An in vitro quantitative analysis of the response of tadpole tissue to thyroxine

Thyroid and Corticosteroid Signaling in Amphibian Metamorphosis

In multicellular organisms, development is based in part on the integration of communication systems. Two neuroendocrine axes, the hypothalamic–pituitary–thyroid and the hypothalamic–pituitary–adrenal/interrenal axes, are central players in orchestrating body morphogenesis. In all vertebrates, the hypothalamic–pituitary–thyroid axis controls thyroid hormone production and release, whereas the hypothalamic–pituitary–adrenal/interrenal axis regulates the production and release of corticosteroids. One of the most salient effects of thyroid hormones and corticosteroids in post-embryonic developmental processes is their critical role in metamorphosis in anuran amphibians. Metamorphosis involves modifications to the morphological and biochemical characteristics of all larval tissues to enable the transition from one life stage to the next life stage that coincides with an ecological niche switch. This transition in amphibians is an example of a widespread phenomenon among vertebrates, where thyroid hormones and corticosteroids coordinate a post-embryonic developmental transition. The review addresses the functions and interactions of thyroid hormone and corticosteroid signaling in amphibian development (metamorphosis) as well as the developmental roles of these two pathways in vertebrate evolution.

Insufficiency of Thyroid Hormone in Frog Metamorphosis and the Role of Glucocorticoids

Thyroid hormone (TH) is the most important hormone in frog metamorphosis, a developmental process which will not occur in the absence of TH but can be induced precociously by exogenous TH. However, such treatments including in-vitro TH treatments often do not replicate the events of natural metamorphosis in many organs, including lung, brain, blood, intestine, pancreas, tail, and skin. A potential explanation for the discrepancy between natural and TH-induced metamorphosis is the involvement of glucocorticoids (GCs). GCs are not able to advance development by themselves but can modulate the rate of developmental progress induced by TH via increased tissue sensitivity to TH. Global gene expression analyses and endocrine experiments suggest that GCs may also have direct actions required for completion of metamorphosis independent of their effects on TH signaling. Here, we provide a new review and analysis of the requirement and necessity of TH signaling in light of recent insights from gene knockout frogs. We also examine the independent and interactive roles GCs play in regulating morphological and molecular metamorphic events dependent upon TH.

Xenopus metamorphosis as a model to study thyroid hormone receptor function during vertebrate developmental transitions

A hormone-dependent developmental transition from aquatic to terrestrial existence occurs in all tetrapod vertebrates, such as birth, hatching, and metamorphosis. Thyroid hormones (TH) and their receptors (TRs) are key players in the tissue transformations comprising vertebrate developmental transitions. The African clawed frog, Xenopus, is a premier model for the role of TRs in developmental transitions because of the numerous and dramatic TH-dependent tissue transformations during metamorphosis and because of the endocrine, molecular, and genomic resources available. TRs are nuclear receptors that repress TH-response genes when plasma TH is minimal and that activate those same genes to induce tissue-specific gene regulation cascades when TH plasma levels increase. Tissue-specific TR expression levels help determine tissue sensitivity and responsivity to TH thereby regulating the initiation and rate of developmental change in TH-sensitive tissues which govern the tissue developmental asynchrony observed during metamorphosis. This review highlighting Xenopus presents the key experimental findings underpinning the roles TRs play in control of vertebrate developmental transitions.

Xenopus Tadpole Tissue Harvest

The procedures described here apply to Xenopus tadpoles from the beginning of feeding through the major changes of metamorphosis and are appropriate for downstream postoperative snap freezing for molecular analysis, fixation for histological analysis, and sterile organ culture. To the uninitiated, the most difficult aspects of tadpole tissue dissections are likely knowing the appearance and location of organs, and the difficulty manipulating and holding tadpoles in place to carry out the oftentimes fine and precise dissections. Therefore, images and stepwise instructions are given for the harvest of external organs (tail, head, eyes, tail skin, back skin, gills, thymus, hind limbs, forelimbs) and peritoneal organs (intestine, pancreas, liver, spleen, lungs, fat bodies, kidney/gonad complex), as well as brain, heart, and blood. Dissections are typically done under a dissection stereomicroscope, and two pairs of fine straight forceps, one pair of fine curved forceps, and one pair of microdissection scissors are sufficient for most tissue harvests.

A morphometric and molecular study of the apoptosis observed on tadpoles' tail explants under the exposition of triiodothyronine in different homeopathic dilutions

BACKGROUND

As a therapeutic system, homeopathy is supported by: i) similitude and experimentation in healthy individuals, ii) potentization. A challenge for researchers consists in looking for signals in water (or vehicle) to explain the storage of information in extremely high dilutions and the transfer of such information to the living systems. Anuran amphibian metamorphosis is controlled by thyroid hormones (TH), including the resorption of the tadpole tail. Apoptosis is a genetically regulated form of cell death that can be triggered by various extracellular and intracellular stimuli resulting in coordinated activation of a family of cysteine proteases called caspases.

METHODS

This study was blind and randomized. It performed in three stages: I) the identification of the most effective T3 homeopathic dilution to induce apoptotic reactions in Rana (Lithobates) catesbeianus tadpole tail explants stimulated by T3 in substantial, II) study of different controls and III) detection in explants under the action of the most effective dilution of T3, as established in Stage I.

RESULTS

There was no statistically significant difference between tail macroscopic dimensions between the groups. T3 10cH decreased the expression of caspase 3/7 mRNA, in explants treated with T3 20 nM.

CONCLUSION

The present experiment is in agreement with the hypothesis that T3, at a 10cH homeopathic dilution, changes the metamorphosis molecular network.

Variation in thyroid hormone action and tissue content underlies species differences in the timing of metamorphosis in desert frogs

Hormonal control of post-embryonic morphogenesis is well established, but it is not clear how differences in developmental endocrinology between species may underlie animal diversity. We studied this issue by comparing metamorphic thyroid hormone (TH) physiology and gonad development across spadefoot toad species divergent in metamorphic rate. Tissue TH content, in vitro tail tip sensitivity to TH, and rates of TH-induced tail tip shrinkage correlated with species differences in larval period duration. Gonad differentiation occurred before metamorphosis in species with long larval periods and after metamorphosis in the species with short larval periods. These differences in TH physiology and gonad development, informed by phylogeny and ecology of spadefoot metamorphosis, provide evidence that selection for the short larval periods in spadefoot toads acted via TH physiology and led to dramatic heterochronic shifts in metamorphic climax relative to gonad development.

Programmed cell death in the anuran tadpole tail requires expression of a cell surface glycoprotein

Programmed cell death is generally perceived as a suicide process involving activation of an internal death program thought to be common to all cells. We have previously presented evidence supporting the view that, at least in the tadpole tail, programmed cell death may involve assassination by cytotoxic cells such as resident macrophages. In this report, we show that regression of tadpole tail slices in culture is blocked by tunicamycin and brefeldin A, demonstrating that the intracellular protein trafficking machinery must be intact. Regression is also blocked by concanavalin A and fucose, suggesting a requirement for a cell surface glycoprotein. These observations are consistent with our hypothesis that programmed cell death requires expression of specific markers on the surfaces of cells destined to die, identifying the cells bearing those markers as targets for destruction.

Flounder metamorphosis: its regulation by various hormones

Metamorphosis in the flounder has often been compared with the transition of tadpoles into frogs. The dorsal fin rays of the Japanese flounder (Paralichthys olivaceus) elongate during prometamorphosis when thyroid hormone levels are low, and are resorbed during metamorphic climax when thyroid hormone levels are high. Using an in vitro system for the culture of the flounder fin rays, we have examined how various hormones affect the resorption process. Both thyroxine (T4) and triiodothyronine (T3) directly stimulated fin ray shortening, T3 being more potent than T4. Other hormones, such as prolactin, cortisol and sex steroids, did not directly affect the resorption process but modified the tissue's response to thyroid hormones. Similar observations were obtained from in vivo studies. We also monitored the changes in the whole body concentrations of various hormones during early development and metamorphosis, and related these with the thyroid hormone profiles in order to get a better picture of their interactions. The gaps in the present status of research on the role of thyroid hormones during metamorphosis in the Japanese flounder are also discussed.

Inhibition of programmed cell death by catalase and phenylalanine methyl ester

1. Programmed cell death proceeds by an unknown mechanism which results in characteristic morphological changes known as apoptosis. 2. We have proposed that, in hormone-induced apoptosis, cell death may be the result of an attack of cells destined to die by cytotoxic macrophages. 3. We have investigated the effects of superoxide dismutase, catalase and the macrophage toxin, phenylalanine methyl ester, on the regression of tadpole tail slices in culture. 4. Our findings, that regression of bullfrog tadpole tails is blocked by catalase and phenylalanine methyl ester, support the working hypothesis.

Changes in cortisol and thyroid hormone concentrations during early development and metamorphosis in the Japanese flounder, Paralichthys olivaceus

Both cortisol and thyroid hormones were detected in newly fertilized eggs of the Japanese flounder, Paralichthys olivaceus. Mean cortisol levels ranged around 2.5 ng/g wet weight. Cortisol concentrations declined to about one-tenth of their initial levels by 2 days before hatching. The concentrations of triiodothyronine (T3; 7 ng/g) were greater than those of thyroxine (T4; 0.4 ng/g). The T3 levels decreased gradually in the eggs until the time of hatching and then decreased rapidly to undetectable levels within 2 days, whereas T4 remained at more or less constant levels during early development. The significance of the "selective" clearance of the three hormones from eggs remains unclear. Tissue concentrations of cortisol during premetamorphosis were about 4 ng/g and increased to a peak level of 11 ng/g at climax. After climax, cortisol declined by 50%. The changes in thyroid hormone levels were in parallel with the changes in cortisol. T4 remained below 1 ng/g during prematamorphosis but increased gradually during prometamorphosis, reaching peak levels (12 ng/g) during metamorphic climax, and then declined by approximately 50%. T3 remained at low levels through most of the metamorphosis. Histologically, the interrenal tissue was activated during the metamorphosis. The coincident increases in cortisol and thyroid hormones support previous results of a synergistic action of both hormones during metamorphosis in the flounder.

Cortisol enhances the stimulating action of thyroid hormones on dorsal fin-ray resorption of flounder larvae in vitro

Effects of thyroid hormones and cortisol on fin-ray resorption of the larval flounder were studied in vitro. Dorsal fin rays were dissected from the prometamorphic larvae of the Japanese flounder (Paralichthys olivaceus) and were cultured for 7-8 days in a medium supplemented with different concentrations of thyroxine (T4) or triiodothyronine (T3) or combinations of either thyroid hormone or cortisol (0.1 micrograms/ml). Both T4 and T3 induced shortening of the second fin ray. T3 was more potent than T4; doses of 0.1 and 1.0 micrograms/ml T4 significantly accelerated shortening of the fin rays, whereas T3 was effective at concentrations ranging from 0.001 to 1.0 micrograms/ml. Cortisol further enhanced the effects of both T4 and T3 but was ineffective in the absence of thyroid hormones. When the prometamorphic larvae were reared in water containing T4 (0.01 micrograms/ml), T4 + cortisol (0.10 micrograms/ml), or cortisol alone for 15 days, both the T4- and T4 + cortisol-treated groups showed an advancement of the morphological changes associated with metamorphosis relative to the control group without hormone treatment. However, no clear synergistic effect of cortisol was observed in the in vivo experiment; a sufficient quantity of cortisol may be produced by the larval interrenal.

Temperature, thyroxine, and induced metamorphosis in tadpoles of a primitive frog, Ascaphus truei

To understand the environmental factors and endocrine mechanisms which regulate the extended period (2 to 4 years) of larval development in Ascaphus, the effect of temperature and thyroxine (T4 by immersion) on metamorphic changes of the tail, limb buds, operculum, anal fold, oral disc, and teeth were studied in premetamorphic and prometamorphic tadpoles. (1) Control tadpoles did not undergo metamorphic change at any of the temperatures tested. (2) When tadpoles were kept in thyroxine solutions at 5 degrees, there were no changes in developmental morphology, and tadpoles kept at 18 and 20 degrees showed significant metamorphic changes in 5 to 6 days, but there was a high mortality at the higher T4 concentrations. The optimum temperature is about 10 degrees, and significant changes were completed in 3 weeks. (3) The premetamorphic tadpoles were less sensitive to T4 than prometamorphic tadpoles. The most responsive tissues are teeth and operculum, and the tail tissue shows the lowest sensitivity. (4) Duration of exposure to hormone and concentration also affect the growth or regression of larval tissue.

An analysis of the influence of thyroid hormone on the synthesis of proteins in the tail fin of bullfrog tadpoles

By incubation of explants of tail fin from tadpoles of Rana catesbeiana in a solution of 35S-methionine for 4 h, newly synthesized proteins were labeled isotopically. After separation by two-dimensional polyacrylamide gel electrophoresis, those proteins were visualized by fluorography. Exposure of explants to culture medium containing thyroxine (T4) (150 nM) increased the incorporation of 35S-methionine into several proteins with 48 h. Effects of T4 on the relative abundance of two of these newly synthesized proteins were detected after 8 h of hormonal treatment. Very similar patterns of newly synthesized proteins were observed when proteins from explants of tail fin removed from tadpoles at metamorphic climax and immediately incubated with 35S-methionine were compared with proteins produced in fin derived from premetamorphic animals. These results are interpreted to indicate that both treatment of explants with T4 and elevation of endogenous levels of thyroid hormones during spontaneous metamorphosis increased the relative rates of synthesis of several identical proteins. The potential involvement of those proteins in early phases of metamorphic action which eventually lead to cell death and resorption is discussed.

An epidermal factor which induces thyroid hormone-dependent regression of mesenchymal tissues of the tadpole tail

Explants of tadpole tail skin secreted a factor which induces the thyroid hormone-dependent regression of the tail mesenchyme. The activity of the factor was not sensitive to digestion with trypsin or pronase. Heating at 120 degrees C but not at 100 degrees C for 20 min destroyed the activity. The factor came out gradually through a dialysis membrane. The factor was retarded on a Sephadex G-10 column and eluted with water after the salts fraction. We suggest that the active principle is a non-proteinaceous substance with a low molecular weight.

Investigations on the role of cAMP in regulating resorption of the tail fin from tadpoles of Rana catesbeiana

Conflicting reports have appeared regarding the role of cAMP in regulating resorption of the tadpole tail during anuran metamorphosis. That cyclic nucleotide has been suggested as a mediator of the effects of both the thyroid hormones and prolactin. We tested the effects of cAMP and its derivatives dibutyryl-cAMP and 8-bromo-cAMP on explants of tail fin from tadpoles of Rana catesbeiana maintained in tissue culture. Unmodified cAMP (0.1, 2 mM) did not influence resorption. Dibutyryl-cAMP (0.1, 1 mM) and 8-bromo-cAMP (1 mM) inhibited resorption of explants induced by thyroxine (T4). The phosphodiesterase inhibitor isobutylmethylxanthine similarly inhibited regression of explants cultured with T4. None of these agents affected the increase in specific activity of hexosaminidase brought about by T4. Although the effects of cAMP in antagonizing tail resorption were similar to those of prolactin, we found no direct effect of prolactin on levels of cAMP in cultured tail fin. Thus, the effects of prolactin appear not to be mediated by increased levels of cAMP. We conclude, however, that the elevation of cellular levels of cAMP does inhibit the resorptive action of T4.

Observations on the interaction of prolactin and thyroxine in the tail of the bullfrog tadpole

Explants of tail fin from tadpoles of Rana catesbeiana maintained in tissue culture underwent resorption when exposed to a medium containing 150 nM thyroxine (T4). Increases in the specific activity of hexosaminidase (Hex) and of acid phosphatase (AP) were associated with this response. Ovine prolactin inhibited resorption of explants in a dose-responsive manner; however, prolactin had no influence on the specific activity of either Hex or AP cultured explants. This result contrasted with the effect of prolactin in vivo, where it inhibited both the resorption of the tail and the increase in specific activity of hydrolytic enzymes induced by T4. The actions of both hormones, when applied in vivo, were more pronounced in the tail fin than in the muscular core of the tail at the stage of regression tested, indicating that the failure of prolactin to influence enzyme induction in vitro did not result from an incapacity of the fin to respond to prolactin in this fashion. It is suggested that the inhibition of T4-induced enzymic activity by prolactin which is observed in the intact animal may come about in response to some related but indirect systemic action of prolactin that is lacking when explants are cultured in vitro. It is concluded that inhibition of the induction of hydrolytic enzymes is not an essential factor in the inhibitory effect of prolactin on the resorption of the tail fin.

Control of the development of the ipsilateral retinothalamic projection in Xenopus laevis by thyroxine: results and speculation

The ipsilateral retinothalamic projection of the frog Xenopus laevis is formed by the axons of a subset of retinal ganglion cells which are found throughout peripheral and non-nasodorsal retina. Unlike the crossed retinotectal and retinothalamic projections, which begin to form during early embryonic stages, the ipsilateral projection does not begin to develop until late in tadpole life, at stages when thyroxine first becomes detectable in the circulation. Blocking the production of thyroid hormone in tadpoles prevents the development of the ipsilateral projection, in a reversible manner. Intraocular injection of thyroxine can "rescue" the development of the projection in tadpoles which otherwise remain premetamorphic. In addition, the projection from one eye of a metamorphically-blocked tadpole can be induced to form by an intraocular injection of thyroxine at a dose which has no detectable effect on retinal development in the other, untreated eye. These results indicate that the development of the ipsilateral retinothalamic projection is dependent upon thyroxine, and strongly suggest that the hormone acts at the level of the eye, rather than at the optic chiasm or thalamic target, to bring about the development of a new pathway. A number of ways in which thyroxine might act in the system are discussed.

Hormonal regulation of growth and life span of bullfrog tadpole tail epidermal cells cultured in vitro

Epidermal cells were dissociated from tails of the bullfrog tadpole, Rana catesbeiana, and cultured to investigate their response to steroid and thyroid hormones. Charcoal-treated serum (CTS) was used in the growth medium when cells were to be grown in the absence of steroid and thyroid hormones. The cells could be maintained for 2 weeks with a small increase in cell number in medium that contained CTS (CTS medium). Addition of cortisol to CTS medium increased both cellular attachment to the culture dishes and the proliferation of the attached cells with an optimum concentration of 5 X 10(-7) M. The cells remained viable and attached for at least a week. Cortisol stimulated the rate of protein synthesis 1.8-fold but did not alter the rate of DNA synthesis. The cells did not proliferate in the medium containing triiodothyronine (T3) and detached themselves from the dish within 5 days, which occurred in a dose-dependent manner with a maximum effect at 10(-8) M. It drastically decreased the rate of DNA synthesis but did not influence the rate of protein synthesis. These responses of cells to cortisol and T3 may reflect growth and death of tail epidermal cells in vivo at metamorphosis.

Wound healing in tadpole tailfin pieces in vitro

Explants of tail fins from R. catesbeiana tadpoles undergo reepithelialization of their cut surfaces (healing) when cultured in vitro in Hanks' balanced salt solution at 22 degrees C. Healing is initiated early and closure of the wound is complete by 12 to 24 hours. Morphogenesis continues for several days as further reorganization and migration of epidermal cells from the regions adjacent to the wound margins take place. The addition of serum to the culture media improves the general appearance of these tissues and promotes healing. The rate of healing is affected by temperature. Tail fins maintained at 10 degrees C do not heal while fins maintained at 30 degrees and 37 degrees, although healing more rapidly than at 22 degrees, undergo progressive degeneration in culture. Epidermal cell movements were also studied in explants consisting of a combination of intact tail fin plus tail fin deprived of its epithelium. Rapid and extensive migration of epidermal cells from the intact tail fin across the collagen lamella of the stripped fin is observed.

Changes in the tail of Ambystoma maculatum at different stages of metamorphosis: observations on tissue remodeling and its relationship to hydrolytic enzymes

A system for staging A. maculatum during growth and metamorphosis was devised, based on several parameters of body size; body length, tail length and tail width. Animals at various stages of metamorphosis were employed to study the relationship between specific biochemical and histological changes that occur in the tail of this urodele during metamorphosis. The specific and total activity of two hydrolytic enzymes, acid phosphatase and beta-N-acetyl-glucosaminidase, were measured in tail tissues at progressive stages of development. The activities of these enzymes increased in both the fins and muscular portion of the tail during metamorphosis. These activities can be correlated with resorption of the tail fins and the remodeling of tissues in the muscular portion of the tail.

The effect of prolactin and thyroxine on tail resorption in R. pipiens: in vivo and in vitro

Thyroxine induces tail resorption in anurans both in vivo and in vitro. This event is accompanied by an increase in the activity of the hydrolytic enzymes acid phosphtase and B-N-acetylglucosaminidase. Prolactin treatment promotes tail growth and directly antagonizes the action of thyroxine on tail tissues. This is evident at the gross morphological level as well as at the biochemical level where prolactin inhibits the increase in activity on the hydrolytic enzymes that occurs during thyroid hormone induced resorption.