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

Abstract

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.

Development, structure, and protein composition of the corneous beak in turtles

“…Scanty information is available on the development, cell structure, and biochemical composition of the turtle beak that in general shows similar histology, ultrastructure and variable immunolabeling for corneous proteins, as in claws (Alibardi, 2016a(Alibardi, , 2020a. The development of the beak reported in the present Review has been mainly studied in two species, the Australian Murray river pleurodirian turtle, Emydura macquarii and the cryptodirian Hermans's turtle, Testudo hermanni (Alibardi, 2016a, 2020b, andunpublished observations).…”

“…As the embryonic keratinocytes mature at stages 23-25, the reticulate bodies disappear and the entire cytoplasm becomes condensed and evenly filled with thin keratin filaments (IFKs) among a dense matrix (Figure 2f). This process can be termed keratinization as the main proteins present are represented by IFKs among a scarce matrix material of unknown composition (Alibardi, 2016a(Alibardi, , 2016b. When the embryonic epidermis is fully keratinized, around hatching, it is shed leaving underneath the stratum corneum of the definitive beak in hatchlings and in juvenile turtles.…”

“…While numerous detailed information are now available on the development, histology, ultrastructure, coding genes and protein composition of the horny carapace, plastron and of the claws (Alibardi, 1999(Alibardi, , 2002(Alibardi, , 2005(Alibardi, , 2013(Alibardi, , 2020aAlibardi & Dipietrangelo, 2005;Alibardi, Spisni, & Toni, 2004;Alibardi & Thompson, 1999a, 1999bCherepanov, 1997;Dalla Valle, Michieli, Benato, Skobo, & Alibardi, 2013;Dalla Valle, Nardi, Toni, Emeera, & Alibardi, 2009;Holthaus et al, 2015;Homer et al, 2001;Li, Kong, Ponting, & Haerty, 2013), relatively little is known on the origin and morphogenesis of the turtle beak, a characteristic feature of turtles. Only few recent studies have provided some information on the formation and fine structure of the corneous component of the turtle beak (Alibardi, 2016a(Alibardi, , 2020b. The present Review is a summary on what is presently known on the formation and cell structure of the corneous beak in turtles, and compares this skin appendage with the horny beaks present in other sauropsids.…”

“…Detailed descriptions of the methods here utilized are described in previous publications (Alibardi and Toni, 2009;Alibardi, 2014bAlibardi, , 2014cAlibardi, , 2016aAlibardi, , 2020b. Briefly, immunohistochemistry was conducted on embryonic material or on beaks of juveniles turtles after fixation in 4-5% paraformaldehyde or Carnoy, and the following embedding of the embryos or head of juveniles into wax or plastic resin (Lowcryl K4M).…”

Review: Evolution and diversification of corneous beta‐proteins, the characteristic epidermal proteins of reptiles and birds

“…This unique novel structure in turtles is the horny shell, composed of a dorsal carapace and a ventral plastron. Other epidermal structures that contain CBPs in turtles are limb scutes or scales, claws and a turtle specific kind of beak, the rhamphotheca (Alibardi et al, ; Alibardi, ; Alibardi, , ; Dalla Valle, Michieli, Benato, Skobo, & Alibardi, ; Dalla Valle, Nardi, Toni, Emera, & Alibardi, ; Toni et al, ). Soft‐shelled turtles differ from hard shelled, classical turtles, due to loss of the hard cornified scutes of the shell and may be a valuable model to identify correlations between these morphological changes and loss or sequence changes of CBP genes (Dalla Valle, Michieli, Benato, Skobo, & Alibardi, ; Dalla Valle, Nardi, Toni, Emera, & Alibardi, ; Holthaus et al, ; Li et al, ).…”