Characterization of factors in routine laboratory protocols that significantly influence the Feulgen reaction

We investigated the parameters that could affect the cytophotometric analysis of cell nuclei stained by the Feulgen reaction. These parameters included: the hydrolysis temperature (in the normal "room temperature" range); the composition of the Schiff's reagent; the speed of centrifugation of the cell suspensions; the mode of preservation [air-drying or ethanol-formalin-acetic acid (EFA) fixation]; the fixation time; the pronase digestion time; and the concentration of pronase used to obtain cell suspensions from archival (formalin-fixed, paraffin-embedded) materials. Relatively homogeneous material was studied: the MXT mouse mammary adenocarcinoma growing in vivo as tumors with both small and hyperchromatic cell nuclei and in vitro as monolayers with larger and less hyperchromatic cell nuclei. The results of these investigations demonstrate the necessity for the precise definition of a protocol for such procedures as sampling, fixation, and staining of cell nuclei if computerized cell image analyses are to be objective and reproducible. For present purposes this protocol differs depending on whether fresh or archival material is studied. For fresh tissue the protocol is immersion of the sample in EFA within 10 sec, fixation for 30 min, and staining by the Feulgen reaction in which hydrolysis is performed with 6 N HCl at 24 degrees C for 60 min. For archival tissue, the protocol becomes fixation in formol (or EFA), embedding, sectioning at 80 microns, digestion with 0.05% pronase for 2 hr, centrifugation at 1200 x g on glass slides, and staining by the Feulgen reaction in which hydrolysis is performed with 6 N HCl for 60 min at 24 degrees C.