Pigment Cell Types Research Articles

Structural and ultrastructural aspects of the skin of large yellow croaker Larimichthys crocea.

The skin color of the large yellow croaker (Larimichthys crocea) is a crucial indicator to determine its economic value. However, the location of pigment cells in the skin structure is uncertain. To determine the pigment cell type in the skin, the vertical order and ultrastructure of pigment cells were examined using light microscopy and transmission electron microscopy. Both dorsal and ventral skins comprise the epidermis, dermis, and hypodermis. Xanthophores, melanophores, and iridophores were observed in the dermis of the dorsal skin, whereas the latter two were in the dermis of the ventral skin. Interestingly, the size of xanthophores in the dorsal skin was significantly smaller than that of xanthophores in the ventral skin; however, the density of dorsal xanthophores was significantly higher than that of ventral xanthophores. The type L-iridophores with large crystalline structures were observed in the uppermost area of the upper pigment layer, which contributed to the strikingly metallic luster shown by the ventral skin. The melanophores were exclusively found in the dorsal skin, offering the purpose of camouflage. Taken together, our results indicated that the pigment cells display different arrangement patterns between dorsal and ventral skin, and the golden color in the ventral skin results from the coexistence of light-reflecting iridophores and light-absorbing xanthophores.

Transcriptional background effects on a tumor driver gene in different pigment cell types of medaka.

The Xiphophorus melanoma receptor kinase gene, xmrk, is a bona fide oncogene driving melanocyte tumorigenesis of Xiphophorus fish. When ectopically expressed in medaka, it not only induces development of several pigment cell tumor types in different strains of medaka but also induces different tumor types within the same animal, suggesting its oncogenic activity has a transcriptomic background effect. Although the central pathways that xmrk utilizes to lead to melanomagenesis are well documented, genes and genetic pathways that modulate the oncogenic effect and alter the course of disease have not been studied so far. To understand how the genetic networks between different histocytes of xmrk-driven tumors are composed, we isolated two types of tumors, melanoma and xanthoerythrophoroma, from the same xmrk transgenic medaka individuals, established the transcriptional profiles of both xmrk-driven tumors, and compared (1) genes that are co-expressed with xmrk in both tumor types, and (2) differentially expressed genes and their associated molecular functions, between the two tumor types. Transcriptomic comparisons between the two tumor types show melanoma and xanthoerythrophoroma are characterized by transcriptional features representing varied functions, indicating distinct molecular interactions between the driving oncogene and the cell-type-specific transcriptomes. Melanoma tumors exhibit gene signatures that are relevant to proliferation and invasion, while xanthoerythrophoroma tumors are characterized by expression profiles related to metabolism and DNA repair. We conclude the transcriptomic backgrounds, exemplified by cell-type-specific genes that are downstream of xmrk effected signaling pathways, contribute the potential to change the course of tumor development and may affect overall tumor outcomes.

Generation of translucent Xenopus tropicalis through triple knockout of pigmentation genes.

Amphibians generally have three types of pigment cells, namely, melanophores (black and brown), xanthophores (yellow and red), and iridophores (iridescent). Single knockout of the tyr, slc2a7, and hps6 genes in Xenopus tropicalis results in the absence of melanophores, xanthophores, and iridophores, respectively. The generation of triple- knockout (3KO) X. tropicalis for these three genes could allow for observation of internal organs without sacrificing the animals, which would be transparent due to the absence of pigments. In this study, we generated 3KO X. tropicalis, which is one of the most widely used model amphibians, through crossing of a slc2a7 single-knockout frog with a tyr and hps6 double-knockout frog, followed by intercrossing of their offspring. The 3KO tadpoles had transparent bodies like the nop mutant and the frogs had translucent bodies. This translucency allowed us to observe the heart, lungs, stomach, liver, and digestive tract through the ventral body skin without surgery. After intravital staining, 3KO X. tropicalis showed much clearer fluorescent signals of mineralized tissues compared with the wild type. These 3KO X. tropicalis provide a useful mutant line for continuous observation of internal organs and fluorescent signals in the body. In particular, such 3KO frogs would revolutionize fluorescence monitoring in transgenic tadpoles and frogs expressing fluorescent proteins.

A gene regulatory network combining Pax3/7, Sox10 and Mitf generates diverse pigment cell types in medaka and zebrafish.

Neural crest cells generate numerous derivatives, including pigment cells, and are a model for studying how fate specification from multipotent progenitors is controlled. In mammals, the core gene regulatory network for melanocytes (their only pigment cell type) contains three transcription factors, Sox10, Pax3 and Mitf, with the latter considered a master regulator of melanocyte development. In teleosts, which have three to four pigment cell types (melanophores, iridophores and xanthophores, plus leucophores e.g. in medaka), gene regulatory networks governing fate specification are poorly understood, although Mitf function is considered conserved. Here, we show that the regulatory relationships between Sox10, Pax3 and Mitf are conserved in zebrafish, but the role for Mitf is more complex than previously emphasized, affecting xanthophore development too. Similarly, medaka Mitf is necessary for melanophore, xanthophore and leucophore formation. Furthermore, expression patterns and mutant phenotypes of pax3 and pax7 suggest that Pax3 and Pax7 act sequentially, activating mitf expression. Pax7 modulates Mitf function, driving co-expressing cells to differentiate as xanthophores and leucophores rather than melanophores. We propose that pigment cell fate specification should be considered to result from the combinatorial activity of Mitf with other transcription factors.

Leukocyte Tyrosine Kinase (Ltk) Is the Mendelian Determinant of the Axolotl Melanoid Color Variant.

The great diversity of color patterns observed among amphibians is largely explained by the differentiation of relatively few pigment cell types during development. Mexican axolotls present a variety of color phenotypes that span the continuum from leucistic to highly melanistic. The melanoid axolotl is a Mendelian variant characterized by large numbers of melanophores, proportionally fewer xanthophores, and no iridophores. Early studies of melanoid were influential in developing the single-origin hypothesis of pigment cell development, wherein it has been proposed that all three pigment cell types derive from a common progenitor cell, with pigment metabolites playing potential roles in directing the development of organelles that define different pigment cell types. Specifically, these studies identified xanthine dehydrogenase (XDH) activity as a mechanism for the permissive differentiation of melanophores at the expense of xanthophores and iridophores. We used bulked segregant RNA-Seq to screen the axolotl genome for melanoid candidate genes and identify the associated locus. Dissimilar frequencies of single-nucleotide polymorphisms were identified between pooled RNA samples of wild-type and melanoid siblings for a region on chromosome 14q. This region contains gephyrin (Gphn), an enzyme that catalyzes the synthesis of the molybdenum cofactor that is required for XDH activity, and leukocyte tyrosine kinase (Ltk), a cell surface signaling receptor that is required for iridophore differentiation in zebrafish. Wild-type Ltk crispants present similar pigment phenotypes to melanoid, strongly implicating Ltk as the melanoid locus. In concert with recent findings in zebrafish, our results support the idea of direct fate specification of pigment cells and, more generally, the single-origin hypothesis of pigment cell development.

Light Microscopy and Ultrastructure of Body Wall in Leech Haemadipsa zeylanica.

Leeches are widely distributed worldwide in a diversity of habitats, such as freshwater, seas, desert, and oases. Since literature does not have an elaborate study on histology of leeches, thus emphasis is given to study the histology. The aim of this study is to examine the histology of free-living leeches Haemadipsa zeylanica under light microscopy and extended to ultrastructural study under transmission electron microscopy. Tissue was fixed in 10% formalin, dehydrated with alcohol, infiltrated and embedded in wax, and thin section was cut using a rotary microtome. Sections were stained with hematoxylin and eosin. The epithelial layer of cells in H. zeylanica had a thin layer of cuticles covering it. In light microscopy, the epidermis has three types of epidermal cells, namely, glandular cells (Type I, Type II, and Type III), supporting cells (Type IV), and pigment cells (Type V). When the three glandular cells of the body wall of H. zeylanica epidermis were examined under an electron microscope, three types of distinct electron-dense granules were identified. Type I cells with granules, Type II cells with course granules close to the dermis were identified as pear-shaped secretory cells, and Type III cells with electron-dense granules.

Comparison of vertebrate skin structure at class level: A review.

The skin is a barrier between the internal and external environment of an organism. Depending on the species, it participates in multiple functions. The skin is the organ that holds the body together, covers and protects it, and provides communication with its environment. It is also the body's primary line of defense, especially for anamniotes. All vertebrates have multilayered skin composed of three main layers: the epidermis, the dermis, and the hypodermis. The vital mission of the integument in aquatic vertebrates is mucus secretion. Cornification began in apmhibians, improved in reptilians, and endured in avian and mammalian epidermis. The feather, the most ostentatious and functional structure of avian skin, evolved in the Mesozoic period. After the extinction of the dinosaurs, birds continued to diversify, followed by the enlargement, expansion, and diversification of mammals, which brings us to the most complicated skin organization of mammals with differing glands, cells, physiological pathways, and the evolution of hair. Throughout these radical changes, some features were preserved among classes such as basic dermal structure, pigment cell types, basic coloration genetics, and similar sensory features, which enable us to track the evolutionary path. The structural and physiological properties of the skin in all classes of vertebrates are presented. The purpose of this review is to go all the way back to the agnathans and follow the path step by step up to mammals to provide a comparative large and updated survey about vertebrate skin in terms of morphology, physiology, genetics, ecology, and immunology.

Genetic and correlative light and electron microscopy evidence for the unique differentiation pathway of erythrophores in brown trout skin

Based on their cell ultrastructure, two types of erythrophores in the spotted skin regions of brown trout (Salmo trutta) were previously described. To test the hypothesis regarding the origin of a new cell type following genome duplication, we analysed the gene and paralogue gene expression patterns of erythrophores in brown trout skin. In addition, the ultrastructure of both erythrophore types was precisely examined using transmission electron microscopy (TEM) and correlative light microscopy and electron microscopy (CLEM). Ultrastructural differences between the sizes of erythrophore inclusions were confirmed; however, the overlapping inclusion sizes blur the distinction between erythrophore types, which we have instead defined as cell subtypes. Nevertheless, the red spots of brown trout skin with subtype 2 erythrophores, exhibited unique gene expression patterns. Many of the upregulated genes are involved in melanogenesis or xanthophore differentiation. In addition, sox10, related to progenitor cells, was also upregulated in the red spots. The expressions of paralogues derived from two genome duplication events were also analysed. Multiple paralogues were overexpressed in the red spots compared with other skin regions, suggesting that the duplicated gene copies adopted new functions and contributed to the origin of a new cell subtype that is characteristic for red spot. Possible mechanisms regarding erythrophore origin are proposed and discussed. To the best of our knowledge, this is the first study to evaluate pigment cell types in the black and red spots of brown trout skin using the advanced CLEM approach together with gene expression profiling.

Tfap2b specifies an embryonic melanocyte stem cell that retains adult multifate potential.

SummaryMelanocytes, the pigment-producing cells, are replenished from multiple stem cell niches in adult tissue. Although pigmentation traits are known risk factors for melanoma, we know little about melanocyte stem cell (McSC) populations other than hair follicle McSCs and lack key lineage markers with which to identify McSCs and study their function. Here we find that Tfap2b and a select set of target genes specify an McSC population at the dorsal root ganglia in zebrafish. Functionally, Tfap2b is required for only a few late-stage embryonic melanocytes, and is essential for McSC-dependent melanocyte regeneration. Fate mapping data reveal that tfap2b+ McSCs have multifate potential, and are the cells of origin for large patches of adult melanocytes, two other pigment cell types (iridophores and xanthophores), and nerve-associated cells. Hence, Tfap2b confers McSC identity in early development, distinguishing McSCs from other neural crest and pigment cell lineages, and retains multifate potential in the adult zebrafish.

Water column use by reef fishes of different color patterns

ABSTRACT Color in animals responds to selective pressures and mediates the relationship between organism and environment. Reef fishes have the amplest variety of pigment cell types. This color patterns’ variety may function as camouflage and be related to spatial use. We tested the hypothesis that the coloration of reef fish relates to water column stratum occupation. We predicted that sedentary animals connected to the background take advantage of background matching or disruptive patterns; more mobile demersal species apply disruptive coloration or motion-dazzle; and that pelagic species tend to have silvery bodies. We classified color patterns and categorized the water column stratum use for the Brazilian reef fishes in FishBase. Our analyses confirmed that irregular contrasting contour breaks, suggestive of disruptive coloration, occurs in benthic species and that silvering color was more prevalent in the pelagic stratum. Our raw data suggested a higher frequency of contrasting regular stripes, typical of motion-dazzle, in demersal species. However, the considerable uncertainty around estimates did not confirm this pattern. Reef fishes coloration is correlated to occupation of different strata in the water column. This can be interpreted as fishes being adapted to these habitats and partially explaining the richness of color patterns among them.

Analysis of pigment cell composition, pigment content, tyrosinase content and activity of three kinds of loaches Misgurnus anguillicaudatus from Poyang Lake.

Pigment cell composition, pigment content, tyrosinase content and activity analysis were investigated on three kinds of loaches Misgurnus anguillicaudatus: big blackspot loaches (BBL), small blackspot loaches (SBL) and non-blackspot loaches (NBL), from Poyang Lake. Results showed that there were three types of skin pigment cells, namely melanophores, xanthophores and iridophores. Melanophores in dorsum were more than those in abdomen. Melanophore cytosomes in BBL were larger than those in SBL and NBL, and melanosomes were the largest in stage four. The melanophores in dorsal skin of SBL or NBL were small cell bodies, spindle-like and in chain distribution. There was an extremely significant difference in melanin content in BBL between the dorsum and abdomen (P <0.01). There were no significant differences in melanin abdominal content, lutein and carotenoid contents among three kinds of loaches (P >0.05). In dorsal skin, tyrosinase content was the highest in BBL, and it was significantly lower in NBL than in BBL and SBL (P <0.01). This study reveals the differences in pigment and tyrosinase content in three kinds of loaches and provides a theoretical basis for further study of the mechanism of black spot formation.

Tfap2b Specifies an Embryonic Melanocyte Stem Cell That Retains Adult Multi-Fate Potential

Melanocytes, our pigment producing cells, are replenished from multiple stem cell niches in adult tissues. Although pigmentation traits are known risk-factors for melanoma, we know little about melanocyte stem cell (MSC) populations other than hair follicle MSCs, and lack key lineage markers with which to identify MSCs and study their function. Here, we discover that Tfap2b, and a select set of its target genes, specifies an MSC population at the dorsal root ganglia in zebrafish. Functionally, Tfap2b is required for only a few late-stage embryonic melanocytes, and instead is essential for MSC-dependent melanocyte regeneration. Fate-mapping data reveal that tfap2b-expressing MSCs have multi-fate potential, and are the cell-of-origin for large patches of adult melanocytes, and two other pigment cell types, iridophores and xanthophores. Hence, Tfap2b confers MSC identity in early development, thereby distinguishing MSCs from other neural crest and pigment cell lineages, and retains multi-fate potential in the adult zebrafish.

Fish pigmentation and coloration: Molecular mechanisms and aquaculture perspectives

AbstractFish coloration, evolved from selection and adaptation, is a prominent feature of remarkable aquaculture merits. Fish possess more pigment cell types than any other vertebrates and are an excellent model for the study of mechanisms underlying skin coloration and pigmentation at levels from molecular genetics to system biology. How to effectively improve skin colour in ornamental and aquaculture fish has long been a focus on selective breeding programmes. In this review article, we introduce different types of chromatophore and early development of pigmentation, describe cellular mechanisms related to morphological and physiological changes of body colour and demonstrate applications of single‐sex control and molecular marker–assisted breeding in body colour selection. We also reviewed genes that have been found involving in neural crest migration and development, as well as those involved in melanin‐based coloration and other types of pigmentation, and explored how to use transgenic technology for enhancing fish body colour. The applications of genome editing and other omics technologies in relation to fish coloration are also illustrated. Finally, we present our perspectives about future fish coloration research and practice in aquaculture. Taken together, this review is expected to provide an update on cellular and molecular mechanisms in fish coloration and pigmentation, which will likely promote body colour–based molecular breeding programmes in aquaculture.

Nile Tilapia: A Model for Studying Teleost Color Patterns.

The diverse color patterns of cichlid fishes play an important role in mate choice and speciation. Here we develop the Nile tilapia (Oreochromis niloticus) as a model system for studying the developmental genetics of cichlid color patterns. We identified 4 types of pigment cells: melanophores, xanthophores, iridophores and erythrophores, and characterized their first appearance in wild-type fish. We mutated 25 genes involved in melanogenesis, pteridine metabolism, and the carotenoid absorption and cleavage pathways. Among the 25 mutated genes, 13 genes had a phenotype in both the F0 and F2 generations. None of F1 heterozygotes had phenotype. By comparing the color pattern of our mutants with that of red tilapia (Oreochromis spp), a natural mutant produced during hybridization of tilapia species, we found that the pigmentation of the body and eye is controlled by different genes. Previously studied genes like mitf, kita/kitlga, pmel, tyrb, hps4, gch2, csf1ra, pax7b, and bco2b were proved to be of great significance for color patterning in tilapia. Our results suggested that tilapia, a fish with 4 types of pigment cells and a vertically barred wild-type color pattern, together with various natural and artificially induced color gene mutants, can serve as an excellent model system for study color patterning in vertebrates.

From Melanocytes to Melanoma Cells: Characterization of the Malignant Transformation by Four Distinctly Different Melanin Fluorescence Spectra (Review).

The melanin fluorescence emitted by pigment cells of the human skin has been a central research topic for decades, because melanin, on the one hand, protects against (solar) radiation in the near-UV range, whereas on the other hand, melanocytes are the starting point for the malignant transformation into melanoma. Until recently, however, melanin fluorescence was not accessible in the context of conventional spectroscopy, because it is ultraweak and is overshadowed by the more intense so-called autofluorescence of endogenous fluorophores. The advent of a new method of laser spectroscopy has made this melanin fluorescence measurable in vivo. A stepwise two-photon absorption with 800 nm photons is used, which more selectively excites melanin (dermatofluoroscopy). Our review summarizes the experimental results on melanin fluorescence of the four types of cutaneous pigment cells from healthy and malignant tissues. Outstanding is the finding that different types of melanocytes (i.e., melanocytes of common nevi, versus dysplastic nevi or versus melanoma cells) show characteristically different fluorescence spectra. The possibilities of using this melanin fluorescence for melanoma diagnosis are shown. Moreover, the uniform fluorescence spectra emitted by different melanoma subtypes are essential. Conclusions are drawn about the molecular processes in the melanosomes that determine fluorescence. Finally, experimental suggestions for further investigations are given.

The MITF paralog tfec is required in neural crest development for fate specification of the iridophore lineage from a multipotent pigment cell progenitor.

Understanding how fate specification of distinct cell-types from multipotent progenitors occurs is a fundamental question in embryology. Neural crest stem cells (NCSCs) generate extraordinarily diverse derivatives, including multiple neural, skeletogenic and pigment cell fates. Key transcription factors and extracellular signals specifying NCSC lineages remain to be identified, and we have only a little idea of how and when they function together to control fate. Zebrafish have three neural crest-derived pigment cell types, black melanocytes, light-reflecting iridophores and yellow xanthophores, which offer a powerful model for studying the molecular and cellular mechanisms of fate segregation. Mitfa has been identified as the master regulator of melanocyte fate. Here, we show that an Mitf-related transcription factor, Tfec, functions as master regulator of the iridophore fate. Surprisingly, our phenotypic analysis of tfec mutants demonstrates that Tfec also functions in the initial specification of all three pigment cell-types, although the melanocyte and xanthophore lineages recover later. We show that Mitfa represses tfec expression, revealing a likely mechanism contributing to the decision between melanocyte and iridophore fate. Our data are consistent with the long-standing proposal of a tripotent progenitor restricted to pigment cell fates. Moreover, we investigate activation, maintenance and function of tfec in multipotent NCSCs, demonstrating for the first time its role in the gene regulatory network forming and maintaining early neural crest cells. In summary, we build on our previous work to characterise the gene regulatory network governing iridophore development, establishing Tfec as the master regulator driving iridophore specification from multipotent progenitors, while shedding light on possible cellular mechanisms of progressive fate restriction.

Periodic albinism of a widely used albino mutant of Xenopus laevis caused by deletion of two exons in the Hermansky-Pudlak syndrome type 4 gene.

The periodic albino mutant of Xenopus laevis is a recessive mutant, in which reduced amounts of melanin appear in the retinal pigment epithelium (RPE) and in melanophores at the late embryonic stage, after which both RPE and melanophores gradually depigment. Three types of pigment cells (melanophores, iridophores and xanthophores) have been reported to be affected in this albino. However, the causative gene of the periodic albinism remains unknown. Hermansky–Pudlak syndrome (HPS) is an autosomal recessive disorder that affects humans and mice, which is caused by defective biogenesis of lysosome‐related organelles (LROs). Two subgenomes (L and S) are present in the allotetraploid frog X. laevis. Comparison of genes between the chromosomes 1L and 1S revealed that the HPS type 4 (hps4) gene was present only in chromosome 1L. In the albino mutant, a 1.9 kb genomic deletion in the hps4.L gene including exons 7 and 8 caused a premature stop codon to create a truncated Hps4 protein. Injection of wild‐type hps4.L mRNA into mutant embryos rescued the albino phenotype. These findings indicate that hps4 is a causative gene for the periodic albinism in X. laevis. The phenotype of this mutant should be reassessed from the perspective of LRO biogenesis.

Airinemes: thin cellular protrusions mediate long-distance signalling guided by macrophages.

Understanding the mechanisms of cell-to-cell communication is one of the fundamental questions in biology and medicine. In particular, long-range signalling where cells communicate over several cell diameters is vital during development and homeostasis. The major morphogens, their receptors and intracellular signalling cascades have largely been identified; however, there is a gap in our knowledge of how such signalling factors are propagated over a long distance. In addition to the diffusion-based propagation model, new modalities of disseminating signalling molecules have been identified. It has been shown that cells can communicate with direct contact through long, thin cellular protrusions between signal sending and receiving cells at a distance. Recent studies have revealed a type of cellular protrusion termed ‘airinemes’ in zebrafish pigment cell types. They share similarities with previously reported cellular protrusions; however, they also exhibit distinct morphology and features. Airinemes are indispensable for pigment pattern development by mediating long-distance Delta-Notch signalling between different pigment cell types. Notably, airineme-mediated signalling is dependent on skin-resident macrophages. Key findings of airineme-mediated intercellular signalling in pattern development, their interplay with macrophages and their implications for the understanding of cellular protrusion-mediated intercellular communication will be discussed.

Developmental and Cellular Basis of Vertical Bar Color Patterns in the East African Cichlid Fish Haplochromis latifasciatus.

The East African adaptive radiations of cichlid fishes are renowned for their diversity in coloration. Yet, the developmental basis of pigment pattern formation remains largely unknown. One of the most common melanic patterns in cichlid fishes are vertical bar patterns. Here we describe the ontogeny of this conspicuous pattern in the Lake Kyoga species Haplochromis latifasciatus. Beginning with the larval stages we tracked the formation of this stereotypic color pattern and discovered that its macroscopic appearance is largely explained by an increase in melanophore density and accumulation of melanin during the first 3 weeks post-fertilization. The embryonal analysis is complemented with cytological quantifications of pigment cells in adult scales and the dermis beneath the scales. In adults, melanic bars are characterized by a two to threefold higher density of melanophores than in the intervening yellow interbars. We found no strong support for differences in other pigment cell types such as xanthophores. Quantitative PCRs for twelve known pigmentation genes showed that expression of melanin synthesis genes tyr and tyrp1a is increased five to sixfold in melanic bars, while xanthophore and iridophore marker genes are not differentially expressed. In summary, we provide novel insights on how vertical bars, one of the most widespread vertebrate color patterns, are formed through dynamic control of melanophore density, melanin synthesis and melanosome dispersal.

Axanthism in amphibians: A review and the first record in the widespread toad of the Bufotes viridis complex (Anura: Bufonidae)

Axanthism in amphibians is a relatively rare color aberration reported less often than leucism or albinism. It is caused by lack of specific types of pigment cells in the skin, namely xanthophores, erythrophores, and iridophores. Here, we present the first case of occurrence of this aberration in a widely distributed toad of the green toad (Bufotes viridis) complex and provide an extensive review of axanthism occurrence in amphibians. So far it has been reported in more than 20 species from nine families of amphibians with the highest occurrence in the family Ranidae.