A Small-Molecular-Weight Growth Inhibitory Factor (SGIF) in Stationary-Phase Supernatants of Tissue Cultures

Abstract

Tissue cultures stop growing and in time self-destroy when the cell concentration in the medium reaches a certain maximum. This phenomenon has been known for as long as tissue cultures have been used. Periodic addition of medium is a prerequisite for maintaining the cultured cells in adequate growing phase. The common explanation for this phenomenon is that contact inhibition occurs in overgrown cultures. Depletion of nutrients in the culture medium by the growing and metabolizing cells is an alternative explanation. The contact inhibition mechanism was self-suggested by the observation that cells growing in monolayers come in very intimate contact with each other in overgrown cultures, which do not grow further. This explanation of growth arrest was not adequate, in our opinion, to account for the arrest of growth of cells in suspension cultures, such as the human lymphoid cells. In such cultures, the total mass of cells is insignificant compared to the volume of medium in which they float. The depletion of nutrients, on the other hand, is an equally inadequate explanation, for we, and others, have found that cells can grow in media diluted with saline or other solutions for various purposes. We considered, therefore, the hypothesis that the lack of cell growth in stationary-phase cultures is due to the accumulation of materials produced by the cells themselves, which are capable of arresting cell growth when reaching a critical concentration. In fact, the effect of addition of tissue culture medium to keep a cell culture growing may be only the dilution effect on the growth inhibitory factor.