Retrospective assessment of liver cell proliferation via PCNA: a comparison with tritiated thymidine

Cell proliferation (S phase response) in archival liver tissues of partially hepatectomized rats was determined via proliferating cell nuclear antigen (PCNA) immunohistochemistry. These results were compared with the S phase response assessed previously in the same tissues via tritiated thymidine (Tdr) autoradiography. The effect of prolonged tissue fixation on PCNA immunohistochemistry was compared in two studies: study A, the liver was fixed for a maximum of 7 days and then embedded in paraffin and stored for approximately 18 months, while in study B, the liver was fixed in formalin for 7 years and then embedded in paraffin and stored for approximately 18 months until sectioning and immunostaining. PCNA immunostaining was successful in the liver sections of both studies, irrespective of the length of formalin fixation. Furthermore, the S phase labeling indices (LI) determined via PCNA and Tdr were comparable, although not identical, in the two studies. Therefore, use of PCNA immunohistochemistry should allow retrospective staining of rodent tissues for the assessment of cell proliferative activity in formalin-fixed organs from previously conducted toxicity and carcinogenicity studies.