Light-induced pigment aggregation in xanthophores of the medaka, Oryzias latipes

Protein Kinase A Signaling Inhibits Iridophore Differentiation in Zebrafish

In zebrafish (Danio rerio), iridophores are specified from neural crest cells and represent a tractable system for examining mechanisms of cell fate and differentiation. Using this system, we have investigated the role of cAMP protein kinase A (PKA) signaling in pigment cell differentiation. Activation of PKA with the adenylyl cyclase activator forskolin reduces the number of differentiated iridophores in wildtype larvae, with insignificant changes to melanophore number. Inhibition of PKA with H89 significantly increases iridophore number, supporting a specific role for PKA during iridophore development. To determine the effects of altering PKA activity on iridophore and melanophore gene expression, we examined expression of iridophore marker pnp4a, melanophore marker mitfa, and the mitfa repressor foxd3. Consistent with our cell counts, forskolin significantly decreased pnp4a expression as detected by in situ hybridization and quantification of pnp4a+ cells. Forskolin had the opposite effect on mitfa and foxd3 gene activity, increasing the area of expression. As mitfa/nacre mutants have extra iridophores as compared to wildtype larvae, we examined the function of mitfa during PKA-sensitive iridophore development. Forskolin treatment of mitfa/nacre mutants did significantly reduce the number of iridophores but to a lesser extent than that observed in treated wildtype larvae. Taken together, our data suggests that PKA inhibits iridophore development in a subset of iridophore precursors, potentially via a foxd3-independent pathway.

Gross anatomy and ultrastructure of Moorish Gecko, Tarentola mauritanica skin

The epidermis of Tarentola mauritanica in the skin regions of back, flank and belly has been described using light and electron microscopy. This animal model was useful to give an insight of the functional pattern involved in pigmentation, cryptism and photosensitivity. Skin from back and flanks, in electron microscopy, shows a high concentration of chromatophores, among those melanophores, xanthophores and iridophores have been reported. Interestingly, in the flank-back transition region electron microscopy reveals the presence of nerve endings. Our contribution adds new knowledge about the skin of this species, and it could be useful to study in deep the mechanism of cryptic colour change in reptiles.

Zebrafish stripes as a model for vertebrate colour pattern formation

Colour patterns are prominent features of many animals and have important functions in communication, such as camouflage, kin recognition and mate choice. As targets for natural as well as sexual selection, they are of high evolutionary significance. The molecular mechanisms underlying colour pattern formation in vertebrates are not well understood. Progress in transgenic tools, in vivo imaging and the availability of a large collection of mutants make the zebrafish (Danio rerio) an attractive model to study vertebrate colouration. Zebrafish display golden and blue horizontal stripes that form during metamorphosis as mosaics of yellow xanthophores, silvery or blue iridophores and black melanophores in the hypodermis. Lineage tracing revealed the origin of the adult pigment cells and their individual cellular behaviours during the formation of the striped pattern. Mutant analysis indicated that interactions between all three pigment cell types are required for the formation of the pattern, and a number of cell surface molecules and signalling systems have been identified as mediators of these interactions. The understanding of the mechanisms that underlie colour pattern formation is an important step towards deciphering the genetic basis of variation in evolution.

Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapia Oreochromis niloticus

Non-visual photoreceptors with diverse photopigments allow organisms to adapt to changing light conditions. Whereas visual photoreceptors are involved in image formation, non-visual photoreceptors mainly undertake various non-image-forming tasks. They form specialised photosensory systems that measure the quality and quantity of light and enable appropriate behavioural and physiological responses. Chromatophores are dermal non-visual photoreceptors directly exposed to light and they not only receive ambient photic input but also respond to it. These specialised photosensitive pigment cells enable animals to adjust body coloration to fit environments, and play an important role in mate choice, camouflage and ultraviolet (UV) protection. However, the signalling pathway underlying chromatophore photoresponses and the physiological importance of chromatophore colour change remain under-investigated. Here, we characterised the intrinsic photosensitive system of red chromatophores (erythrophores) in tilapia. Like some non-visual photoreceptors, tilapia erythrophores showed wavelength-dependent photoresponses in two spectral regions: aggregations of inner pigment granules under UV and short-wavelengths and dispersions under middle- and long-wavelengths. The action spectra curve suggested that two primary photopigments exert opposite effects on these light-driven processes: SWS1 (short-wavelength sensitive 1) for aggregations and RH2b (rhodopsin-like) for dispersions. Both western blot and immunohistochemistry showed SWS1 expression in integumentary tissues and erythrophores. The membrane potential of erythrophores depolarised under UV illumination, suggesting that changes in membrane potential are required for photoresponses. These results suggest that SWS1 and RH2b play key roles in mediating intrinsic erythrophore photoresponses in different spectral ranges and this chromatically dependent antagonistic photosensitive mechanism may provide an advantage to detect subtle environmental photic change.

Possible involvement of cone opsins in distinct photoresponses of intrinsically photosensitive dermal chromatophores in tilapia Oreochromis niloticus

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.

The signaling pathway in photoresponses that may be mediated by visual pigments in erythrophores of Nile tilapia

The ability to increase the synthesis or vary the distribution of pigment in response to light is an important feature of many pigment cells. Unlike other light-sensitive pigment cells, erythrophores of Nile tilapia change the direction of pigment migration depending on the peak wavelength of incident light: light near 365, 400 or 600 nm induces pigment aggregation, while dispersion occurs in response to light at 500 nm. How these phenomena are achieved is currently unknown. In the present study, the phototransduction involved in the pigment dispersion caused by light at 500 nm or the aggregation by light at 600 nm was examined, using pertussis toxin, cholera toxin, blockers of ion channels, various chemicals affecting serial steps of signaling pathways and membrane-permeable cAMP analog. The results show that light-induced bidirectional movements in tilapia erythrophores may be controlled by cytosolic cAMP levels via Gi- or Gs-type G proteins. In addition, RT-PCR demonstrated for the first time the expression of mRNAs encoding red and green opsins in tilapia fins, only where erythrophores exist. Here, we suggest that multiple cone-type visual pigments may be present in the erythrophores, and that unique cascades in which such opsins couple to Gi or Gs-type G proteins are involved in the photoresponses in these pigment cells. Thus, tilapia erythrophore system seems to be a nice model for understanding the photoresponses of cells other than visual cells.

Esrom, an ortholog of PAM (protein associated with c-myc), regulates pteridine synthesis in the zebrafish

Zebrafish esrom mutants have an unusual combination of phenotypes: in addition to a defect in the projection of retinal axons, they have reduced yellow pigmentation. Here, we investigate the pigment phenotype and, from this, provide evidence for an unexpected defect in retinal neurons. Esrom is not required for the differentiation of neural crest precursors into pigment cells, nor is it essential for cell migration, pigment granule biogenesis, or translocation. Instead, loss of yellow color is caused by a deficiency of sepiapterin, a yellow pteridine. The level of several other pteridines is also affected in mutants. Importantly, the cofactor tetrahydrobiopterin (BH4) is drastically reduced in esrom mutants. Mutant retinal neurons also appear deficient in this pteridine. BH4-synthesizing enzymes are active in mutants, indicating a defect in the regulation rather than production of enzymes. Esrom has recently been identified as an ortholog of PAM (protein associated with c-myc), a very large protein involved in synaptogenesis in Drosophila and C. elegans. These data thus introduce a new regulator of pteridine synthesis in a vertebrate and establish a function for the Esrom protein family outside synaptogenesis. They also raise the possibility that neuronal defects are due in part to an abnormality in pteridine synthesis.

Direct Effects of Visible and UVA Light on Pigment Migration in Erythrophores of Nile Tilapia

Erythrophores derived from Nile tilapia (Oreochromis niloticus) are sensitive to visible light of defined wavelengths in primary culture in the same manner as erythrophores in the skin. Cultured erythrophores aggregate their pigment in response to light with peak wavelengths near 400 or 600 nm, while dispersion is caused by light near 500 nm. In this study, we report that ultraviolet A (UVA) with a peak wavelength near 365 nm also induces pigment aggregation in erythrophores in the skin and in primary culture. The responses of erythrophores in the skin or in culture depend on the light intensity, although the photo-sensitivity differs among individual cells. From the results, we conclude that the action of visible light and UVA light on tilapia erythrophores is direct, and that multiple types of visual pigments may coexist in individual erythrophores.

Cutaneous biology and diseases of fish

The normal structure and function of the piscine integument reflects the adaptation of the organism to the physical, chemical, and biological properties of the aquatic environment, and the natural history of the organism. Because of the intimate contact of fish with the environment, cutaneous disease is relatively more common in fish than in terrestrial vertebrates and is one of the primary disease conditions presented to the aquatic animal practitioner. However, cutaneous lesions are generally nonspecific and may be indicative of disease that is restricted to the integument or a manifestation of systemic disease. Regardless, a gross and microscopic examination of the integument is simple to perform, but is highly diagnostic and should always be included in the routine diagnostic effort of the aquatic animal practitioner, especially since various ancillary diagnostic procedures are either not practical or lack predictive value in fish. The purpose of this article is to provide an overview of normal cutaneous biology prior to consideration of specific cutaneous diseases in fish.