Spleen and liver pigmented macrophages of Rana esculenta L. A new melanogenic system?

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

The amphibian's spleen as a source of biomarkers for ecotoxicity assessment: Historical review and trends

Amphibians are very sensitive to many environmental changes, so these animals are considered good bioindicator models for ecotoxicology. Given the importance of the amphibian spleen for hematopoietic and immune responses, this can be a key organ for the evaluation of biomarkers to monitor the health of individuals in nature or in captivity. In this systematic review, we searched databases and summarized the main findings concerning the amphibian spleen as a source of possible biomarkers applied in different scientific fields. The searches resulted in 83 articles published from 1923 to 2022, which applied the use of splenic samples to evaluate the effects of distinct stressors on amphibians. Articles were distributed in more than twenty countries, with USA, Europe, and Brazil, standing out among them. Publications focused mainly on anatomical and histomorphological characterization of the spleen, its physiology, and development. Recently, the use of splenic biomarkers in pathology and ecotoxicology began to grow but many gaps still need to be addressed in herpetological research. About 85 % of the splenic biomarkers showed responses to various stressors, which indicates that the spleen can provide numerous biomarkers to be used in many study fields. The limited amount of information on morphological description and splenic anatomy in amphibians may be a contributing factor to the underestimated use of splenic biomarkers in herpetological research around the world. We hope that this unprecedented review can instigate researchers to refine herpetological experimentation, using the spleen as a versatile and alternative source for biomarkers in ecotoxicology.

Synchrotron-Based Imaging Reveals the Fate of Selenium in Striped Marsh Frog Tadpoles

Synchrotron-based X-ray fluorescence microscopy (XFM) coupled with X-ray absorption near-edge structure (XANES) imaging was used to study selenium (Se) biodistribution and speciation in Limnodynastes peronii tadpoles. Tadpoles were exposed to dissolved Se (30 μg/L) as selenite (Se^IV) or selenate (Se^VI) for 7 days followed by 3 days of depuration. High-resolution elemental maps revealed that Se partitioned primarily in the eyes (specifically the eye lens, iris, and retinal pigmented epithelium), digestive and excretory organs of Se^IV-exposed tadpoles. Speciation analysis confirmed that the majority of accumulated Se was converted to organo-Se. Multielement analyses provided new information on Se colocalization and its impact on trace element homeostasis. New insights into the fate of Se on a whole organism scale contribute to our understanding of the mechanisms and risks associated with Se pollution.

Ultrastructural Evidence of Melanomacrophagic Centers and Lipofuscin in the Liver of Zebrafish (Denio rerio)

Melanomacrophagic centers (MMCs) were studied in the liver of zebrafish using transmission electron microscope (TEM). The MMCs were located in the space of Disse (SD), and their pseudopodia protruded into the lumen of sinusoids. The degree of extension of body structure of MMCs in the SD was determined by the size of the phagocytosed content. An irregular or amoeboid nucleus was present. Vacuoles were occasionally present, both, in endothelium and MMCs. The cytoplasm of MMCs showed several engulfed structures. The most common structure was the presence of mitochondria of small to giant size and distorted shape with inconspicuous cristae. The product of mitochondrial degeneration accompanied by lysosomes contributed to the formation of lipofuscins. Besides, changes were also observed in rough endoplasmic reticulum (rER), the Golgi complex, and lysosomes. Occasionally, small to large fragments of the erythrocytes were found in the cytoplasm of MMCs. The rER encompassed the mitochondria and lipid droplets forming a membrane-like structure. Golgi complex were dilated. Lysosomes fused with such membrane-bound structures contributed to the formation of the lipofuscin. The results provide evidence of the role of liver-resident MMCs of zebrafish in phagocytosis of damaged organelles, clearance of the worn-out erythrocytes, and lipofuscin formation.

Splenic development and injury in premature lambs supported by the artificial placenta

INTRODUCTION

The purpose of this study is to evaluate splenic effects during artificial placenta (AP) support.

METHODS

AP lambs (118-121 d, n = 14) were delivered and placed on the AP support for a goal of 10-14 days. Cannulation used right jugular drainage and umbilical vein reinfusion. Early (ETC; 115-120 d; n = 7) and late (LTC; 125-131 d; n = 7) tissue controls were delivered and immediately sacrificed. Spleens were formalin fixed, H&E stained, and graded for injury, response to inflammation, and extramedullary hematopoiesis (EMH). CD68 and CD163 stains were used to assess for macrophage activation and density. Clinical variables were correlated with splenic scores. Groups were compared using Fisher's Exact Test and descriptive statistics. p < 0.05 indicated significance.

RESULTS

Mean survival for AP lambs was 12 ± 5 d. There was no necrosis found in any of the groups. Vascular congestion and sinusoidal histiocytosis did not significantly differ between AP and control groups (p = 0.72; p = 0.311). There were significantly more pigmented macrophages (p = 0.008), CD163 (p = <0.001), and CD68 (p = <0.001) stained cells in the AP group. ETC and LTC demonstrated more EMH than AP spleens (p = <0.001).

CONCLUSIONS

During AP support, spleens appear to develop normally and exhibit an appropriate inflammatory response. After initiation of AP support, EMH transitions away from the spleen.

STUDY TYPE

Research Paper/Therapeutic Potential.

LEVEL OF EVIDENCE

N/A.

LPS-induced alterations in reproductive organs and liver pigmentation in the toad Rhinella diptycha (Bufonidae)

Anurans can be naturally infected by the bacterium Escherichia coli (Migula, 1895) Castellani and Chalmers, 1919, which contain lipopolysaccharides (LPS) in their external membrane. LPS affects reproductive functions in mammals and extracutaneous pigmentary system of anurans. Besides the functional gonad, bufonid amphibians have an undeveloped female organ (Bidder’s organ), whose function is poorly known. We aimed to investigate the effects of LPS on testis and Bidder’s organ germ cells and on liver pigmentation in the toad Rhinella diptycha (Cope, 1862). Animals were inoculated with 0.1 mL of saline solution (control) or LPS solution (18 mg/kg) and then euthanized after 24 or 48 h. The organs were processed for histology and then evaluated with morphometric analysis. The LPS decreased the area of bundle spermatozoa and increased lumen spermatozoa, while in Bidder’s organ, the number of degenerating oocytes decreased. Liver melanin increased after 24 h and decreased after 48 h. Our results suggest that LPS delays the release of spermatozoa, possibly due to alterations in serum levels of testosterone and (or) gonadotropins, which may also explain fewer degrading cells in Bidder’s organ. Increased melanin in the liver was expected due to the bactericidal role of this pigment. Thus, LPS affected reproductive organs and induced cytoprotective responses in the liver of amphibians.

Melanomacrophage functions in the liver of the caecilian Siphonops annulatus

Melanomacrophages are phagocytes that synthesize melanin. They are found in the liver and spleen of ectothermic vertebrates, and in the kidney of fish. In agnathan and elasmobranch fish, melanomacrophages are seen as isolated cells, and forming clusters in all the other vertebrates. The natural phagocytic activity of melanomacrophages is poorly characterized, as most of the research works have focused on induced phagocytic activity only. Furthermore, little is known about amphibian melanomacrophages, mainly about those in caecilians - wormlike amphibians in the order of Gymnophiona, which is the least known group of terrestrial vertebrates. The present research work aimed at the structure and function of hepatic melanomacrophages of Siphonops annulatus, a species largely found in South America. We identified the role of these cells in the control of circulating basophils (pro-melanogenic cells), in the turnover of liver collagen stroma and in the hemocatheresis, interrelated physiological mechanisms.

Administration of a polyphenol-enriched feed to farmed sea bass (Dicentrarchus labrax L.): Kidney melanomacrophages response

The reinforcement of the defense mechanism of fish, through the administration of immunostimulants, is considered as a promising alternative to vaccines. Natural immunostimulants such as polyphenols, flavanoids, pigments and essential oils can modulate the innate immune response. In lower vertebrates, melano-macrophage centres, i.e. clusters of pigment-containing cells forming the extracutaneous pigment system, are wide-spread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. In the present study, we evaluated the effect of a polyphenol-enriched diet on the health status of European sea bass (Dicentrarchus labrax L.). Farmed sea bass were administered a feed containing a phytocomplex, rich in catechins and epigallocatechins, which was obtained from the seeds of Canosina Nero di Troia Vitis vinifera and mixed with conventional feed at two different concentrations. The effects of such a diet were investigated in juvenile and commercial size samples, i.e. undergoing a short- and long-term period of diet, respectively, focusing on their extracutaneous pigmentary system and, in more detail, on the enzymatic activities leading to melanin biosynthesis. Our results show that prolonged dietary treatments with higher concentration of polyphenols might modulate tyrosinase activity and gene expression in commercial size fishes. An increase of melano-macrophage activity is correlated to a stimulation of cytoprotective functions against antigenic stimulants and pathogens, as an expression of a robust and protective adaptive immune response.

Adipose tissue macrophages develop from bone marrow-independent progenitors in Xenopus laevis and mouse

ATMs have a metabolic impact in mammals as they contribute to metabolically harmful AT inflammation. The control of the ATM number may have therapeutic potential; however, information on ATM ontogeny is scarce. Whereas it is thought that ATMs develop from circulating monocytes, various tissue-resident Mϕs are capable of self-renewal and develop from BM-independent progenitors without a monocyte intermediate. Here, we show that amphibian AT contains self-renewing ATMs that populate the AT before the establishment of BM hematopoiesis. Xenopus ATMs develop from progenitors of aVBI. In the mouse, a significant amount of ATM develops from the yolk sac, the mammalian equivalent of aVBI. In summary, this study provides evidence for a prenatal origin of ATMs and shows that the study of amphibian ATMs can enhance the understanding of the role of the prenatal environment in ATM development.

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.

A multidisciplinary study of the extracutaneous pigment system of European sea bass (Dicentrarchus labrax L.). A possible relationship between kidney disease and dopa oxidase activity level

Infectious diseases and breeding conditions can influence fish health status. Furthermore it is well known that human and animal health are strongly correlated. In lower vertebrates melano-macrophage centres, clusters of pigment-containing cells forming the extracutaneous pigment system, are widespread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. Hence, they are employed as biomarker of fish health status. We have investigated this cell system in the European sea bass (Dicentrarchus labrax L.) following the enzyme activities involved in melanin biosynthesis. We have found a possible relationship between kidney disease of farmed fishes and dopa oxidase activity level, suggesting it as an indicator of kidney disease. Moreover variations of dopa oxidase activity in extracutaneous pigment system have been observed with respect to environmental temperature. At last, for the first time, using femtosecond transient absorption spectroscopy (Femto-TA), we pointed out that pigment-containing cells of fish kidney tissue present melanin pigments.

Melanogenesis in visceral tissues ofSalmo salar. A link between immunity and pigment production?

Melanogenesis is mostly studied in melanocytes and melanoma cells, but much less is known about other pigment cell systems. Liver, spleen, kidney, and other organs of lower vertebrates harbour a visceral pigment cell system with an embryonic origin that differs from that of melanocytes. In teleosts, melanin-containing cells occur in the reticulo-endothelial system and are mainly in the kidney and spleen. The Atlantic salmon ( Salmo salar L.) is an ichthyic breeding species of considerable economic importance. The accumulation of pigments in salmon visceral organs and musculature adversely affects the quality of fish products and is a problem for the aquaculture industry. With the aim to reveal novel functions and behaviour of the salmonid extracutaneous pigment system, we investigated aspects of the melanogenic systems in the tissues of Atlantic salmon, as well as in SHK-1 cells, which is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney. We demonstrate that a melanogenic system is present in SHK-1 cells, head-kidney, and spleen tissues. As teleosts lack lymph nodes and Peyer’s patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs. The detection of tyrosinase activity in lymphoid organs indicates that a link exists between the extracutaneous pigmentary system and the immune system in salmon.

Liver histopathology in the cane toad, Rhinella marina (Amphibia: Bufonidae), induced by Ortleppascaris sp. larvae (Nematoda: Ascarididae)

Exposure to parasites is considered to be an important factor in the development of many diseases and histopathologies which are the result of the parasite-host interaction. The present study evaluated the impact of natural infection by larvae of Ortleppascaris sp. (Nematoda: Ascaridida) in the liver of the cane toad Rhinella marina (Linnaeus, 1758). Larvae were encysted in nodules delimited by collagenous fibers and fibroblasts or freely within the hepatic parenchyma, provoking a clear response from the host. The histological examination of the liver revealed viable larvae in a number of different developmental stages, as well as cysts filled with amorphous material and cell residues and surrounded by dense fibrotic tissue. The infection of the liver by these larvae induces a significant increase in the area occupied by melanomacrophages and a reduction or deficit in the vascularization of the liver, hypertrophy of the hepatocytes, vacuolar bodies, and cytoplasmatic granules. Focal concentrations of inflammatory infiltrates were observed enclosing the unencapsulated early-stage larvae. These results indicate that infection by Ortleppascaris sp. induces severe physiological problems and histopathological lesions in the liver of R. marina .

Retention of the cyanobacterial neurotoxin beta-N-methylamino-l-alanine in melanin and neuromelanin-containing cells--a possible link between Parkinson-dementia complex and pigmentary retinopathy

beta-N-methylamino-l-alanine (BMAA), a neurotoxic amino acid produced by cyanobacteria, has been suggested to be involved in the etiology of a neurodegenerative disease complex which includes Parkinson-dementia complex (PDC). In PDC, neuromelanin-containing neurons in substantia nigra are degenerated. Many PDC patients also have an uncommon pigmentary retinopathy. The aim of this study was to investigate the distribution of (3)H-BMAA in mice and frogs, with emphasis on pigment-containing tissues. Using autoradiography, a distinct retention of (3)H-BMAA was observed in melanin-containing tissues such as the eye and neuromelanin-containing neurons in frog brain. Analysis of the binding of (3)H-BMAA to Sepia melanin in vitro demonstrated two apparent binding sites. In vitro-studies with synthetic melanin revealed a stronger interaction of (3)H-BMAA with melanin during synthesis than the binding to preformed melanin. Long-term exposure to BMAA may lead to bioaccumulation in melanin- and neuromelanin-containing cells causing high intracellular levels, and potentially changed melanin characteristics via incorporation of BMAA into the melanin polymer. Interaction of BMAA with melanin may be a possible link between PDC and pigmentary retinopathy.

Evidence for the ectopic synthesis of melanin in human adipose tissue

Melanin is a common pigment in animals. In humans, melanin is produced in melanocytes, in retinal pigment epithelium (RPE) cells, in the inner ear, and in the central nervous system. Previously, we noted that human adipose tissue expresses several melanogenesis-related genes. In the current study, we confirmed the expression of melanogenesis-related mRNAs and proteins in human adipose tissue using real-time polymerase chain reaction and immunohistochemical staining. TYR mRNA signals were also detected by in situ hybridization in visceral adipocytes. The presence of melanin in human adipose tissue was revealed both by Fontana-Masson staining and by permanganate degradation of melanin coupled with liquid chromatography/ultraviolet/mass spectrometry determination of the pyrrole-2,3,5-tricarboxylic acid (PTCA) derivative of melanin. We also compared melanogenic activities in adipose tissues and in other human tissues using the L-[U-(14)C] tyrosine assay. A marked heterogeneity in the melanogenic activities of individual adipose tissue extracts was noted. We hypothesize that the ectopic synthesis of melanin in obese adipose may serve as a compensatory mechanism that uses its anti-inflammatory and its oxidative damage-absorbing properties. In conclusion, our study demonstrates for the first time that the melanin biosynthesis pathway is functional in adipose tissue.

Pigmentation in Black-boned sheep (Ovis aries): association with polymorphism of the Tyrosinase gene

Measurements were made in Black-boned (n = 40) and normal (n = 23) sheep (Ovis aries) from a flock in Nanping County of Yunnan Province, China, as well as a group (n = 21) of Romney Marsh sheep (O. aries) with the view to explaining the basis of the dark pigmentation occurring in the Black-boned animals. Plasma colour was significantly darker (P < 0.01) in Black-boned sheep than in their normal flock mates, which in turn had significantly darker plasma (P < 0.01) than the Romney Marsh sheep. Similar significant (P < 0.01) differences were measured for plasma tyrosinase activity and both groups of sheep from Nanping County had similar plasma concentrations of glutathione which were significantly smaller (P < 0.01) than for the Romney Marsh sheep.A partial fragment of 750 bp of exon 1 of the gene encoding tyrosinase was constructed and found to contain two silent mutation sites (G192C and C462T) but there was no effect on amino acid sequences of tyrosinase. Using restriction fragment length polymorphism analyses two allelic variants of site G192C were identified giving rise to the genotypes GG, GC and CC; the frequencies of allele G being 0.914, 0.824 and 0.286 in the Black-boned sheep, their flock mates and the Romney Marsh sheep respectively. Plasma tyrosinase activity was similar for genotypes GG and GC and for both genotypes significantly higher (P < 0.05) than for genotype CC. The sheep from Nanping County displayed only the GG and GC genotypes and had predominantly black or black and white coat colour whereas the Romney Marsh sheep were of either genotype GC or CC and exhibited only white coat colouration. It is not appears that the dark pigmentation of the Black-boned sheep arises because of polymorphisms in the exon 1 of tyrosinase gene. However, this result could explain the differences between Black-boned and Romney Marsh sheep but not for differences between Black-boned and Nanping Normal sheep. Moreover, this result has provided evidence of genetic markers in the form of polymorphisms of the tyrosinase gene which may help to find the black traits causing mutations. There would be merit in further studies using histochemical and molecular techniques to elucidate the causes of the dark pigmentation in these Black-boned sheep.

Melanocytes in the Testes ofEupemphix nattereri (Anura, Leiuperidae): Histological, Stereological, and Ultrastructural Aspects

Ectothermic vertebrates have a well-developed system of melanin-containing cells, which localize in several organs and tissues and compose an extracutaneous pigmentary system. This research aimed at characterizing histological and ultrastructural patterns of pigmented cells in the testes of the anura Eupemphix nattereri (Steindachner, 1963), including the stereological and quantitative evaluation of this cell type in the gonads. Ten adult males were collected in Nova Itapirema, São Paulo, Brazil, and submitted to morphological studies with light and transmission electron microscopy. The testis presents a great number of large cells with many brown granules and long cytoplasmic processes. The pigmented cells found in the testis are structurally similar to melanocytes, characterized by large amounts of melanosomes. The cells may be in intimate contact with the same cell type, with myoid cells surrounded by a large amount of collagen fibers, Leydig cells, and next to fibroblasts. The distribution and amount of extracutaneous melanocytes is variable when other organs and membranes are analyzed, allowing the establishment of species-specific patterns for the extracutaneous pigmentary system.

Isolation of the Atlantic salmon tyrosinase gene family reveals heterogenous transcripts in a leukocyte cell line

In ectothermic vertebrates, visceral organs harbor melanin-containing cells. Their ability as pigment producers is nevertheless disputed. To address expression of the key genes for melanogenesis in Atlantic salmon (Salmo salar), a tyrosinase-positive leukocyte cell line (SHK-1) and skin were used to obtain full-length tyrosinase (Tyr), tyrosinase-like protein-1 (Tyrp1), and dopachrome tautomerase (Dct) mRNA transcripts. In the SHK-1 cells, two different Tyrp1 transcripts were identified, one lacking exon 1. However, only the full-length version of Tyrp1 was identified in the skin. Sequencing of Tyrp1 genomic region revealed that the two Tyrp1 transcripts might originate from two different loci, possibly a result of pseudo-tetraploidity of the Atlantic salmon genome. Expression of Tyr, Tyrp1 and Dct was investigated by quantitative real-time reverse transcriptase polymerase cain reaction showing highest expression in the SHK-1 cell line and skin, intermediate in pronephros, and negligible or absent in liver and muscle. Histological approaches were used to demonstrate melanin and revealed presence of melanized cells in skin, kidney and liver, and absence of such cells in muscle. In addition to verify melanin synthesis abilities of visceral-located cells, our results indicate loci-specific transcription differences between populations of melanin-producing cells in Atlantic salmon.

Melanogenesis and evidence for melanosome transport to the plasma membrane in a CD83 teleost leukocyte cell line

Visceral organs of ectothermic vertebrates harbour melanin-containing leukocytes termed melanomacrophages. These cells are thought to participate in immune reactions and free-radical trapping. In teleosts, the melanin-producing ability of melanomacrophages has hitherto not been confirmed by molecular techniques. Here, a leukocyte marker and the apparatus for melanosome production and transport were investigated in an Atlantic salmon (Salmo salar) pronephros-derived mononuclear leukocyte (SHK-1) cell line. The SHK-1 cells expressed transcripts specific for a mammalian CD83 homologue, a standard surface marker for activated or differentiated dendritic cells, and dopachrome tautomerase/tyrosinase-related protein-2, a melanocyte specific enzyme essential for melanin production. Reduction potential of melanin or its precursors was demonstrated histochemically after prolonged cultivation. Ultrastructural investigations revealed tyrosinase and acid phosphate activity in identical organelles and BSA-gold co-localized with multilamellar melanosomes after 2 h internalization. Apparently, melanosomes were transported and released through periodically occurring tubules fusing with the plasma membrane. Video monitoring revealed filopodia and macropinocytosis. These results showed that the SHK-1 cell line is capable of melanogenesis and melanosome secretion. Melanin-producing cells in teleost pronephros may represent a distinct CD83(+) leukocyte population consisting of phylogenetically relict multifunctional cells. This is the first report of a melanin-producing leukocyte cell-line.

CHARACTERIZATION OF TESTUDINE MELANOMACROPHAGE LINEAR, MEMBRANE EXTENSION PROCESSES—CABLEPODIA—BY PHASE AND ATOMIC FORCE MICROSCOPY

Melanomacrophages (MMs) are a component of an internal, pigmented cell system in liver and splenic tissues of some fishes, anurans, and reptiles. The cells have been found in centers or aggregates in sinusoids and are associated with cells capable of producing a peptide cytokine and immunoglobulins. A unique cell extension process has been observed in turtle MMs placed into cell culture, and this process has been studied by light and atomic force microscopy. These structures, referred to as cablepodia, are uniquely straight, narrow, and unbranching and appear to originate from growth cones opposite lamellipodia. Cablepodia were found to connect with other turtle MMs and fibroblasts forming cell networks. Dividing fibroblasts to which a cablepodium attached ceased cell division. The observations collectively suggest that a principal reason for aggregations of MMs in internal organs of lower vertebrates is their ability to form interconnected networks of cell processes for trapping and processing of particulate matter, cells, and infectious organisms and, possibly, for the communication of cell signals and transfer of intracellular materials.

Melanogenic response of the Kupffer cells of Rana esculenta L to melanocyte stimulating hormone

We have previously shown that Kupffer cells (KCs) of Rana esculenta L. possess melanogenic ability. The melanogenic enzyme activities in these cells are different from those described in skin melanocytes, and very little is known about their regulation by extracellular signalling molecules. In order to study this regulation, we analysed the effects of NDP-MSH on the levels of expression of the tyrosinase gene and on dopa-oxidase activity, using primary cultures of KCs. Incubation of the cells with NDP-MSH increases tyrosinase gene transcription, within the first 24 h of stimulation. To gain insight into the signalling mechanism involved in the cell response to the hormone, KCs in culture were incubated with IBMX or forskolin. These agents mimic the effects of alpha-MSH on melanocytes by increasing the intracellular level of cAMP. The experimental results showed that while the hormonal treatment always activated the KC tyrosinase system, treatment with IBMX or forskolin never did. Therefore, in KCs the tyrosinase-stimulating action of NDP-MSH was not mimicked by cAMP elevating agents. Assays of cAMP levels in cells stimulated with NDP-MSH demonstrated that the hormone does not produce significant increases in intracellular cAMP. On the contrary, forskolin produced significant increases in cAMP starting from 30 min of incubation. These results suggest that tyrosinase induction by melanocortins in KCs is not mediated by the cAMP pathway, and highlight the existence of substantial differences in the hormone signal transduction mechanisms between amphibian KCs and melanocytes or melanoma cells.

The Spleen Pigment Cells in Some Amphibia

It was demonstrated that the spleen pigment cells of Amphibia are macrophages: they show an ultrastructurally distinctive morphology, are able to phagocytose and react positively for non-specific esterases. These pigmented macrophages express mRNA for tyrosinase and also they show dopa oxidase activity; therefore they are able to synthesize melanins, as Kupffer cells do.

Melano-macrophage centres and their role in fish pathology

Melano-macrophage centres, also known as macrophage aggregates, are distinctive groupings of pigment-containing cells within the tissues of heterothermic vertebrates. In fish they are normally located in the stroma of the haemopoietic tissue of the spleen and the kidney, although in amphibians and reptiles, and some fish, they are also found in the liver. They may also develop in association with chronic inflammatory lesions elsewhere in the body and during ovarian atresia. In higher teleosts, they often exist as complex discrete centres, containing lymphocytes and macrophages, and may be primitive analogues of the germinal centres of lymph nodes. Melano-macrophage centres usually contain a variety of pigments, including melanins, and these increase in range and volume in older fish or in the presence of cachectic disease. Melano-macrophage centres act as focal depositories for resistant intracellular bacteria, from which chronic infections may develop. Iron capture and storage in haemolytic diseases appears to be a primary function, but antigen trapping and presentation to lymphocytes, sequestration of products of cellular degradation and potentially toxic tissue materials, such as melanins, free radicals and catabolic breakdown products are among other functions that have been ascribed. Recent work suggests that they are a site of primary melanogenesis rather than mere storage. Melano-macrophage centres increase in size or frequency in conditions of environmental stress and have been suggested as reliable biomarkers for water quality in terms of both deoxygenation and iatragenic chemical pollution.

Biogenesis of melanosomes in Kupffer cells of Proteus anguinus (Urodela, Amphibia)

The ultrastructural characteristics of melanosomes and premelanosomes observed during the biogenesis of melanosomes in liver pigment cells of the neotenic cave salamander Proteus anguinus (Proteidae) are described. It is well known that amphibian liver pigment cells, also known as Kupffer cells (KC), contain melanosomes and are able to synthesize melanin. Liver pigment cells of P. anguinus contain numerous siderosomes and melanosomes. The melanosomes are grouped together within single-membrane-bounded bodies, named as 'clusters of melanosomes' or 'melanosomogenesis centers'. Inside such clusters, different structures are present: (1) filament-like structures, characteristic of the initial stage of melanosome biogenesis, (2) medium electron-dense melanosomes in different stages of melanization, (3) melanosomes with an electron-dense cortical area and a less electron-dense medullar area, and (4) uniformly highly electron-dense mature melanosomes or melanin granules. Histochemical and cytochemical dihydroxyphenylalanine (DOPA) oxidase reactions in pigment cells were positive. Our results confirm the ability of amphibian KC to synthesize melanin and contribute to this little known subject.