Microscopic and immunohistochemical study on the cornification of the developing beak in the turtle Emydura macquarii

The development and cornification of the ramphoteca (beak) in turtles are not known. The microscopic aspects of beak formation have been analyzed in the pleurodirian turtle Emydura macquarii using histological, immunocytochemical and ultrastructural methods. At embryonic Stage 15 the maxillar beak is originated from discontinuous placodes (one frontal and two oral) formed in the epidermis above and below the mouth that later merge into the epidermis of the beak. The mandibular beak is formed by two lateral placodes. In the placodes, basal keratinocytes in contact with local mesenchymal condensations become columnar, and generate suprabasal cells forming 5-6 layers of embryonic epidermis at Stages 17-20 and a compact shedding alpha-layer at the base of the embryonic epidermis. These keratinocytes contain irregular or aggregated reticular bodies made of 30-40 nm thick strands of coarse filaments, mixed with tonofilaments and sparse lipid droplets. Beneath the shedding layer are present 3-4 layers of keratinocytes accumulating coarse filaments mixed with beta-corneous packets, and underneath spindle-shaped beta-cells differentiate where beta-corneous packets completely replace the reticulate bodies. Differently from scales where corneocytes partially merge, beak corneocytes remain separated but they are joined by numerous interlocking spines. The production of beta-cells in the thick corneous layer of the developing beak, like in claws, occurs before the differentiation of beta-cells in the body scutes. This indicates that a massive mesenchymal condensation triggers beta-differentiation before this process is later activated in most of body scutes of the carapace and plastron. J. Morphol. 277:1309-1319, 2016. © 2016 Wiley Periodicals, Inc.