Fine structure of nuptial pad surface of male ranid frogs

The nuptial pad, a cutaneous secondary sexual characteristic, developed on the ventrolateral aspect of the first digit (the thumb) of the male ranid frogs was observed. Under scanning electron microscopy, numerous small elevations were observed rising above the pad's surface; they were rounded in Rana brevipoda porosa and R. rugosa, conical in R. nigromaculata, and rather tall and gradually tapering in R. ornativentris. These elevations were densely crowded, and in some cases several elevations were seen to have fused to form ridges in the R. rugosa pads. Accessory microprocesses completely covered the outermost layer of cells of the elevations. Numerous pile-like microprocesses gradually shortened from the top toward the side of the elevation in R. ornativentris and R. nigromaculata. Those on the top were noticeably longer than those on the side in R. rugosa. The bundles of thick processes radically projected above the top in R. brevipoda porosa. In R. brevipoda porosa nuptial pads were observed under transmission electron microscopy, and the outermost monolayer cells of an elevation were usually keratinized, devoid of organelles and containing closely packed, fine filaments within a dense matrix. The accessory processes also contained dense fibrous matrices.