Development, structure, and protein composition of reptilian claws and hypotheses of their evolution

Abstract

Here, we review the development, morphology, genes, and proteins of claws in reptiles. Claws likely form owing to the inductive influence of phalangeal mesenchyme on the apical epidermis of developing digits, resulting in hyperproliferation and intense protein synthesis in the dorsal epidermis, which forms the unguis. The tip of claws results from prevalent cell proliferation and distal movement along most of the ungueal epidermis in comparison to the ventral surface forming the subunguis. Asymmetrical growth between the unguis and subunguis forces beta‐cells from the unguis to rotate into the apical part of the subunguis, sharpening the claw tip. Further sharpening occurs by scratching and mechanical wearing. Ungueal keratinocytes elongate, form an intricate perimeter and cementing junctions, and remain united impeding desquamation. In contrast, thin keratinocytes in the subunguis form a smooth perimeter, accumulate less corneous beta proteins (CBPs) and cysteine‐poor intermediate filament (IF)‐keratins, and desquamate. In addition to prevalent glycine–cysteine–tyrosine rich CBPs, special cysteine‐rich IF‐keratins are also synthesized in the claw, generating numerous SS bonds that harden the thick and compact corneous material. Desquamation and mechanical wear at the tip ensure that the unguis curvature remains approximately stable over time. Reptilian claws are likely very ancient in evolution, although the unguis differentiated like the outer scale surface of scales, while the subunguis might have derived from the inner scale surface. The few hair‐like IF‐keratins synthesized in reptilian claws indicate that ancestors of sauropsids and mammals shared cysteine‐rich IF‐keratins. However, the number of these keratins remained low in reptiles, while new types of CBPs function to strengthen claws.

Turtles creep through the pages of The Anatomical Record

“…The work by Lorenzo Alibardi and colleagues has revealed developmental and histological details of the turtle beak (Alibardi, 2021a) and claws (Alibardi, 2021b). Turtles lack teeth, and instead, their jaws are covered with a corneous rhamphotheca, or “beak.” During development, placodes arise from the maxillary and mandibular epidermis that merge with horny laminae via cell proliferation (Alibardi, 2021a).…”

“…Nearby, the palate of turtles has both keratinized and nonkeratinized regions, with the former concentrated near the rhamphotheca (Heiss et al, 2008). Similarly, turtle claws arise from proliferation and protein synthesis in the dorsal epidermis of the digits (Alibardi, 2021b). Turtles use their claws for swimming, digging, and in reproduction (Evans, 1952).…”

“…The work by Lorenzo Alibardi and colleagues has revealed developmental and histological details of the turtle beak (Alibardi, 2021a) and claws (Alibardi, 2021b). Turtles lack teeth, and instead, their jaws are covered with a corneous rhamphotheca, or "beak."…”

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

“…As a result of the interaction (masking or degradation) of IFKs and CBPs in the upper layers of the skin, terminal differentiation occurred, and with the death of corneocytes (corneoptosis, a unique mode of cell death) the skin became cornified with the resistant, covalent bonds, thus gaining the ability to prevent water loss, microbial diseases, and mechanical injuries [ 35 , 43 , 44 , 45 ]. The process from keratinization to cornification can be observed in amniotic embryos, where the embryonic epidermis of the skin appendages accumulates keratins in the first place, but the maturation takes place by the jointure of a high portion of CBPs [ 41 , 46 ].…”

The histology and growth rate of Nile crocodile (Crocodylus niloticus) claws

“…The chemical composition, as well as a detailed description of the development and structure of various reptilian claws, appear elsewhere (Alibardi, 2021;Chang et al, 2009); however, notable differences exist during reptilian claw development between different taxa. In crocodilians, the cornification process differs from that of other taxa by the region of cell proliferation and growth, which is not localized in a nail groove, but dividing cells are equally distributed along the epidermis on the last phalange from the base, proximally, to the tip, distally (Alibardi, 1998(Alibardi, , 2016Alibardi & Thompson, 2002;Maderson & Alibardi, 2000;Rutland et al, 2019).…”