The establishment of criteria for "quiescence" in ageing human embryo cell cultures and their response to a proliferative stimulus.

Experimental observations have shown that late passage cultured cells have a different growth pattern from early passage cells. For a meaningful comparison of the response of "quiescent" ageing cells to a proliferative stimulus it was, therefore, necessary to use cells in comparable states of "quiescence". Criteria have been established for defining quiescence in ageing cells which include alterations in cell number, ability to transport non-metabolizable substrates and inability to incorporate 3H-thymidine. Using cells of different ages in comparable quiescent states, it has been shown that a smaller proportion of older cells reacted to a proliferative stimulus, and the time taken to respond was longer.