Pachymedusa Research Articles

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.

Evaporative water loss in anuran amphibia: A comparative study

1. 1. Evaporative water loss, at different rates of air flow, was measured in the anurans, Rana pipiens, Bufo bufo, Xenopus laevis and Agalychnis dacnicolor. It was much less in the latter tree frog, but was similar in the other three species. 2. 2. Inhibition of pulmonary respiration did not significantly change total evaporation nor could the interspecific difference in water loss be related to O 2 consumption. Water loss corresponded to 60–100 mg H 2O/cm 3O 2 used. 3. 3. Evaporation from the amphibian integument was less than that from a planar free-water surface. The difference from three of the species may however be due to the animal's shape. Agalychnis, behaved quite differently and water appeared to have a restricted access to the outer surface of the skin in this frog. 4. 4. The observed variation could not be accounted for by differences in the thickness or lipid content of the skin.

Melanophore differentiation in leaf frogs

In melanosomes of Pachymedusa (Agalychnis) dacnicolor and other leaf frogs, the pteridine dimer, pterorhodin, is found in fibers concentric to a kernel of eumelanin. The kernel is a remnant mature larval melanosome that is renovated at metamorphic climax and on which pterorhodin is deposited at the completion of metamorphosis. When pterorhodin is initially detected by chemical means in the skin of stage 25 individuals, flocculations of fibers are first seen in melanosomes. At stage 25+ a more intense chemical demonstration is accompanied by larger flocculations. These larval melanosomes are smaller than those of other vertebrates, but are formed from classical premelanosomes. At metamorphosis, the melanosome's limiting membrane is elevated from the surface of the eumelanin, and small spheroids are seen in the space and on the pigment surface. The Golgi complex is extremely active, numerous small vesicles are seen in the cytoplasm, and blebbing of the outer membrane of the nuclear envelope occurs. At stage 25 small thick-walled vesicles appear in the cytoplasm in contact with or within the melanosome; they may represent the transport of pterorhodin or elements necessary for its formation.

Infrared reflectance in leaf-sitting neotropical frogs.

Two members of the glass-frog family Centrolenidae (Centrolenella fleischmanni, C. prosoblepon) and the hylid subfamily Phyllomedusinae (Agalychnis moreletii, Pachymedusa dacnicolor) reflect near-infrared light (700 to 900 nanometers) when examined by infrared color photography. Infrared reflectance may confer adaptive advantage to these arboreal frogs both in thermoregulation and infrared cryptic coloration.

Heterochromatin dispersion and blebbing of the nuclear membrane induced by melanophore stimulating hormone.

Stimulation (in vivo and in vitro) of dermal melanophores of the leaf frog, Agalychnis dacnicolor, by melanophore stimulating hormone (MSH) elicits two responses in addition to the dispersion of melanosomes: (1) dispersion of heterochromatin; and (2) blebbing of the outer membrane of the nuclear envelope. The latter leads to the formation of dilated rough endoplasmic reticulum with the involvement of 80–100 A microfilaments. N6,O2-dibutyryl adenosine 3′,5′-monophosphate (db-cAMP) elicits a similar response. Actinomycin D prevents both heterochromatin dispersion and membrane blebbing while cytochalasin B (CB) prevents only the latter.

Asymmetrical permeability of the integument of tree frogs (Hylidae).

The skin from four species of North American tree frogs (Hylidae) was found in vitro, to be remarkably permeable to water in the presence or absence of the neurohypophysial hormone vasotocin. This property was, however, only seen in preparations from the ventral integument; osmotic water movement across the dorsal skin was negligible. The ionic permeability, reflected by the electrical resistance, was also much greater in the ventral skin. Unidirectional fluxes of 22Na, 36Cl, 14C-urea and tritiated water were measured in vitro in a single species, Agalychnis dacnicolor Cope. The ventral skin was 10-20 times more permeable to these molecules than the dorsal skin. The short-circuit current (usually reflecting active transmural Na transport) across the ventral skin was increased by aldosterone and vasotocin but these responses were absent on the dorsal side. The lipid content of the dorsal skin was four times greater than that on the ventral surface.

Skin Structure and Wiping Behavior of Phyllomedusine Frogs

Four species of the genus Phyllomedusa (sauvagei, iherengi, pailona and hypochondrialis) were found to have previously undescribed integumental alveolar glands which contain lipid material. No lipid glands were observed in Agalychnis annae and very few in Pachymedusa dacnicolor, two other members of the subfamily Phyllomedusinae. The four species of Phyllomedusa demonstrate a complex and stereotyped wiping of the body surface immediately after the secretion of lipid. These frogs demonstrate very low evaporative water loss through the skin. The low water loss is associated with a constellation of behavioral and physiological characters: the selection of a permanent perch where the frogs remain during the day, the secretion of lipid by skin glands, wiping behavior and the assumption of torpor. Light and electron microscopy of the integument of these frogs demonstrated a typical anuran skin, with the exception of the presence of lipid glands. The impermeability of the skin is attributed to the formation of a lipid layer on the skin surface.

An analysis of color change phenomena in the leaf frog, Agalychnis dacnicolor

AbstractIn vivo and in vitro studies of integumental color change in the leaf frog, Agalychnis dacnicolor, are reported. A color index is established for the assessment of color change in the absence of melanophore visibility. Green leaf frogs adapt rapidly to background by controlling MSH release. Some members of the population are always brown regardless of background. Naturally brown specimens respond to localized reductions in illumination by becoming green in the affected area. Either skin or chromatophores in these areas are photosensitive. Isolated skins darkened with theophylline are not photosensitive; however, masking of areas of isolated skins prevented darkening of the masked area. Contact between skin and mask may inhibit darkening by physically preventing melanosome dispersion. Neither melanosome aggregation nor dispersion occur in isolated skins incubated in oxygen‐free media. Experiments involving denervation and electrical stimulation both in vivo and in vitro fail to implicate a chromatophore control by innervation.

The tail‐darkening reaction of phyllomedusine tadpoles

AbstractA tail‐darkening reaction is described for tadpoles of the phyllomedusine frogs, Agalychnis dacnicolor and Phyllomedusa trinitatus. Tails are pale in the light because melanophores in the dermis of the fin are punctate. In darkness the melanophores become dendritic and the tail appears black. This photosensitivity is local since tail‐darkening in isolated tails and in intact tadpoles occurs equally well. Tails cultured for many days retain fully the capacity for tail darkening. Correlations between tail darkening and filter feeding and swimming are made from comparisons with Xenopus tadpoles.

Color changes, unusual melanosomes, and a new pigment from leaf frogs.

Melanosomes of phyllomedusid frogs are unusually large and are composed of an amorphous matrix of thick fibers. Their hitherto undescribed dark red pigment is neither phaeomelanin nor eumelanin, but seems to be related to melanins. Melanophores of at least one of these species, Agalychnis dacnicolor, exhibit color change in direct response to illumination, and it is suggested that these chromatophores are innervated.

Ionic requirements for melanophore-stimulating hormone (MSH) release

In vitro studies on ionic requirements for melanophore-stimulating hormone (MSH) release were carried out on a variety of vertebrate species. Frog ( Rana pipiens, Agalychnis dacnicolor), toad ( Bufo marinus), lizard ( Anolis carolinensis), mouse, and rat pituitaries release an MSH-like substance into the incubation medium. incubation of the separated pars distalis or pars intermedia of the frog or rat reveal that most MSH-like activity is confined to the intermediate lobe, suggesting that the hormone is MSH. Removal of the Ca 2+ ion from the medium results in an almost total inhibition of MSH release from the pituitary of each of the species studied. Similarly, removal of the K + ion reduces secretion to about half that of control values, except in the mouse and the toad ( B. marinus), where such an effect could not be demonstrated. High (5×) K + failed to increase the rate of MSH release in either normal or Ca 2+-deficient media. Long-term (4–8 hr) incubation of frog or rat pituitaries in a Ca 2+-deficient medium did not decrease the level of pituitary MSH over that of pituitaries incubated for similar lengths of time in normal media. Excess (10×) Ca 2+ failed to stimulate MSH release from frog or rat pituitaries over that released into normal media. Ouabain reversibly blocked the release of MSH from frog ( R. pipiens) and rat pituitaries. Ouabain failed to block MSH release from the mouse and the toad, B. marinus (the two species which were similarly unresponsive to the absence of K + from the medium). MSH release from the frog was inhibited in a Li +-Ringer or an isotonic sucrose medium. These results suggest that MSH release from pars intermedia cells may be regulated by a Na +K + pump (active transport) mechanism. Although Ca 2+ is ultimately necessary for MSH release (in all species studied), its role may be secondary to that of an initial action of MSH release-inhibiting factors on cellular permeability to the K + and/or Na + ion. The experimental evidence presently suggests that some species may not possess an active transport system linked to the Ca 2+ ion requirement for hormone release.