Grain count threshold dependence of FLM curve pattern and cell cycle parameters of hepatocytes in vivo.

FLM curves from hepatocytes of regenerating rat liver in vivo were compared at different grain count thresholds. Estimates of cell cycle phases derived from curves with thresholds decreasing from 15 to 1 grain (background 0-2 grains per nuclear area) revealed a prolongation of ts from 6-6 to 9-5 hr, at the expense of tG2M, and tG1, whereas tc remained constant. A similar pattern was observed in FLM curves at various threshold levels for hepatocytes localized in subunits of the liver lobule along the vascular axis from afferent to efferent pole. The shapes of these FLM curves indicated an intralobular gradient of reutilizable labelled material. The use of two different threshold levels is crucial for proper selection of FLM curves to evaluate cell cycle phases in regenerating rat liver: first, threshold to exclude the autoradiographic background, and a second one to avoid errors due to reutilization of labelled DNA precursors. Each threshold has its own implications for the estimation of cell cycle phases.