For determination of apoptosis in single cells or tissues we use the non-radioactive TUNEL assay from Roche detecting DNA strand breaks. The method works immunohistochemically with cells and with cryo sections of tumour tissues of interest.
Cells or cryo-sections are fixed as well as permeabilised and incubated with fluorescein-dUTP to label DNA damages. For visualisation of dUTP-binding it is possible to chose between DAB (3,3’-diaminobenzidine tetrahydrochloride, substrate of horseradish peroxidase) [Fig. 1A] and fluorescence [Fig. 1B] staining. Analysis is performed by fluorescence or light microscopy, respectively.
 

      

Fig.1: TUNEL assay performed with cryo-sections of a neuroblastoma (A) and a glioma (B) cell line growing as xenografts in immunodeficient nude mice. Figure A shows DAB staining and Figure B fluorescence labelling.