Papillary and Callous Scales in the Integument of Agamid Lizards (Agamidae, Sauria) as a Phenomenon of Extraordinary Development of the Corneous Layers

TL;DR

This study explores unusual thick scales in male agamid lizards, which may help with protection or mating, and compares them to similar structures in other animals.

Contribution

The paper identifies and describes two new types of modified scales in agamid lizards and their potential developmental and functional significance.

Findings

Two types of modified scales with thick corneous layers were found in male agamid lizards.

These scales are absent in juveniles and less developed in some females and subadults.

The thick corneous layer is linked to a well-developed papillary dermal layer promoting keratinocyte production.

Abstract

Using visual inspection, light and scanning electron microscopy, we studied the integument of six species of the genus Acanthocercus and Laudakia nupta (family Agamidae). Two types of unusual scales were found. Scales covered with an extremely thick corneous layer were detected around the tail base and on the palmar and plantar limb surfaces of all species. When the corneous layer was removed, the surface of such scales remained covered with numerous high papillae (“papillary scales”). Non-overlapping callous-like scales (“callous scales”) were placed in front of cloaca of all Acanthocercus species and in front of the cloaca and in the central part of the belly of L. nupta. Modified scales were certainly male characteristics, developed to a lesser degree and only in some females and subadut specimens, and absent in juveniles. The dermis of modified scales had a well-developed papillary layer that expended the total surface area of stratum germinativum, created a pool of proliferated cells, and promoted a higher production of intensively differentiating keratinocytes with final formation of thick corneous layer that distinguish both types of scales. The functions of modified scales in lizards and some analogies with specialized areas of normal and pathological epidermis of birds and mammals are discussed.

Scaled integument of six species of the genus Acanthocercus and Laudakia nupta, family Agamidae was studied using light and scanning electron microscopy. Gross observation revealed the presence of two types of modified scales in the males. The enlarged scales covered with an extremely thick β-corneous layer were detected in the dorsal and ventral surfaces of the tail base and on the palmar and plantar limb surfaces of all species. After detachment of the β-layer, the surface of such scales was covered with high papillae (“papillary scales”). The callous scales were found in the precloacal region of Acanthocercus species and in both precloacal and mid-ventral regions of L. nupta. Modified scales were found in some females and subadut specimens, and absent in juveniles. A prominent papillary layer characterized the dermis of both scale types. It was assumed that well-developed dermal papillae in such scales expended the total surface area of stratum germinativum and created a pool of proliferated cells in the interpapillary loops to increase the production of differentiating keratinocytes. These processes were undoubtedly associated with the formation of a thick and resistant corneous layer that distinguished both types of scales. Functional role, a taxonomic value, and analogies with normal and pathological epidermis in birds and mammals are discussed.

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