The Trichohyalin-Like Protein Scaffoldin Is Expressed in the Multilayered Periderm during Development of Avian Beak and Egg Tooth

Abstract

Scaffoldin, an S100 fused-type protein (SFTP) with high amino acid sequence similarity to the mammalian hair follicle protein trichohyalin, has been identified in reptiles and birds, but its functions are not yet fully understood. Here, we investigated the expression pattern of scaffoldin and cornulin, a related SFTP, in the developing beaks of birds. We determined the mRNA levels of both SFTPs by reverse transcription polymerase chain reaction (RT-PCR) in the beak and other ectodermal tissues of chicken (Gallus gallus) and quail (Coturnix japonica) embryos. Immunohistochemical staining was performed to localize scaffoldin in tissues. Scaffoldin and cornulin were expressed in the beak and, at lower levels, in other embryonic tissues of both chickens and quails. Immunohistochemistry revealed scaffoldin in the peridermal compartment of the egg tooth, a transitory cornified protuberance (caruncle) on the upper beak which breaks the eggshell during hatching. Furthermore, scaffoldin marked a multilayered peridermal structure on the lower beak. The results of this study suggest that scaffoldin plays an evolutionarily conserved role in the development of the avian beak with a particular function in the morphogenesis of the egg tooth.

Identification of Chicken Transglutaminase 1 and In Situ Localization of Transglutaminase Activity in Avian Skin and Esophagus

“…Surprisingly, there is no annotated region corresponding to TGM1 in the current release of the chicken (Gallus gallus) genome sequence (assembly: GCF_016699485.2), raising the question as to whether epidermal transglutamination might occur by an atypical mechanism in the main avian model species [26,27]. The uncertainty about the conservation of a central enzyme of epidermal transglutamination is worrying, because most of the research on avian epidermal cornification proteins was conducted in the chicken [7,8,[28][29][30][31][32][33][34][35][36][37]. Recently, transglutamination of the EDC protein cornulin was proposed to influence the mechanical properties of the avian esophagus [38].…”

Immunolocalization of epidermal differentiation complex proteins reveals distinct molecular compositions of cells that control structure and mechanical properties of avian skin appendages

“…The study of the last EDC protein investigated, scaffoldin, is an extension of previous immunolocalization studies (Alibardi et al, 2015; Mlitz et al, 2014; Mlitz et al, 2021). The TEM observations showed immunolabeling in cells of the embryonic epidermis, in particular on periderm granules of scale, claws, beak, and in the sheet of feathers, with sparse labeling being also present in the cytoplasm surrounding granules, but not on the diffuse keratin bundles present in these cells (Figure 11(a),(b)).…”

“…Finally, the last EDC gene and protein considered here (Figure 12(b), small blue arrows), scaffoldin, instead appears the first to be expressed in the embryonic epidermis, in particular in the periderm of feathers sheath but also in the numerous periderm or transitional layers of the embryonic epidermis of claws and beak. In fact, it is only expressed in periderm granules and in the keratinizing 2–8 embryonic layers of the epidermis, beak, feather sheath and claws, cells destined to slough in the embryo (Alibardi et al, 2015; Mlitz et al, 2014; Mlitz et al, 2021). This protein likely forms a matrix component of the weakly cornified or keratinized periderm before sloughing, where it may form the scaffold for the deposition of other/later EDC proteins, including CβPs.…”

“…No gold labeling for any EDC protein, aside for feather CβP, was observed in sections incubated with primary antibodies without etching: therefore the etching protocol was needed to reveal some immunolabeling, an indication that aside the relatively low number of antigenic sites for most of these proteins, also the amino acid sequence shown in Figure 1 The study of the last EDC protein investigated, scaffoldin, is an extension of previous immunolocalization studies (Alibardi et al, 2015;Mlitz et al, 2014;Mlitz et al, 2021). The TEM observations showed immunolabeling in cells of the embryonic epidermis, in particular on periderm granules of scale, claws, beak, and in the sheet of feathers, with sparse labeling being also present in the cytoplasm surrounding granules, but not on the diffuse keratin bundles present in these cells (Figure 11…”

The normal stages of development for the California valley quail

“…Periderm is thick and beginning to peel off due to the egg tooth’s cornification [ 19 ]. Scales begin to overlap on inferior and superior surfaces of the leg.…”