Short-term effects of α-melanocyte-stimulating hormone in three distinct melanin-pigmented cell types of Anura

Ectothermic animals present melanin-containing cells in their integument and viscera. Besides cutaneous melanophores, amphibians have melanomacrophages in the hepatic parenchyma and melanocytes in the viscera, which are also present in their testicular stroma. The native melanocyte stimulating hormone (α-MSH) is the main hormone that modulates the color change in melanophores. However, we still know too little about how the α-MSH acts in vivo on visceral melanin-containing cells. In this study, we collected 30 adult males of Physalaemus nattereri (Anura, Leptodactylidae) to evaluate the short-term effects of α-MSH on melanophores, melanocytes and melanomacrophages under light microscopy. For this, we injected 0.05 ml of a single intraperitoneal dose containing 2.5x10-7 mmol/10g of α-MSH, diluted in ringer solution, in five experimental groups with five individuals each one. The different groups were analyzed after 1, 3, 6, 12 and 24h. The control group with five other individuals received only 0.05 ml of ringer solution. The skin pigmentation increased quickly after animals received the hormone α-MSH with the consequent darkening of the body (body darkness). Melanophores, melanocytes and melanomacrophages responded similarly to the test, with an increase in the area containing melanin. However, melanophores and melanomacrophages reached their darkest pigmentation in a shorter period of time in comparison to the testicular melanocytes, probably due to specific metabolic characteristics of each organ. Thus, we verified that the three types of cells, although present in different organs, are responsive to the native hormone α-MSH, which enables us to treat them as a pigmentary system.

Melanophores inside Frogs

Melanocytes/melanophores were known for some decades as pigment cells in skin. The origin of these cells in embryogenesis from neural crest cells is actively investigated now. Some melanocytes/melanophores were described inside adult vertebrates. Historically, these internal melanocytes have been largely ignored, until recently. In frogs, the melanophores populate not only the skin, but all the inner connective tissues: epineurium, peritoneum, mesentery, outer vascular layer and skin underside. In adult avian, melanocytes were also found in visceral connective tissues, periostea, muscles, ovaries and the peritoneum. In mammals and humans, melanocytes are also revealed in eyes, ears, heart and brain. A black-brownish pigment, which can be found in brains of humans and some mammals, was called neuromelanin. Currently, attempts are being made to treat neurodegenerative diseases and various nerve injuries with medications containing melanin. In this micro-review, we wanted to remind again about the inner melanophores on visceral organs and lining blood vessels and nerves, their importance in organisms resistance to adverse environmental factors.

Melanophores inside Frogs

Melanocytes/melanophores were known for some decades as pigment cells in skin. The origin of these cells in embryogenesis from neural crest cells is actively investigated now. Some melanocytes/melanophores were described inside adult vertebrates. Historically, these internal melanocytes have been largely ignored, until recently. In frogs, the melanophores populate not only the skin, but all the inner connective tissues: epineurium, peritoneum, mesentery, outer vascular layer and skin underside. In adult avian, melanocytes were also found in visceral connective tissues, periostea, muscles, ovaries and the peritoneum. In mammals and humans, melanocytes are also revealed in eyes, ears, heart and brain. A black-brownish pigment, which can be found in brains of humans and some mammals, was called neuromelanin. Currently, attempts are being made to treat neurodegenerative diseases and various nerve injuries with medications containing melanin. In this micro-review, we wanted to remind again about the inner melanophores on visceral organs and lining blood vessels and nerves, their importance in organisms resistance to adverse environmental factors.

Environmental correlates of internal coloration in frogs vary throughout space and lineages

Internal organs of ectotherms have melanin‐containing cells that confer different degrees of coloration to them. Previous experimental studies analyzed their developmental origin, role in immunity, and hormonal regulation. For example, melanin increases with ultraviolet radiation (UV) and temperature in frogs and fish. However, little is known about how environmental variables influence the amount of coloration on organs among amphibian species over a large spatial extent. Here, we tested how climatic variables (temperature, UV, and photoperiod) influence the coloration of internal organs of anurans. We recorded the level of melanin pigmentation using four categories on 12 internal organs and structures of 388 specimens from 43 species belonging to six anuran families. Then, we tested which climatic variables had the highest covariation with the pigmentation on each organ after controlling for spatial autocorrelation in climatic variables and phylogenetic signal in organ coloration using the extended version of the RLQ ordination. Coloration in all organs was correlated with the phylogeny. However, the coloration of different organs was affected by different variables. Specifically, the coloration of the heart, kidneys, and rectum of hylids, Rhinella schneideri, some Leptodactylus, and Proceratophrys strongly covaried with temperature and photoperiod, whereas that of the testicle, lumbar parietal peritoneum, lungs, and mesenterium of Leiuperinae, Hylodidae, Adenomera, and most Leptodactylus had highest covariation with UV‐B and temperature. Our results support the notion that melanin pigmentation on the surface of organs of amphibians has an adaptive function conferred by the protective functions of the pigment. But most importantly, internal melanin seems to respond differently to climatic variables depending on the lineage and locality in which species occur.

Internal pigment cells respond to external UV radiation in frogs

Fish and amphibians have pigment cells that generate colorful skins important for signaling, camouflage, thermoregulation, and protection against ultraviolet radiation (UVR). However, many animals have also pigment cells inside their bodies, on their internal organs and membranes. In contrast to external pigmentation, internal pigmentation is remarkably little studied and its function is not well known. Here, we tested genotoxic effects of UVR and its effects on internal pigmentation in a Neotropical frog, Physalaemus nattereri. We found an increased body darkness and internal melanin pigmentation in testes and heart surfaces, in the mesenterium and lumbar region after just a few hours of UVR exposure. The melanin dispersion in melanomacrophages in the liver and melanocytes in testes increased after UV exposure. In addition, the amount of melanin inside melanomacrophages cells increased too. While mast cells were quickly activated by UVR, only longer UVR exposures resulted in genotoxic effects inside frogs, by increasing the frequency of micronuclei in red blood cells. This is the first study to describe systemic responses of external UVR on internal melanin pigmentation, melanomacrophages, and melanocytes in frogs and thus provide a functional explanation to the presence of internal pigmentation.