ONGOING PROLIFERATION OF MELANOCYTES IN THE STRIA VASCULARIS OF ADULT GUINEA-PIGS

The intermediate cells of the stria vascularis are melanocytes derived from the neural crest. These internalized pigment cells have been thought to be a static population, distinct from those found in the skin. However, this investigation demonstrates that the melanocytes of the adult stria vascularis undergo continuous replication. Cell proliferation was studied using [H-3]-thymidine autoradiography and bromodeoxyuridine (BrdU) immunohistochemistry. Single or multiple injections of [H-3]-thymidine within a six hour period labeled a mean of 9 intermediate cells. In pigmented guinea pigs, single daily injections of [H-3]-thymidine for 2 or 4 days labeled a mean of 24 and 69 intermediate cells, respectively; Pigmented guinea pigs given BrdU once daily for 2 or 4 days labeled a mean of 38 and 75 intermediate cells. By contrast, albino littermates also given BrdU averaged only 23 and 42 labeled intermediate cells in the same 2 and 4 day experiments. The mean number of proliferating cells/mm of stria per 24 h was 1.54 in the pigmented animals and 0.88 in the albinos. Both the total number and density of labeled intermediate cells were significantly smaller in the albino than the pigmented guinea pigs. These results demonstrate that the melanocytes in the stria vascularis undergo continuous baseline mitosis, and at a rate comparable to the melanocytes of the skin. This surprising similarity promotes the speculation that the proliferative rate of the strial melanocytes may be influenced by some of the same factors known to affect replication and pigment production in the skin.