Cell proliferation, adhesion, and differentiation of keratinocytes in the developing beak and egg-tooth of the turtle Em
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ORIGINAL ARTICLE
Cell proliferation, adhesion, and differentiation of keratinocytes in the developing beak and egg-tooth of the turtle Emydura macquarii Lorenzo Alibardi 1 Received: 24 March 2020 / Accepted: 29 May 2020 # Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract The development of the beak in turtles is poorly known. Beak development has been analyzed by immunofluorescent methods for studying cell proliferation and localization of specific proteins. The flat two-layered epidermis covering the turtle embryo at mid stage of development becomes columnar in the oral region and is associated with an increase of mesenchymal density as in placodes. Using 5BrdU, an intense cell proliferation is observed in the oral and epidermal cells covering the maxilla and mandibular bones, probably stimulated by the underlying mesenchyme in continuation with maxillary and mandibular bones. Expansion and fusion of these placodes give rise to the corneous beak. Beta catenin, mainly junctional but also sparsely detected in nuclei of the germinal layer of the beak epithelium, maintains united the differentiating keratinocytes that form a stratified corneous epithelium. This is initially composed of some layers of large cells that accumulate intermediate filament keratins (IFKs) and give rise to a keratinized embryonic epidermis destined to slough around hatching. The following corneocytes accumulate IFKs but mainly type I/II corneous beta proteins (CBPs) and form a corneous beak. These CBPs appear present with lower intensity in the beak than in the shell, but the higher intensity obtained with a general antibody against CBPs indicates that other CBPs contribute to the composition and stiffness of beak corneous material. The egg-tooth in continuation with the stratum corneum of the maxillary beak develops from a localized proliferation and comprises a thick embryonic epidermis accumulating IFKs under which large beta-cells connected by adhesion proteins accumulate CBPs contributing to hardening of this temporary organ. Keywords Turtle . Embryo . Beak . Egg tooth . Development . Immunolabeling
Introduction Together with birds and platypus, turtles are the only amniotes that during evolution replaced their dentition with a beak or rhamphoteca, a hard corneous lamina that covers their maxilla and mandibular bones (Davit-Beal et al. 2009). The origin of the thick corneous layer of the beak is related to the loss or alteration of some signaling proteins stimulating teeth development, such as Shh and Msx, but little is specifically known for turtles (Tokita et al. 2012).
Handling Editor: Margit Pavelka * Lorenzo Alibardi [email protected] 1
Dipartimento di Biologia, University of Bologna, via Selmi 3, 40126 Bologna, Italy
Previous studies have shown that the adult beak (rhamphoteca) in turtles is formed by a thickened epidermis whose keratinocytes accumulate a large amount of nonkeratin proteins presently termed corneous beta proteins (CBPs, Alibardi 2020), formerly indicated as hard keratins (Frenkel and
