Research Article |
Corresponding author: Bilal A. Paray ( bparay@ksu.edu.sa ) Academic editor: Carolina Arruda Freire
© 2017 Bilal A. Paray, Mohammed K. Al-Sadoon.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Paray BA, Al-Sadoon MK (2017) Ultrastructure of the dermal chromatophores in the Fringe-toed lizard, Acanthodactylus orientalis. Zoologia 34: 1-7. https://doi.org/10.3897/zoologia.34.e11923
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Histology and electron microscopic studies of the dorsal skin of the Fringe-toed lizard, Acanthodactylus orientalis Angel, 1936, showed three types of dermal chromatophores: xanthophores, iridophores and melanophores. These pigment cells were observed in vertical combination, with an uppermost layer of xanthophores, an intermediate layer of iridophores and a basal layer of melanophores. The ultrastructure of the melanophore is characterized by oval nucleus and numerous pigment granules, the melanosomes of different stages that remain scattered in the cytoplasm. The chromatophores of this species contain significant information of anatomical similarity with lower as well as higher vertebrates. They can help to better understand the inter relationships between vertebrate pigment cells and their role in skin dysfunctions.
Histology, Xanthophores, lridophores, Melanosomes, anatomical significance
The stratifying, multilayered epidermis that forms the outermost layer of the skin of all vertebrates is a barrier protecting the body from abrasion, dehydration and microbial infections (
In the dermal skin of reptiles four basic types of pigment cells have been recognized: xanthophores, erythrophores, iridophores, and melanophores (
Earlier ultrastructural studies on the melanophores of various classes of vertebrates such as teleosts and lung fishes (
Despite the body of literature available on melanophores, there have been no studies on the fine structure of the dorsal skin chromatophores of the fringe-toed lizard, Acanthodactylus orientalis Angel, 1936. This species is commonly known from central and southern Syria, northern Jordan and western and central Iraq (
Eleven specimens (average SVL = 8.25 cm; average weight 9.02 g) of A. orientalis were captured by the noosing method during the spring of 2014, in the northern part of the region of Turaif (31°40’39”N 38°39’11”E), Kingdom of Saudi Arabia. Each specimen was measured to record SVL to the nearest 0.1 mm and weighed to the nearest 0.1 g. The captured lizards were transported to the Reptilian laboratory of the Zoology Department, College of Science, King Saud University, where all experimental procedures were performed. Animals were kept at ambient temperature (23 ± 1.5 °C) and with natural photoperiod i.e. 12 hours of light-dark cycle. All field data such as locations of the lizards and their altitude were recorded. All animals were euthanized in accordance with the standards set forth in the guidelines for the care and use of experimental animals by the King Saud University, Riyadh; Kingdom of Saudi Arabia.
For the histological studies the dorsal skin of animals were quickly removed from the trunk region with a sharpened steel scissor under ethyl ether anesthesia and fixed in 10% neutral buffered formalin for 72 hours. The fixed specimens were processed overnight for dehydration, clearing and impregnation using an automatic tissue processor (Sakura, Japan). The specimens were embedded in paraffin blocks using embedding station (Sakura, Japan) and sections of 4 µm thickness were cut using rotary microtome (Leica-RM2245, Germany) and an Autostainer (Leica5020, Germany) was used for Hematoxylin and Eosin staining. The stained sections were observed under light microscopy Eclipse BOi (Nikon, Japan) and the images were taken with digital microscopic mounted camera (OMX1200C Nikon, Japan).
For Transmission electron microscopic studies, 2 x 4 mm skin samples were collected from the dorsal region of animals for microscopic observation. The skin samples were immediately fixed in 2.5% glutaraldehyde and 1% paraformaldehyde in 0.1 M phosphate buffer (pH 7.3) overnight at 4 °C. The tissues were rinsed with 0.25 M sucrose in 0.1 M phosphate buffer (pH 7.3) and were then fixed in 0.1 M buffer containing 1% osmium tetraoxide (OsO4) for 1.30 hours at room temperature. Dehydration of the fixed tissue was performed using increasing concentrations of ethanol. The specimens were substituted in propylene oxide before embedding in pure resin (SPI, Toronto; Canada) (
The clear epidermal and dermal layers showed the general reptile skin structure in cross sections of fixed tissue, under the light and the transmission electron microscopes. The epidermal layer consisted of the outer epidermal generation and the stratum germinativum, in cross sections of the dorsal skin when observed under the light microscope (Fig.
Histological structure of dorsal skin in A. orientalis. The chromatophore layer is located just below the basal cell layer in the epidermis. (HL) Horny epidermal layer, (E) epidermis, (SG) stratum germinativum, (I) iridophore, (M) melanophore, (X) xanthophore, (D) dermis, (OD) osteoderm.
Electron microscopic observations of the chromatophores of Acanthodactylus confirm earlier descriptions from light microscopy. In A. orientalis with four pairs of dark gray longitudinal strips on a beige to sandy reddish background of the trunk, epidermal melanophores and three types of dermal chromatophores (xanthophores, iridophores and melanophores) were observed under TEM (Figs
(4-5) Ultrastructural features of chromatophores of dorsal skin in A. orientalis. The vertical combination of dermal chromatophores is xanthophores at the top, iridophores in the middle, and melanophores at the bottom. (6-7) Electron photomicrograph showing the combination of dermal chromatophores in the skin of A. orientalis. (E) Epidermal layer, (I) iridophores, (M) melanophores, (nu) nucleolus, (N) nucleus, (PT) pterinosomes, (X) xanthophores. Scale bar: 2 μm.
Epidermal melanophores with a nucleus at the center were detected by TEM (Figs
Complex skin pigmentation patterns are exhibited by various vertebrate animals. The distribution of skin pigments is the main factor determining the ultimate pigmentation pattern of a species (
Under the light microscope it was observed that the melanophores of A. orientalis are located just below the epidermis, while some melanophores were found scattered in the dermal matrix of the skin. This finding was confirmed by electron microscope. Ultrastructural observations of the epidermal melanophores of this species also revealed the presence of a prominent oval nucleus, consistent with the findings of Ali and Naaz (2014) for the Indian toad, Bufo melanostictus Schneider, 1799. Melanosomes of varying degree of pigmentation were found in the cytoplasm of the melanophore around the nucleus. Mitochondria, vacuolar endoplasmic reticulum and Golgi apparatus were also observed in the cytoplasm. Finding spherical pre-melanosomes near the Golgi apparatus of A. orientalis species confirmed the findings of Seui et al. (1961), who suggested that Golgi vesicles are possible precursors of melanosomes. The immature and developing melanosomes were also observed in the melanophores of this species as described by
Electron microscopic studies of the chromatophores of A. orientalis confirm the scattered combination of dermalchromatophores with the uppermost layer of xanthophores, the intermediate layer with iridophores and melanophores in the basal layer. This is in agreement with the findings of
The present findings throw some light on the morphoanatomic and phylogenetic details of reptilian melanophores. Here, we conclude that studies of the ultrastructure of the dorsal skin melanophores of A. orientalis resemble the condition found in higher vertebrates, including humans. No significant differences were observed. In conjugation with earlier studies, the present data on the ultrastructure of pigment cells continue to suggest that the process of melanin biogenesis is associated with different phases of melanosome development.
The authors would like to express their sincere appreciation to the Deanship of Scientific Research at the King Saud University, Riyadh, Saudi Arabia for funding this Research Group project no RGP-289.