The Story of the Finest Armor: Developmental Aspects of Reptile Skin

Abstract

The reptile skin is a barrier against water loss and pathogens and an armor for mechanical damages. The integument of reptiles consists of two main layers: the epidermis and the dermis. The epidermis, the hard cover of the body which has an armor-like role, varies among extant reptiles in terms of structural aspects such as thickness, hardness or the kinds of appendages it constitutes. The reptile epithelial cells of the epidermis (keratinocytes) are composed of two main proteins: intermediate filament keratins (IFKs) and corneous beta proteins (CBPs). The outer horny layer of the epidermis, stratum corneum, is constituted of keratinocytes by means of terminal differentiation or cornification which is a result of the protein interactions where CBPs associate with and coat the initial scaffold of IFKs. Reptiles were able to colonize the terrestrial environment due to the changes in these epidermal structures, which led to various cornified epidermal appendages such as scales and scutes, a beak, claws or setae. Developmental and structural aspects of the epidermal CBPs as well as their shared chromosomal locus (EDC) indicate an ancestral origin that gave rise to the finest armor of reptilians.

Keywords: development; epidermis; keratin; origin; protein; reptile.

Figure 1

Evolutionary tree of amniotes (adapted from Ref. [5]. 2009, Shedlock, A.M., et al.).

Figure 2

The formation of CBP (A) Chromosomal EDC locus within the nucleus (n) of a beta-cell. (B) The CBP cluster within the EDC locus. (C) Closer look to CBP cluster showing one intron and two exons of which only the second one has a coding region (cr). (D,E) Monomer and dimer of CBP, respectively, showing beta-sheet conformation. (F) CBP filaments covered and inter-filamentous matrix. (G) Different horny epidermal appendages made of CBPs, such as beak, scale, claw and scutes (The figure is modified from [20], Reprinted/adapted with permission from Ref. [20]. 2016, Elsevier).

Figure 3

A general illustration showing layers of lepidosaurian skin. The Shedding Plane (SP) is separating the inner (IEG) and the outer epidermal generation (OEG) apart. Oberhäutchen, beta (β), mesos (µ) and alpha (α) layers are present in both generations. However, the outer generation also has a clear layer between the outer alpha and inner Oberhäutchen layers at the time of shedding. Dermis contains chromatophores giving lepidosaurians multifarious appearances (Adapted from Yenmiş, 2022 [88]).