Effect of cyclic AMP and cytochalasin B on tissue cultured melanophores of Xenopus laevis

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.

Actin microfilaments in melanophores of Fundulus heteroclitus. Their possible involvement in melanosome migration

In melanophores of Fundulus heteroclitus, hormone-stimulated melanosome aggregation is accompanied by cytoplasmic flow from the cellular processes to the perikaryon, and reversal of these events takes place upon hormone-induced melanosome dispersion. These cells contain parallel arrays of microtubules, the majority of which are located in the perikaryon and in cortical regions of the processes. Studies with heavy meromyosin binding demonstrated two types of actin filaments: 11 a decorated meshwork of filaments similar to those usually found in close association with plasma membranes, and 2) filaments decorated in a manner similar to that of stress fibers. There is an apparent increase in the amount of filaments during melanosome aggregation. These results are discussed in relation to intracellular movement.

The action of melatonin on single amphibian pigment cells in tissue culture

1. Neural crest material from neurula stage Xenopus embryos was tissue cultured as small aggregates of cells or a single cell suspension. Isolated pigment cells differentiated after 2 days in culture. 2. Melatonin (10(-15) to 10(-13) M) induced pigment granule condensation; it was 10,000 times more effective than any other compound tested. 3. Tests with appropriate agents showed the pigment cells to have beta-adrenoceptors and 5-hydroxytryptamine receptors; these sites could be blocked without affecting the response to melatonin. Phentolamine blocked the effect of melatonin. 4. Removal of Na+ or Ca2+ from the bathing medium inhibited melatonin-induced pigment granule condensation; 10 mM K+ induced pigment granule condensation. D600, Mn2+ and La3+, which inhibit Ca2+ entry, blocked the effect of melatonin. 5. Cyclic GMP induced pigment condensation and cyclic AMP pigment dispersion (10(-2) to 10(-4) M). 6. It is suggested that the action of melatonin is accompanied by depolarization and the entry of calcium ions. 7. Pigment cells in tissue culture could provide a useful bioassay method for melatonin.