transformed cells was to subcultivate the cells every third day using an inoculum of 50 X 10^4^ cells per cm^2^ culture vessel area. Normal cells could be carried between 20–27 passages before they were lost in a degenerative phase similar to that described for normal human and other fibroblast‐like cells (Hayflick and Moorhead, 1961). The cumulative number of cells from a second passage inoculum of 2 × 10^6^ cells varied between 10^9^ and 10^12^. RSV‐SR transformed cells differed from normal cells in showing a decrease of the average population doubling time from 21 to 15 h and a two‐fold increase of their maximal terminal cell density. The life‐span of the RSV‐transformed populations was reduced to about 50% of the number of passages obtained by normal cells. The net result was a decrease of the cumulative cell number after transformation by RSV‐SR. No established RSV‐transformed or ‐untransformed lines could be developed.
When RSV‐SR‐transformed cells of one sex were cocultivated with normal cells of the opposite sex a rapid transformation of the latter was observed. The proportion of dividing cells belonging to the originally transformed population gradually diminished until the cultures were completely dominated by freshly transformed cells.
The results fit a previous hypothesis on the growth mechanism of avian Rous sarcoma (Pontén, 1964). RSV induces a change which enables the cell to proliferate at a faster than normal rate, to grow in an irregular manner in vitro, to reach higher than normal local densities in vitro and to invade surrounding normal tissues in vivo. None of these properties is permanent and the transformed or neoplastic cell populations will have a reduced total life span compared to their normal counterparts. Only if normal cells are constantly transformed and added to the proliferating pool of transformed or neoplastic cells will progressive growth prevail. The model is strictly applicable to stable systems (chicken, bovine, human); unstable species with a high spontaneous rate of transformation into established lines (mouse, rat, hamster) will react with the development of autonomously proliferating RSV cells capable of an infinite number of cell divisions.