The centriolar antigen expression in TC7 cells is dependent on growth conditions and occurs at a particular time point in G1

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Abstract
The correlation between growth conditions and centriolar antigen (Cag) expression in TC7 cells, a subline of African green monkey kidney cells, was studied. TC7 cells became quiescent when their number reached a high cell density, or when serum factors were depleted from media containing a low concentration of fetal bovine serum (FBS). There was a stoichiometric relationship between the concentration of serum present and the number of new cells produced. During proliferation, the projected cell area decreased as a function of cell density with two abrupt transitions. The first transition appeared to be independent of cell‐cell contact. However, the second transition seemed to occur mainly as a result of the limitation of the available substratum surface on which the cells could grow. The appearance of Cag in TC7 cells was found to be associated with the cells' growth conditions as well as with the particular phase in cell cycle. In an exponentially growing culture of cells with 10% FBS and in cells that were growth restricted due to a high density (above 2–3 × 105 cells/cm2), the incidence of cells with Cag‐positive staining was about 10–20%. It increased, however, to about 40–60% at cell densities between 2 × 104 cells/cm2 and 1 × 105 cells/cm2. The frequency of cells with positive Cag staining was as high as 80% in TC7 cells that were growth restricted by depletion of serum factor(s). Thus, the quiescent states attained by the two different growth restrictions seem to be different in their ability to express Cag. The frequency of Cag could be further increased by stimulating the quiescent cell population by FBS. In mitotically selected TC7 cells, Cag staining appeared about 4 hr after mitosis and about 2–4 hr before the onset of DNA synthesis. Thus, expression of Cag in TC7 cells is related to their growth conditions, and is characteristic of a part of the G1 phase of the cell cycle.