Effects of ACTH on melanophores and iridophores isolated from bullfrog tadpoles

Hormonal regulation of colour change in eyes of a cryptic fish

Colour change of the skin in lower vertebrates such as fish has been a subject of great scientific and public interest. However, colour change also takes place in eyes of fish and while an increasing amount of data indicates its importance in behaviour, very little is known about its regulation. Here, we report that both eye and skin coloration change in response to white to black background adaptation in live sand goby Pomatoschistus minutes, a bentic marine fish. Through in vitro experiments, we show that noradrenaline and melanocyte concentrating hormone (MCH) treatments cause aggregation of pigment organelles in the eye chromatophores. Daylight had no aggregating effect. Combining forskolin to elevate intracellular cyclic adenosine monophosphate (cAMP) with MCH resulted in complete pigment dispersal and darkening of the eyes, whereas combining prolactin, adrenocorticotrophic hormone (ACTH) or melanocyte stimulating hormone (α-MSH) with MCH resulted in more yellow and red eyes. ACTH and MSH also induced dispersal in the melanophores, resulting in overall darker eyes. By comparing analysis of eyes, skin and peritoneum, we conclude that the regulation pattern is similar between these different tissues in this species which is relevant for the cryptic life strategy of this species. With the exception of ACTH which resulted in most prominent melanophore pigment dispersal in the eyes, all other treatments provided similar results between tissue types. To our knowledge, this is the first study that has directly analysed hormonal regulation of physiological colour change in eyes of fish.

Control of melanosome movement in intact and cultured melanophores in the bitterling, Acheilognathus lanceolatus

The melanophores in the dermis on scales in the bitterling, Acheilognathus lanceolatus were studies to obtain information about the control mechanism of aggregation and dispersion using intact, membrane-permeabilized and cultured cells. The cultured melanophores showed supersensitivity, namely, they responded to norepinephrine with much higher sensitivity than intact cells. The cultured melanophores failed to respond to high KCl. Melatonin aggregated and adenosine dispersed melanosomes within a cell. Digitonin permeabilized cells showed aggregation with Ca ions and dispersion by cyclic adenosine 3',5'-monophosphate (cAMP) in the presence of ATP. Movement of melanosomes was observed under the high magnification of light microscope and the tracks of each pigment granule were followed. The granules moved fast and linearly during aggregation, whereas they showed to-and-fro movement during dispersion.

Anterior/posterior influences on neural crest-derived pigment cell differentiation

The neural crest of vertebrate embryos has been used to elucidate steps involved in early embryonic cellular processes such as differentiation and migration. Neural crest cells form a ridge along the dorsal midline and subsequently they migrate throughout the embryo and differentiate into a wide variety of cell types. Intrinsic factors and environmental cues distributed along the neural tube, along the migratory pathways, and/or at the location of arrest influence the fate of neural crest cells. Although premigratory cells of the cranial and trunk neural crest exhibit differences in their differentiation potentials, premigratory trunk neural crest cells are generally assumed to have equivalent developmental potentials. Axolotl neural crest cells from different regions of origin, different stages of development, and challenged with different culture media have been analyzed for differentiation preferences pertaining to the pigment cell lineages. We report region-dependent differentiation of chromatophores from trunk neural crest at two developmental stages. Also, dosage with guanosine produces region-specific influences on the production of xanthophores from wild-type embryos. Our results support the hypothesis that spatial and temporal differences among premigratory trunk neural crest cells found along the anteroposterior axis influence developmental potentials and diminish the equivalency of axolotl neural crest cells.

Cellular plasticity among axolotl neural crest-derived pigment cell lineages

Many of the factors and mechanisms guiding the migration/differentiation of neural crest cells that give rise to a number of distinguishable cell types, including all dermal and epidermal pigment cells, remain unknown. The axolotl possesses three pigment cell types that differentiate according to specific developmentally programmed sequences and contribute to pigment pattern in the adult. A single lineage of the crest that becomes restricted to one of three pigment cell types gives us the opportunity to examine the existence of a neural crest stem cell population and the potential for trans-differentiation events. Interpretations of experiments involving drug-treated and mutant axolotls implicate cellular plasticity leading to observed phenotypes. We present results from recent in vitro studies designed to identify parameters influencing differentiation events of individual neural crest-derived pigment cell lineages. We demonstrate that the differentiation of xanthophores is enhanced, while that of the melanophores are inhibited in guanosine-supplemented neural crest cell cultures. Data suggest that the increase in one pigment cell population is at the expense of another, indicative of cellular plasticity. Videomicroscopy used in this study agrees with an abundance of correlative evidence supporting the hypothesis of transdifferentiation events among neural crest-derived pigment cell populations. The embryonic neural crest-derived pigment cell system is an ideal model to study differentiation of multipotential stem cells that play critical roles in patterning.

Adrenergic mechanisms associated with the movement of platelets in iridophores from the freshwater goby, Odontobutis obscura

1. Cultured iridophores from the freshwater goby, Odontobutis obscura, were used to investigate adrenergic mechanisms of movement of platelets in the iridophores. 2. Norepinephrine, which was assumed to be the transmitter of the iridophore nerves, induced dispersion of platelets within the cells. 3. The effect of norepinephrine was inhibited by an alpha-adrenergic antagonist, yohimbine, but not by a beta-adrenergic antagonist, propranolol. 4. Isoproterenol, a beta-adrenergic agonist, failed to bring about aggregation of platelets. 5. Forskolin, an activator of adenylate cyclase, was effective in inducing aggregation of platelets. 6. 8-Br-cAMP caused the aggregation of platelets and inhibited the norepinephrine-induced dispersion of platelets. 7. It appears that the adrenoceptors of the iridophores of this species are solely of the alpha type; they mediate the dispersion of platelets; and an increase in intracellular levels of cAMP induces the aggregation of platelets.

Physiological color change in squid iridophores. I. Behavior, morphology and pharmacology in Lolliguncula brevis

Cephalopods generally are thought to have only static iridophores, but this report provides qualitative and quantitative evidence for active control of certain iridescent cells in the dermis of the squid Lolliguncula brevis. In vivo observations indicate the expression of iridescence to be linked to agonistic or reproductive behavior. The neuromodulator acetylcholine (ACh) induced dramatic opitcla changes in active iridophores in vitro, whereas ACh had little effect on passive iridophores elsewhere in the mantle skin. Bath application of physiological concentrations of ACh (10(-7)M to 10(-6)M) to excised dermal skin layers transformed the active iridophores from a non-reflective diffuse blue to brightly iridescent colors, and this reaction was reversible and repeatable. The speed of change to iridescent in vitro corresponded well to the speed of changes in the living animal. Pharmacological results indicate the presence of muscarinic receptors in this system and that Ca++ is a mediator for the observed changes. Although ACh is present in physiological quantities in the dermal iridophore layer, it is possible that ACh release is not controlled directly by the nervous system because electrophysiological stimulation of major nerves in the periphery resulted in no iridescence in L. brevis; nor did silver staining or transmission electron microscopy reveal neuronal elements in the iridophore layer. Thus, active iridophores may be controlled by ACh acting as a hormone.

Effects of exogenous guanosine on chromatophore differentiation in the axolotl

Guanosine is shown to dramatically alter the pigment phenotype of axolotls by suppressing melanization and enhancing the biosynthesis and deposition of purine-derived pigments. Phenotypic changes caused by guanosine are manifested by altered chromatophore differentiation patterns such that few black pigment cells (melanophores) differentiate (and those that do are punctate and necrotic in appearance), whereas the development of yellow (xanthophore) and reflecting (iridophore) pigment cells is enhanced. Mechanisms for changes in chromatophore differentiation, and thus pattern formation, are discussed, including the possibility that pigment cells may undergo transdifferentiation in vivo.

Fish melanin-concentrating hormone disperses melanin in amphibian melanophores

Melanin-concentrating hormone (MCH), which was isolated from salmon pituitary and caused melanin concentration in fish scale melanophores, has been tested on cultured chromatophores of an amphibian, the bullfrog tadpole. MCH induced dispersion of melanin in cultured melanophores of the tadpole. The duration of the dispersing effect of MCH was relatively short compared with that of alpha-melanocyte-stimulating hormone (alpha-MSH). MCH also induced the concentration of cultured iridophores of bullfrog tadpole.

A new in vitro melanophore bioassay for MSH using tail-fins of Xenopus tadpoles

A new in vitro melanophore system is described, which employs pieces from the ventral tail-fin of Xenopus laevis tadpoles. Tail-fin melanophores in vitro retain the ability to disperse their pigment in darkness and to reaggregate it upon illumination. In the light, alpha-MSH, cAMP, dibutyryl-cAMP and theophylline induce a concentration-dependent pigment dispersion. The log dose-response curve obtained with alpha-MSH is sigmoidal with a linear portion between 0.5 and 2.0 ng alpha-MSH/ml. In this range, the log dose-response curve can be used as the standard curve in a bioassay for melanotropic activity, applying either the melanophore index (assay I) or a photometric transmittance measurement (assay II) for the quantification of the melanophore response. To prevent interference from the light/darkness response, light of 400-500 nm (to which the melanophores are most sensitive) was used during the assay. Both assays show high precision (lambda I = 0.13, lambda II = 0.11). Several peptides derived from alpha-MSH were tested for their melanotropic activity. The in vitro Xenopus melanophore system offers unique properties for the study of alpha-MSH action: (1) the melanophore system is uncontaminated with other chromatophores; (2) to date it is the only system suitable for photoaffinity labelling of alpha-MSH receptors; and (3) the melanophore receptor requirements differ from those of Rana.

The differentiation of leaf frog melanophores in culture

During early larval development of mexican leaf frog, Pachymedusa dacnicolor, dermal melanophores are typically black, but a few brown melanophores appear about stage 18. During metamorphic climax brown melanophores begin to dominate and by stage 23 they are the exclusive type. The synthesis of the pigment, a red pteridine-dimer, occurred in the melanophores of organ-cultured back skin isolated from tadpoles. The development of this pigmentation in organ culture was independent of thyroxine at least after the onset of metamorphic climax (stage 19). Isolated melanophores in cell culture conditions proliferated only in the presence of MSH, but even these proliferating melanophores, if derived from larvae of stage 21 or younger, showed no indication of pterorhodin synthesis. Melanophores derived from a stage 22 larva and cultured for 18 days synthesized and deposited pterorhodin.

Hormone-induced pigment translocations in amphibian dermal iridophores, in vitro: changes in cell shape

Hormone-induced pigment translocation studies were conducted at both the light and electron microscopic levels on cultured dermal iridophores from the Mexican leaf frog, Pachymedusa dacnicolor. Two distinct types of dermal iridophores were characterized which differed in (1) their in vivo locations, (2) their overall morphologies in vitro, (3) their responses to alpha-MSH, ACTH, c-AMP or theophylline, (4) their physical alterations of light, and (5) certain ultrastructural features. One iridophore (Type I) was found to be physiologically responsive to the above hormones or agents by a reversible retraction of cellular processes and a thickening of the cell body, an event which is inhibited by cytochalasin B. The other iridophore (Type II) appeared to be unresponsive. Type I iridophores contain cube-like pigmentary organelles, refractosomes, while Type II iridophores contain larger, bar-shaped refractosomes. In addition, both iridophore types contain 60 and 100 A microfilaments as well as microtubules. By in large, micorfilaments were found within microvilli, beneath and parallel to the plasma membrane and in the perinuclear region. Occasionally, bundles of 100 A microfilaments were found between layers of refractosomes in Type I iridophores. These results are discussed in relation to hormone-induced changes in cell shape.