To the ongoing discussion on markers that can identify Go cells let me add few remarks. The concept of Go cells, introduced over four decades ago, was based on functional criterium, namely, it defined cells that did not replicate DNA for the duration equivalent of two generation times. Since then, many attempts have been made to find morphological, metabolic, biochemical and/or molecular markers of the noncycling cells. One of such markers is cellular RNA content. In many cell systems (e.g. mitogen stimulated lymphocytes, fibroblasts, keratinocytes) Go cells have > 10 fold fewer ribosomes (rRNA) compared to their cycling counterparts (G1 cells). Because rRNA consists over 90% of total RNA, the total cellular RNA is a reliable marker discriminating Go from G1 cells (Cytometry, 1: 98-108, 1980) in these cell types. It makes not much sense, therefore, to use expensive antibodies to identify Go cells (e.g. to measure Go to G1 transition during mitogenic stimulation of lymphocytes) when for a fraction of cost and much faster (2-3 min) cell staining with AO or Hoechst/pyronin Y provides the same information. It should be mentioned, however, that in some tumors the difference between Go and G1 cells in RNA content is not as great as in the case of e.g. lymphocytes. Furthermore, it is still debatable whether tumor cells have genuine "Go" state. During differentiation cellular RNA content may be increased (e.g. plasma cells), or decreased (e.g. granulocytes), depending on particular cell phenotype. In such situations cellular RNA content may also be a marker of differentiation, but one should know whether its increase or decrease characterizes the particular pathway of cell differentiation. Numerous other markers of Go vs. G1 cells are used, of which the most notable are Ki-67 and PCNA expression. Because pRB phosphorylation is the critical step committing the cell to initiate DNA replication, we have recently proposed that the status of pRB phosphorylation should be considered as a marker discriminating Go from G1 cells (Exp. Cell Res., 239: 104-110, 1998). Antibody that specifically reacts with underphosphorylated pRB is commercially available (e.g. from PharMingen). It is possible, therefore, to immunocytochemically probe phosphorylation of pRB. Zbigniew Darzynkiewicz
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