Histology, ultrastructure, and pigmentation in the horny scales of growing crocodilians

Abstract

Alibardi L. 2011. Histology, ultrastructure, and pigmentation in the horny scales of growing crocodilians. -Acta Zoologica (Stockholm) 92: 187-200.The present morphological study describes the color of hatchling, juvenile, and adult crocodilian skin and the origin of its pigmentation. In situ hybridization and immunostaining indicate that crocodilian scales grow as an expansion of the proliferating epidermis of the hinge region that form thin lateral rings. In more central areas of growing scales, new epidermal layers contribute to increase the thickness of the stratum corneum. The dark pigmentation and color pattern derive from the different distribution of epidermal and dermal chromatophores. The more intensely pigmented stripes, irregular patches and dot-like spots, especially numerous in dorsal scales, derive from the incorporation of the eumelanosomes of epidermal melanocytes in differentiating beta cells of the epidermis. Dermal melanophores, mainly localized in the loose upper part of the dermis, also contribute to the formation of the dark or gray background of crocodilian scales. The eumelanosomes of dermal melanophores determine the darkening of the skin pattern in association with the epidermal melanocytes. Iridophores are infrequent, while xantophores are present in the species analyzed with a sparse distribution in the superficial dermis among melanophores. The presence of xantophores and of the few iridophores in areas where epidermal melanocytes are absent appear to determine the brown or the light yellow-orange background observed among the darker regions of crocodilian scales.

Cytology and localization of chromatophores in the skin of the Tuatara (Sphenodon punctaus)

“…Although it is not known whether melanosomes in S. punctatus contains eu-or ⁄ and pheo-melanins, it is thought that the dark coloration of reptilian skin only derives from the presence of eumelanosomes, a characteristic of lower vertebrates (Bagnara and Matsumoto 2006;Bagnara et al 2007). This confirms previous studies on lizards, snakes, crocodilian, and turtles where only melanosomes with the typical ultrastructure for eu-melanosomes were found (Breathnach and Poyntz 1966;Landmann 1986;Alibardi 2011). As the chemical content of these organelles is not known, it remains possible that reptilian melanosomes may contain pheomelanines.…”

“…The epidermal and dermal organization and the type of chromatophores involved in the morphological variation in lizards (Breathnach and Poyntz 1966;Sherbrooke and Frost 1989;Kuriyama et al 2006), snakes Ionescu 1972, 1973;Szabo et al 1973;Gosner 1989), crocodilians (Spearman and Riley 1969;Richardson et al 2000;Alibardi 2011), and turtles is known in general. In these reptiles, the skin appears pigmented with black areas forming small or large dots or stripes that are interspersed among the brown, gray or pale yellow areas.…”

Crocodylians evolved scattered multi-sensory micro-organs

“…The toughness of crocodylian skin and scales results from both the strongly collagenous dermis and the presence of a hard β-keratinized, stratified epidermis [4-8]. In addition, crocodylian skin is yet harder and thicker (i) in dorso-caudal regions where it contains osteoderms (bony plates) [9,10] and (ii) on the face and jaws, where the scales are not genetically controlled developmental units, but emerge from a process entirely analogous to physical cracking of a living tissue in a stress field [11]. …”

Structure, innervation and response properties of integumentary sensory organs in crocodilians

“…The stratum corneum is thin over the organ (5m), presumably allowing a range of motions to compress the structure. This layer of three to five -keratin cells (Alibardi, 2011) functions both in structural integrity of the ISO and acts as scaffolding for the most apical of the fine nerve terminals. In transverse sections, highly branched melanocytes can be seen throughout the keratinized layers and underlying collagenous layers and impart the distinctive pigmentation seen in most of the ISO bodies.…”