Fine structure of the horny teeth of the lamprey, Entosphenus japonicus

The fine structure of the horny teeth of the lamprey, Entosphenus japonicus, was examined by light- and electron-microscopy. Most of the horny teeth consisted of two horny and two nonhorny layers. The primary horny layer was well keratinized, and the cells were closely packed and intensely interdigitated, being joined together by many modified desmosomes. The plasma membrane of the horny cell, unlike the membranes of other vertebrates, was not thickened. The intercellular spaces were filled with electron-dense material. Microridges were seen on the free surface. Structures resembling microridges were found on the underside of the primary horny layer. The secondary horny layer displayed various stages of keratinization. The keratinization started at the apex and developed toward the base. In the early stage of keratinization, the superficial cells became cylindrical and were arranged in a row forming a dome-shaped line. Their nuclei were situated in the basal part of the cells. The appearance of the nonhorny layers varied according to the degree of keratinization of the horny layers beneath them. The nonhorny cells were joined together by many desmosomes and possessed many tonofilament bundles. The replacement and keratinization of the horny teeth are discussed in the light of these results.