Cornification of the Beak of Rana dalmatina Tadpoles Suggests the Presence of Basic Keratin-Associated Proteins

Lorenzo Alibardi (2010) Cornification of the beak of Rana dalmatina tadpoles suggests the presence of basic keratin-associated proteins. Zoological Studies 49(1): 51-63. An ultrastructural, immunocytochemical, and electrophoretic study on keratinocytes formation in the horny beak of tadpoles of the frog Rana dalmatina was conducted. The study showed that hard cornification in amphibians occurs with similar cytological details as the hard corneous structures of amniotes. Dense corneous bundles containing scarce keratin intermediate filaments and mainly inter-keratin (matrix) proteins accumulated in central keratinocytes (cone cells) of the horny beak. Ribosomes appeared to be associated with these bundles indicating that newly synthesized matrix molecules immediately aggregated on the keratin filaments, turning the complex into an amorphous and electron-dense material. In the 2 corneous sheaths surrounding the cone cells, the oral and labial sheaths, tonofilaments made up of individual keratin intermediate filaments were initially present. The filaments were later transformed into an amorphous keratin mass as cornification progressed. Two-dimensional electrophoretic analysis determined that most proteins of the beak are acidic to neutral keratins. However protein spots outside the keratin range were also isolated for the 1st time. These proteins of 16-17 or 30 kDa with a isoelectric point of 9-9.5 are believed to represent the 1st keratin-associated proteins found in amphibian corneous tissue, and are likely responsible for the intense cornification of the horny beak. http://zoolstud.sinica.edu.tw/Journals/49.1/51.pdf