Reptile scale paradigm: Evo-Devo, pattern formation and regeneration

Abstract

The purpose of this perspective is to highlight the merit of the reptile integument as an experimental model. Reptiles represent the first amniotes. From stem reptiles, extant reptiles, birds and mammals have evolved. Mammal hairs and feathers evolved from Therapsid and Sauropsid reptiles, respectively. The early reptilian integument had to adapt to the challenges of terrestrial life, developing a multi-layered stratum corneum capable of barrier function and ultraviolet protection. For better mechanical protection, diverse reptilian scale types have evolved. The evolution of endothermy has driven the convergent evolution of hair and feather follicles: both form multiple localized growth units with stem cells and transient amplifying cells protected in the proximal follicle. This topological arrangement allows them to elongate, molt and regenerate without structural constraints. Another unique feature of reptile skin is the exquisite arrangement of scales and pigment patterns, making them testable models for mechanisms of pattern formation. Since they face the constant threat of damage on land, different strategies were developed to accommodate skin homeostasis and regeneration. Temporally, they can be under continuous renewal or sloughing cycles. Spatially, they can be diffuse or form discrete localized growth units (follicles). To understand how gene regulatory networks evolved to produce increasingly complex ectodermal organs, we have to study how prototypic scale-forming pathways in reptiles are modulated to produce appendage novelties. Despite the fact that there are numerous studies of reptile scales, molecular analyses have lagged behind. Here, we underscore how further development of this novel experimental model will be valuable in filling the gaps of our understanding of the Evo-Devo of amniote integuments.

Fig. 1. Potential relationship among amniote skin appendages and key evolutionary novel events

The reptilian integument shows some basal characteristics in comparison to mammalian and avian integuments. Mammals evolved about 225 million years ago from Therapsid-type reptiles, while birds evolved about 175 million years ago from archosaurian Saurospids-type reptiles. Key events that led to evolutionary novelty are shown in blue italic characters.

Fig. 2. Schematic drawings showing different types of reptile scales

Scales (resting phase) are shown in multiple layers with names labeled in panel B. (A) Non-overlapping tuberculate type scales. (B) Overlapping scales commonly seen in squamates. (C) Variations of microstructures from the Oberhäutchen layer illustrating short spines in a, b and long setaes in c (such as those in the adhesive pad lamellae in geckos, Fig. 4B). (D) Pits on the scales of anole, gecko and iguana (mainly epidermal sensory organs; Fig. 4 E,F). (E) Tactile sensory organ on the hinge side of a scale in Agama. Some follicle-like structures have clustered dermal cells associated to their base; Fig. 4G). (F) Scales with ridges are seen on the back of skink or the neck of anole. (G) Frills, or very elongated scales, are seen on the back of iguana (Fig. 3B). (H) The horn on the head of chameleon contains a bony element core (osteoderm). (I) Scales on the limb of crocodilians show only minor overlapping. (J) Keeled scales with a central, elevated corneous ridge are seen on the dorsal body of crocodilians and some armored agamid lizards (e.g. Australian spiny desert lizard or molok). Legends: a, fine ‘hair’ on scales of anoles; b, Micro-ornamentation on scales of snakes; c, Toe pad of anole or gecko;*, dermal cells clustered at the base of sensory organs in Agama; AK, α keratin; BK, β keratin; BP, bone element.

Fig. 3. Arrangement and different types of scales in iguana

(A) An adult iguana showing different scale types in different regions. (B–F) Left column: scales from regions designated in (A). Right column: H&E staining of their histological sections on the right. (B) Frills from the midline of the neck. Note the elongated scales compared with those in (C–F). (C) Scales from the dorsal trunk. (D) Scales from the ventral trunk. (E) Scales from the tail. (F) Tuberculate scales from the lateral neck region. Scale bars, 500 µm.

Fig. 4. Unusual scale types

(A,B) Some scales have specialized surfaces to help them climb. (A) Toe pad of anole. (B) Longitudinal sections of digital pads shown in (A). Note the hairy structures on the setae are variations of the Oberhäutchen layer (Fig. 2C). (C,D) Some scales have dorsal ridges (keels) which increase the protective properties of the scales. (C) Dorsal skink scale with three ridges. (D) Cross section of an anole neck scale which exhibits one central ridge (Fig. 2F). (E–G) Some scales form pits, sensory organs with a simple structure at the dermal-epidermal junction (Fig. 2D). (E) Low power view of the distal edge of a scale. (F) Detail of the pit sensory organ in (E) (arrow indicates the sensorial filament derived from Oberhäutchen layer, Fig. 2D). (G) Agama tactile sensory organ. It shows more of a complex epidermal structure with a nerve terminal in its base (arrow). Scale bars, 100 µm.

Fig. 5. Possible models for feather and hair evolution

(A) Possible models for the evolution of feathers. Experiments show that the barb - rachis model is correct. (B) Possible models for the evolution of hairs. The pit-like structures are shown in Figs. 2 and 4. This drawing is from Fig. 6 B,C of Wu et al., 2004.

Fig. 6. A hypothetical model for reptile scale pattern formation, illustrating the effects of different concentrations and with possible involvement of both epithelium and mesenchyme

Numerical solution of a patterning model, as outlined in the Supplementary Information. We assume that a reaction-diffusion model with Schnakenberg-type kinetics (Schnakenberg, 1979) can be used to describe the concentration of two chemicals v and w in the epidermis. (A,B)The concentration profiles of v and w respectively, using the parameter values given in Table 2 (column 3), with no interaction from the dermis. (C–E) illustrate possible pattern profiles that could arise depending on the threshold level chosen for differentiation. (F,G) The concentration profiles of v and w, respectively, using the parameter values given in Table 2 (column 4), with no interaction from the dermis. (H) Spatial map of the pattern assumed to form in the dermis (Shaw and Murray, 1990). (I,J) Results of numerical solution of the model with interaction from the dermis: it is assumed that production of the chemical v is perturbed according to the patterning shown in (C). We see that the wavelengths of each pattern combine to form a more complex arrangement than in either of the individual cases shown in (F,G,H).

Fig. 7. A hypothetical model for snake pigmentation patterns

This is based on the cell-chemotaxis model proposed by Murray and co-workers (Maini et al., 1991; Murray and Myerscough, 1991). (A–E) The various patterns produced when different modes are isolated by suitable choice of model parameters, (F–H) Possible pigmentation patterns that may arise through variation of the differentiation threshold corresponding to (E). The diagrams were produced by plotting periodic functions taking wave numbers calculated in Maini et al. (1991) and Murray and Myerscough, (1991).

Fig 8. Growth, proliferation, differentiation and shedding cycling in anole scales

(A) Schematic drawing showing the histological changes in the epidermis of squamates. The multi-layered epidermis can be divided into two phases of the sloughing cycle. The resting phase is represented by stage 1, which can be further divided into immediate post-shedding phase (1a), the perfect resting phase (1b) and the pre-renewal phase (1c). The renewal phase consists of a series of stages with new layers being formed and specified. Depending on the layers present, they are divided into stage 2 through stage 6. The outer generation (OG) consists of Obo (Oberhäutchen), βo (β-layer), mo (mesos layer), αo (α-layer), lt (lacunar tissue) and cl (clear layer). The new inner generation (IG) comprises Obi (Oberhäutchen), βi (β-layer), mi (mesos layer), αi (a partially formed α-layer). sg, germinal layer; sb, stratum basale. At the right side, the reverse triangle at the end of stage 6 marks the time of shedding. Panel A is from Landmann, 1986. (B) Schematic drawing to show the three major cellular events. Cell proliferation occurs in stage 1c. Specification and initial differentiation of different epidermal layers takes place from stage 2 to stage 4. Terminal differentiation occurs in stages 5 and 6.