S-phase cells in the guinea pig endolymphatic sac

S-phase (the period of DNA synthesis) cells in the endolymphatic sac (ES) were investigated by immunohistochemical methods. Monoclonal antibodies against 5-bromo-2'-deoxyuridine (BrdUrd) and proliferating cell nuclear antigen (PCNA) were used. After administration of BrdUrd i.p., guinea pigs were sacrificed with acid formalin and the decalcified temporal bones examined immunohistochemically. Scattered cells labelled with anti-BrdUrd or anti-PCNA antibodies were detected within the ES whereas labelled cells were not found in either the cochlea or the vestibule. From the morphological findings, these labelled cells within the ES epithelium were considered to be epithelial cells and the sub-epithelial stained cells looked like fibroblastic cells. Some labelled resident floating cells were found in the ES lumen. Our results reveal that the ES, where resident cells proliferate, is the sole site of the cell-renewing system in the inner ear of an adult animal.