Objective To detemine optimal conditions by using 5 bromodeoxyuridine （BrdU） as a marker of transplanted retinal pigment epithelial （RPE） cells in the subretinal space of albino rabbits Methods Pigmented rabbit RPE cells at second to fifth passage were fed with 20 μmol/L BrdU in Eagle’s minimal essential medium （MEM） for five days After extensive wash with phosphate buffered saline （PBS）

the cells were detached by trypsin and used for transplantation onto Bruch’s membrane of albino rabbits Eyes were enucleated at various times post transplantation Acetone

4% paraformaldehyde

periodate lysine paraformaldehyde （PLP）

or half strength Karnovsky’s fixatives were individually used to fix the tissue in order to find optimal condition for detecting BrdU marker The fixation was followed by embedding in OCT compound

glycol plastic

or paraffin The transplanted area was then sectioned

pepsin digested

and used for immunohistochemical staining with monoclonal antibody against BrdU and avidin biotin alkaline phosphatase complex （ABC AP） Results Frozen sections of acetone or paraformaldehyde fixed tissue gave strong immunostaining of BrdU but the overall morphology was poor Karnovsky’s fixed tissue offered strong staining but this was buried by strong background When using PLP as a fixative

we obtained strongly positive blue staining with very low background

and also excellent morphologic preservation Conclusion In combination with immunohistochemical detection method

BrdU labeling is an excellent long term marker for RPE transplantation one year after surgery To use BrdU as a marker necessitates the use of pepsin digestion to make the BrdU antigen in the nuclei accessible to antibody But the pepsin digestion may damage other tissues and result in overall poor morphology Among the fixatives tested in this study

PLP fixed tissues offered both strong BrdU staining and good preservation of structural integrity

particularly the fine structure of photoreceptor and RPE cells