Studies in which 5-bromo-2'-deoxyuridine (BrdU) is used to quantify rates o
f cell proliferation are;conducted prospectively. Therefore, the opportunit
y exists to select conditions that optimize detection of the BrdU epitope.
The objective of this study was to quantify the extent to which the BrdU ep
itope was masked by formalin vs methacarn fixation in the assessment of cel
l proliferation. Mammary carcinomas from animals pulse-labeled with BrdU we
re trisected. A portion was frozen and the remaining two portions were fixe
d in 10% neutral buffered formalin or methacarn far 24 hr, processed, embed
ded in paraffin, and sections stained for incorporated BrdU using a peroxid
ase immunohistochemical staining technique. Antigen retrieval techniques al
so were applied to formalin-fixed sections. Fixation in methacarn gave the
highest labeling index (16.4%), which was comparable to that observed in un
fixed frozen sections (17.5%). Formalin fixation alone dramatically suppres
sed the labeling index (0.3%), which was only partially recovered using var
ious antigen retrieval techniques (2.1-8.1%). Methacarn fixation is recomme
nded for prospective studies in which BrdU detection is planned because of
the quantitative recovery of epitope and the simplicity of the approach.