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

The role of muscarinic receptors and intracellular Ca2+ in the spectral reflectivity changes of squid iridophores

In this paper we describe changes in spectral reflectivity of the light reflectors (iridophores) of the squid Alloteuthis subulata. The spectral changes that can be seen in living squid, can also be brought about by superfusing whole skin preparations with acetylcholine (ACh) (20 μmol l-1) and muscarine (30 μmol l-1) but not nicotine (up to 50 mmol l-1), suggesting that cholinergic muscarinic receptors are involved. Changing the osmolarity of the external solution had no effect on spectral reflectivity. To study the iridophores at the cellular level,iridophores were isolated enzymatically. Lucifer Yellow filled the iridophores uniformly, showing cellular individuality. Isolated iridophore cells were loaded with Fura-2 AM and cytoplasmic Ca2+ was recorded ratiometrically. Intracellular Ca2+ (resting concentration at 66.16 nmol l-1) increased transiently after addition of ACh (50 μmol l-1), muscarine (25 μmol l-1), but not nicotine (up to 5 mmol l-1). Ca2+ also increased when superfused with potassium chloride (10 mmol l-1) and caffeine (2.5 mmol l-1). Hypo- and hyperosmotic solutions had no effects on the cytoplasmic Ca2+. By presenting direct evidence that iridophores are polarised cellular structures containing Ca2+ stores and that they are activated via cholinergic muscarinic receptors, we demonstrate that Ca2+ is involved in the reflectivity changes of the iridophores of A. subulata. Specimens were prepared for transmission electron microscopy. It was found that the orientations of the plates with respect to the skin surface are in good agreement with the expected orientations based on the prediction that the iridophores act as multilayer reflectors.

Does the introduction of a new player, the endoplasmic reticulum, create more or less confusion in understanding the mechanism(s) of pigmentary organelle translocations?

In 1925, Wilson listed, in his classic third edition of Cell in Development and Heredity, four theories for the morphological and physiological characteristics of cytoplasm; each theory provided some sort of explanation as to the mechanism(s) of organelle translocations. During the past twenty years, cell biologists have focused their attentions on the cell's cytoskeleton, microtrabecular lattice, and associated mechanochemical motors which drive organelles along cytoskeletal tracks. A number of cell types have been used to study organelle translocations, but chromatophores, pigment cells, from cold-blooded vertebrates have been one of the more popular models. This article reviews some of the research findings during the past twenty years, particularly those involving cytoplasmic elements: i.e, microfilaments, intermediate filaments, microtubules, and mechanochemical motors. In addition, it contrasts the proposed involvement of these elements in organelle translocations with the endoplasmic reticulum, a tubulovesicular organelle, which we recently demonstrated is responsible, through its elongation or retraction, for the translocations of carotenoid droplets in goldfish xanthophores and swordtail fish erythrophores. Here, the carotenoid droplets are not free in the cytoplasm and do not translocate via cytoskeletal tracks, but instead are attached to or are a part of the endoplasmic reticulum. On the other hand, carotenoid droplets of squirrel fish erythrophores are free in the cytoplasm and appear to translocate via microtubules. Finally, the rates of pigmentary organelle translocations are reviewed in light of the participation of the cytoskeletal elements with the endoplasmic reticulum.

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.

Physiological color change in squid iridophores. II. Ultrastructural mechanisms in Lolliguncula brevis

Evidence is presented that changes in the optical properties of active iridophores in the dermis of the squid Lolliguncula brevis are the result of changes in the ultrastructure of these cells. At least two mechanisms may be involved when active cells change from non-iridescent to iridescent or change iridescent color. One is the reversible change of labile, detergent-resistant proteinaceous material within the iridophore platelets, from a contracted gel state (non-iridescent) to an expanded fluid or sol state when the cells become iridescent. The other is a change in the thickness of the platelets, with platelets becoming significantly thinner as the optical properties of the iridophores change from non-iridescent to iridescent red, and progressively thinner still as the observed iridescent colors become those of shorter wavelengths. Optical change from Rayleigh scattering (non-iridescent) to structural reflection (iridescent) may be due to the viscosity change in the platelet material, with the variations in observed iridescent colors due to changes in the dimensions of the iridophore platelets.

Purification of Black Moor goldfish melanophores and responses to epinephrine

Two key modifications of the previously reported method for isolation of goldfish xanthophores allowed the isolation and establishment of primary cultures of terminally differentiated melanophores from the Black Moor goldfish (Carassius auratus). First, pretreatment with 10(-4) M epinephrine causing aggregation of the melanosomes and collapse of the dendrites, prevents damage to the melanophores during tissue dissociation and melanophore isolation. Second, maintenance of these cells in culture was successful only when the culture medium was supplemented with fish serum. The purified melanophores attached, flattened, and were maintained in culture for up to 3 mo. Although the morphology of the cultured melanophores is less dendritic than their in vivo counterparts, the melanophores translocate melanosomes in a normal manner except that they exhibit enhanced sensitivity to epinephrine. This epinephrine-induced pigment aggregation, as well as the redispersion of pigment after the removal of epinephrine, can occur in the presence of ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and absence of Ca2+.

Clonal heterogeneity in physiological properties of melanized cells induced from goldfish erythrophoroma cell lines

Cells of the uncloned goldfish erythrophoroma lines, GEM-81 and GEM-218, have been induced to melanize by cultivation in autologous serum. The melanized cells continue to proliferate and exhibit clonal heterogeneity in terms of morphology, growth rates, contact behavior and pigment content, and distribution and translocation in response to hormones. Based on these characteristics and those of their normal counterparts, the melanized tumor cells have been categorized as type-I and type-II melanocytomas, and melanophoromas. The melanophoroma cells are capable of pigment translocation in response to epinephrine, melatonin, and/or MSH, whereas melanocytoma cells are not. The distinguishing characteristics of each type are apparent at the first appearance of melanized cells and appear to be stable except in some type-II melanocytoma clones which contain cells capable of differentiating into melanophoroma cells in long-term cultures. It appears that the parent erythrophoroma lines contain stem cells, melanoblastomas, which are capable of melanogenesis. These stem cells may themselves be a heterogeneous population with respect to the characteristics of the melanized cells to which they give rise.