Metamorphic Climax Research Articles

The immune system is involved in Xenopus metamorphosis

Amphibian metamorphosis provides a model to elucidate the mechanisms underlying how vertebrates reconstitute a body plan and how the immune system develops during ontogeny. In Xenopus, T cells are expanded from the early developmental stages just after hatching. These T cells switch from larval-type in an easily tolerizable state into an adult-type having a potent immune responsiveness comparable to that of mammals. During metamorphosis, tadpoles exhibit morphological changes in skin that completely transforms from larval-type to adult-type. Only tail tissue behaves differently; it remains a larval-type tissue until it disappears at the end of metamorphosis. Thus, at metamorphic climax, four different types of cells co-exist in a tadpole body: larval tissue cells; adult tissue cells; larval immune cells; and adult immune cells. Based on the results showing that tadpole tail skin is rejected by syngeneic adult, it is proposed that the elimination of the larval tissue cells by the adult T cells that occurs during metamorphosis is immunologically mediated. Recent results indicate that the antigenic proteins expressed in the metamorphosing skin cells participate in the process of tail regression. This chapter describes how animals adjust and survive through such crises associated with large scale replacement of entire body cells.

CRITICAL PERIOD OF SENSITIVITY FOR EFFECTS OF CADMIUM ON FROG GROWTH AND DEVELOPMENT

Cadmium is a ubiquitous pollutant in aquatic environments that can alter organismal physiology and ecology. Previous experiments found that ecological Cd exposures increased the growth and development of two North American anurans. However, the generality of these effects among species, the time period over which they occur, and the mechanisms responsible remain conjectural. The goal of the present study was to determine the critical period of sensitivity of Rana pipiens exposed to ecologically relevant levels of Cd. We exposed tadpoles to Cd (0 [control], 1.0, and 10.0 microg/L) from Gosner stage (GS) 25 to metamorphic climax. We assessed effects of Cd on amphibian length, survival, and development during premetamorphosis (GS 25-30) and prometamorphosis (GS 31-42). After 14 d of exposure, we staged tadpoles and recorded snout-vent length. Tadpoles were then pooled according to treatment and stage (GS < or =29 or GS > or =30) and allowed to undergo metamorphic development. Tadpoles exposed to 10 microg/L were significantly larger and more advanced in development by 14 d. Survival to forelimb emergence exceeded 90% in all treatments, and time to metamorphic climax was not different from that in controls. Body burdens of Cd were positively correlated with increasing treatment. Early amphibian development (premetamorphosis) was shown to be the critical period of sensitivity for growth and development. Whereas the freshwater criterion for Cd appears to be protective for survival, a lack of knowledge remains about the sublethal effects of chronic exposures of metal pollutants, especially as they relate to tissue concentrations at various stages of amphibian life history.

Metamorphosis-related changes in leukocyte profiles of larval bullfrogs (Rana catesbeiana)

Metamorphosis in amphibians is a fascinating phenomenon that offers physiologists a number of opportunities to study aspects of tissue growth and lysis in animals. In larval amphibians, tissues are formed early on during larval growth, then are vastly reorganized and broken down during metamorphosis, and the effects of this phenomenon on the relative distribution of white blood cells in circulation (i.e., leukocyte profiles) was first examined in the 1920s. Nearly a century later, there has been little work to continue this research avenue, although our knowledge of the roles of certain leukocyte types has improved. Moreover, recent interest in counts of leukocytes in amphibians for environmental monitoring purposes emphasizes the need to understand how white blood cells naturally vary throughout larval life. In this study, I examined blood smears from wild-caught larval and adult bullfrogs (Rana catesbeiana) of varying developmental stages and quantified the proportions and abundance (estimated from blood smears) of all leukocyte types at each stage. Lymphocytes were the most common cell found and appeared most abundant during the early growth phases of larvae. Neutrophils were the second most common cell and were also associated most with earlier phases. Eosinophils appeared directly associated with metamorphosis, generally increasing in abundance to a peak at metamorphic climax. Monocytes were also found most frequently in individuals experiencing metamorphic climax. Results in this study concerning eosinophils are consistent with those of urodeles and point to their role in the breakdown of tissues, which may resemble their role in modulating inflammation responses.

Predation‐induced plasticity in metamorphic duration inXenopus laevis

Summary 1. Many organisms are able to vary the duration of life-history stages in response to environmental conditions such as predation risk. However, in those undergoing a metamorphosis, it is not known whether the duration of the metamorphic phase itself can change in response to the presence of a predator, and whether this carries costs. 2. In experiments using the amphibian Xenopus laevis , we found that metamorphosis was accelerated in the presence of a predator and this occurred consistently across the natural range of temperatures experienced by Xenopus . 3. Although metamorphic climax was reduced in duration, a functional tail was maintained for longer in the presence of a predator. Furthermore, burst swimming speed was significantly faster for animals metamorphosing in the presence than in the absence of a predator. This suggests that the more rapid development induced by predator presence does not carry costs in terms of ability to escape predators during metamorphosis. 4. There was no evidence of post-metamorphic costs of faster metamorphic climax in terms of escape response since juveniles from the two predator treatments did not differ in burst swimming speed. However, individuals metamorphosing in the presence of predators lost proportionally more mass during metamorphosis, resulting in smaller juveniles than those without predators. This reduced juvenile size represents a potential cost of accelerating metamorphic development. 5. Therefore, conditions experienced during metamorphic climax, independent of larval conditions, can have a significant influence on the life histories of animals with complex life cycles.

Comparative susceptibility to atrazine of three developmental stages of Rhinella arenarum and influence on metamorphosis: Non-monotonous acceleration of the time to climax and delayed tail resorption

Acute and subchronic toxicity of atrazine was evaluated in embryos (stage 4) and in premetamorphosis (stage 25) and prometamorphosis (stage 38–39) larvae of the common South American toad Rhinella arenarum (Anura: bufonidae). The influence of atrazine on the last stages of metamorphosis was also examined by exposing prometamorphosis larvae until completion of metamorphosis. Results obtained revealed that larvae in premetamorphosis are more sensitive than larvae in prometamorphosis and that these are, in turn, more sensitive than embryonic stages. Indeed, concentrations of atrazine as high as 30 mg/L had little effects on embryonic stages, the embryos surviving and developing in a similar manner as controls. LC50s of premetamorphosis larvae equaled 27.16, 7.03 and 2.32 mg/L of atrazine after 4, 14 and 21 days of exposure, respectively, compared to LC50s values of 18.27 and 14.43 mg/L after 14 and 21 days of exposure for larvae in prometamorphosis. In experiments with premetamorphosis larvae, the range of tested concentrations was extended to very low concentrations (down to 0.0001 mg/L) to examine whether recent findings of greater mortality at lower doses than at higher doses were also observed in R. arenarum but no such pattern was found. Exposure of prometamorphosis larvae to concentrations of atrazine of 10 mg/L and above widely prevented completion of metamorphosis and caused important mortality. Alternatively, whereas all animals eventually completed metamorphosis when exposed to concentrations of atrazine between 0.1 and 5 mg/L, the timings of metamorphosis were altered starting from 0.1 mg/L, the lowest concentration tested. Indeed, a significant decrease in the time needed for 50% of the larvae to reach the metamorphic climax (stage 42) was observed within this range of atrazine concentrations, the response presenting a U-shaped non-monotonic dose–response curve. Larvae exposed to these concentrations of atrazine also needed significantly more time for completing tail resorption, this effect being equivalent at all concentrations. Overall, the combination of these two different facets of atrazine influence on metamorphosis resulted in a significant acceleration of metamorphosis at 1 mg/L and a significant increase in the duration of metamorphosis at 5 mg/L, whereas no significant difference was observed with 0.1 mg/L.

Exposure of Xenopus laevis Tadpoles to Cadmium Reveals Concentration-dependent Bimodal Effects on Growth and Monotonic Effects on Development and Thyroid Gland Activity

Xenopus laevis were exposed to 0-855 microg cadmium (Cd)/l (measured concentrations) in FETAX medium from fertilization to 47 days postfertilization. Measurements included embryonic survival and, at 47 days, tadpole survival, snout-vent length, tail length, total length, hindlimb length, weight, Nieuwkoop-Faber (NF) stage of development, initiation of metamorphic climax (> or = NF 58), and thyroid follicle cell height. Embryonic and larval survival were unaffected by Cd. Relative to control tadpoles, reduced tail and total length were observed at 0.1- 8 and at 855 microg Cd/l; and reduced snout-vent length, hindlimb length, and weight were observed at 0.1-1 and at 855 microg Cd/l. Mean stage of development and rate of initiation of climax were unaffected by Cd at 0-84 microg/l; however, none of the tadpoles exposed to 855 microg Cd/l progressed beyond mid-premetamorphosis (NF 51). Thyroid glands with fully formed follicles were observed in all tadpoles > or = NF 49 examined. Follicle cell height was unaffected by Cd at 0-84 microg/l but it was reduced at 855 microg/l; in the latter, cell height was reduced even when compared with NF 49-51 tadpoles pooled from the 0 to 84 microg Cd/l groups. In conclusion, (1) Cd affected tadpole growth in a bimodal pattern with the first and second inhibitory modes at concentrations below and above 84 microg Cd/l, respectively; (2) exposure to high Cd concentrations (855 microg/l) reduced thyroid activity and arrested tadpole development at mid-premetamorphosis; and (3) unlike its effect on growth, Cd inhibited tadpole development and thyroid function in a seemingly monotonic pattern.

Metamorphic changes in localization of sugars in skin of the leopard frog, Rana pipiens

A lectin histochemical study was carried out to determine the distribution of specific sugars in glycoconjugates within an important osmoregulatory organ, amphibian skin. Paraffin sections were made of Rana pipiens skin from dorsal and ventral regions of aquatic larvae in representative developmental stages as well as from several body regions of semiaquatic adult frogs. Sections were incubated with horseradish peroxidase (HRP)-conjugated lectins, which bind to specific terminal sugar residues of glycoconjugates. Such sites were visualized by DAB-H2O2. The following HRP-lectins were used: UEA-1 for alpha-L-fucose, SBA for N-acetyl-D-galactosamine, WGA for N-acetyl-beta-D-glucosamine, PNA for beta-galactose, and Con A for alpha-mannose. We found that lectin binding patterns in larvae change during metamorphic climax as the skin undergoes extensive histological remodeling; this results in adult skin with staining patterns that are specific for each lectin and are similar in all body regions. Such findings in R. pipiens provide additional insight into the localization of molecules involved in osmoregulation in amphibian skin.

Galanin: Presence and distribution in the brain and pituitary of Rhinella arenarum (Amphibia: Anura) during development

The immunohistochemical distribution of galanin (Gal) in the brain and pituitary of Rhinella arenarum was studied during development. Gal-immunoreactivity was first observed in the brain just after hatching in anterior preoptic area, infundibular area, median eminence and pars distalis of the pituitary as well as in the olfactory epithelium. At the beginning of prometamorphosis new Gal-immunoreactive (ir) cells were observed in the olfactory nerve and bulb. Later in prometamorphosis new Gal-ir cells were observed in the telencephalon, suprachiasmatic nucleus, rostral rhombencephalon and in the pars nervosa of the pituitary. The most numerous accumulations of Gal-ir neurons throughout the larval development were observed in the ventral hyphothalamus where numerous Gal-ir cells of cerebrospinal fluid-contacting type were found. During metamorphic climax and soon after we did not detect Gal-ir neurons in the pallium, medial or pretectal dorsal thalamus. In the median eminence and pars distalis of the pituitary many Gal-ir fibers were found during development indicating that Gal may play a role in the modulation of hypophyseal secretion. Furthermore, the distribution of Gal-ir elements observed throughout larvae development indicates that galaninergic system maturation continues until sexual maturity.

Developmental and Regional Patterns of GAP-43 Immunoreactivity in a Metamorphosing Brain

Growth-associated protein-43 is typically expressed at high levels in the nervous system during development. In adult animals, its expression is lower, but still observable in brain areas showing structural or functional plasticity. We examined patterns of GAP-43 immunoreactivity in the brain of the bullfrog, an animal whose nervous system undergoes considerable reorganization across metamorphic development and retains a strong capacity for plasticity in adulthood. Immunolabeling was mostly diffuse in hatchling tadpoles, but became progressively more discrete as larval development proceeded. In many brain areas, intensity of immunolabel peaked at metamorphic climax, the time of final transition from aquatic to semi-terrestrial life. Changes in intensity of GAP-43 expression in the medial vestibular nucleus, superior olivary nucleus, and torus semicircularis appeared correlated with stage-dependent functional changes in processing auditory stimuli. Immunolabeling in the Purkinje cell layer of the cerebellum and in the cerebellar nucleus was detectable at most developmental time points. Heavy immunolabel was present from early larval stages through the end of climax in the thalamus (ventromedial, anterior, posterior, central nuclei). Immunolabel in the tadpole telencephalon was observed around the lateral ventricles, and in the medial septum and ventral striatum. In postmetamorphic animals, immunoreactivity was confined mainly to the ventricular zones and immediately adjacent cell layers. GAP-43 expression was present in olfactory, auditory and optic cranial nerves throughout larval and postmetamorphic life. The continued expression of GAP-43 in brain nuclei and in cranial nerves throughout development and into adulthood reflects the high regenerative potential of the bullfrog’s central nervous system.

An Aryl Hydrocarbon Receptor Repressor from Xenopus laevis: Function, Expression, and Role in Dioxin Responsiveness during Frog Development

Xenopus laevis and other frogs are extremely insensitive to the toxicity of xenobiotic ligands of the aryl hydrocarbon receptor (AHR), including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Premetamorphic life stages are especially insensitive, and they are reported to be refractory to induction of Cytochrome P4501As, which are readily induced in older animals. The AHR repressor (AHRR) is a member of the AHR gene family. AHRR expression is induced by TCDD; it then represses AHR in an apparent negative feedback loop. In this study, we sought to test the hypothesis that constitutive AHRR expression underlies the lack of TCDD responsiveness in frog early life stages. We determined the sequence of an AHRR complimentary DNA encoding an 85.3-kDa protein sharing 52-55% identity with the bHLH/PAS domains of other AHRRs. In transient transfection assays, X. laevis AHRR inhibited TCDD-induced reporter gene expression mediated by either X. laevis AHR paralog, AHR1alpha or AHR1beta. AHRR messenger RNA was expressed at low levels in embryos (Nieuwkoop-Faber stage 33-38; approximately 52 h.p.f.) and was induced approximately twofold following TCDD exposure (42 ng/g wet weight). In contrast, AHRR exhibited higher constitutive expression and was induced more than threefold in tadpoles at stage 52-55 (prometamorphic; approximately 4 weeks postfertilization) and in isolated viscera of stage 62 tadpoles (in the metamorphic climax; approximately 7 weeks postfertilization). Although the magnitude of induction was smaller, the temporal pattern of AHRR expression and inducibility resembled that of CYP1A6. Thus, attenuated transcriptional activation of AHR target genes and low TCDD toxicity in X. laevis embryos cannot be explained by constitutive, high-level expression of AHRR.

Estrogenic exposure affects metamorphosis and alters sex ratios in the northern leopard frog (Rana pipiens): Identifying critically vulnerable periods of development

During the transformation from larval tadpole to juvenile frog, there are critical periods of metamorphic development and sex differentiation that may be particularly sensitive to endocrine disruption. The aim of the present study was to identify sensitive developmental periods for estrogenic endocrine disruption in the northern leopard frog (Rana pipiens) using short, targeted exposures to the synthetic estrogen, ethinylestradiol (EE2). Post-hatch tadpoles (Gosner stage 27) were exposed over five distinct periods of metamorphosis: early (stage 27–30), mid (stage 30–36), early and mid (stage 27–36), late (stage 36–42), and the entire metamorphic period (chronic; stage 27–42). For each period, animals were sampled immediately following the EE2 exposure and at metamorphic climax (stage 42). The effects of EE2 on metamorphic development and sex differentiation were assessed through measures of length, weight, developmental stage, days to metamorphosis, sex ratios and incidence of gonadal intersex. Our results show that tadpoles exposed to EE2 during mid-metamorphosis were developmentally delayed immediately following exposure and took 2 weeks longer to reach metamorphic climax. In the unexposed groups, there was low proportion (0.15) of intersex tadpoles at stage 30 and gonads appeared to be morphologically distinct (male and female) in all individuals by stage 36. Tadpoles exposed early in development displayed a strong female-biased sex ratio compared to the controls. Moreover, these effects were also seen at metamorphic climax, approximately 2–3 months after the exposure period, demonstrating that transient early life-stage exposure to estrogen can induce effects on the reproductive organs that persist into the beginning of adult life-stages.

Involvement of growth hormone-insulin-like growth factor I system in cranial remodeling during halibut metamorphosis as indicated by tissue- and stage-specific receptor gene expression and the presence of growth hormone receptor protein

The role of growth hormone (GH) and insulin-like growth factor-I (IGF-I) in the tissue remodeling associated with the transition of a symmetrical larva to an asymmetrical juvenile during flatfish metamorphosis is unknown. In order to investigate the potential role of these hormones in the remodeling of cranial bone and soft tissue that accompanies eye migration during metamorphosis of Atlantic halibut (Hippoglossus hippoglossus) larvae, tissue-specific gene expression was monitored by in situ hybridization for Atlantic halibut type I growth hormone receptor (hhGHR), type II hhGHR, and insulin-like growth factor-I receptor (hhIGF-IR). Polyclonal antibody generated against the extracellular domain of type I hhGHR was used for the immunohistochemical localization of type I GHR protein. Type I hhGHR, type II hhGHR, and hhIGF-IR mRNA were expressed in fibroblasts, frontal bone osteocytes, and dorsal chondrocytes at the onset of metamorphosis (stage 8), during metamorphic climax (stage 9), and in fully metamorphosed juveniles (stage 10). Type I GHR protein showed similar expression patterns to those of type I hhGHR mRNA, except in chondrocytes in which little GHR protein was detected. The localization of GHR and IGF-IR transcripts and GHR protein in cranial structures that undergo remodeling is intriguing and suggests that, in addition to thyroid hormones, the GH-IGF-I system is involved in morphological transformations during metamorphosis in Atlantic halibut.

Developmental analysis of the extrabulbar olfactory projections in the ranid frog with some phylogenetic considerations

This study has examined the presence and distribution of extrabulbar olfactory projections (nerve fibers that originate from cells in the olfactory epithelium, bypass the olfactory bulbs and project caudally into the brain) in larvae and adult stages of a semi‐aquatic frog, Rana esculenta. This species was chosen because previous work had suggested that the presence of an extrabulbar olfactory system (EBOS) may correlate with the detection of water‐borne odorants. The main result is that this extrabulbar system is present in both larval and adult specimens of this frog species. In stage 26, the earliest one used in our study, the EBOS is well‐developed and projects as far caudal as the rhombencephalon. During successive larval development and until the completion of metamorphosis, there occurs a progressive reduction in the caudal extent of the EBOS. This reduction is more dramatic during the metamorphic climax (stages 31–33). A reduction in the caudal extent of the extrabulbar projections in adult stages is similar to other species previously examined. We are inclined to believe that there is no correlation between a reduction in the caudal extent of the EBOS and transition from water to land.

Temperature-mediated morphology changes during metamorphic climax in the African clawed frog, Xenopus laevis

(1) Investigations of the effect of temperature on body size are largely limited to the larval phase, with our understanding of the effect of temperature during metamorphic climax entirely restricted to the insects. (2) Environmental temperature was manipulated only during metamorphosis in the aquatic amphibian Xenopus laevis. (3) Lower temperatures during metamorphosis resulted in individuals with greater mass, head width and snout–vent length on the completion of metamorphosis. (4) This suggests that temperatures experienced during the relatively short metamorphic phase will play an important part in determining the temperature–size relationship in amphibians.

Pliocene–Pleistocene HT–LP metamorphism during multiple granitic intrusions in the southern branch of the Larderello geothermal field (southern Tuscany, Italy)

This work presents the results of a multidiscipinary study carried out in the southern branch of the Larderello geothermal field (Travale–Montieri area), which was based on the integration of field data with information on the deeper structures as derived from interpretation of seismic reflection profiles and the P – T – t history of the metamorphic substratum as reconstructed from borehole data. Our data document that the structural and metamorphic signature of the metamorphic substratum is chiefly related to a prograde HT–LP metamorphic overprint, ranging in age from c . 2.8 to 0.7 Ma. The metamorphic climax was attained within the K-feldspar zone and equilibrated at P – T conditions of about 0.2 GPa and 650 °C. We interpret these new findings as evidence of Pliocene–Pleistocene thermal metamorphism associated with multiple granite intrusions at shallow levels in the crust. These results (1) add strength to the interpretations emphasizing the contribution of Neogene thermal metamorphism to the tectonometamorphic signature of the basement rocks found at depth in the Larderello geothermal field, and (2) provide insights on the thermal structures of the crustal section hosting the geothermal field and on the modes through which the long-lived positive thermal anomaly operated in southern Tuscany.

Thyroid hormone receptor expression during metamorphosis of Atlantic halibut ( Hippoglossus hippoglossus)

Flatfish such as the Atlantic halibut ( Hippoglossus hippoglossus) undergo a dramatic metamorphosis that transforms the pelagic, symmetric larva into a benthic, cranially asymmetric juvenile. In common with amphibian metamorphosis, flatfish metamorphosis is under endocrine control with thyroid hormones being particularly important. In this report we confirm that tri-iodothyronine (T 3) levels peak at metamorphic climax during halibut metamorphosis. Moreover, we have isolated cDNA clones of TRα and TRβ genes and confirmed the presence in halibut of two TRα isoforms (representing the products of distinct genes) and two TRβ isoforms (generated from a single gene by alternative splicing). Real-time PCR was used to assess expression of these genes during metamorphosis. TRβ shows the most dramatic expression profile, with a peak occurring during metamorphic climax.

Cloning and expression of xP1-L, a new marker gene for larval surface mucous cells of tadpole stomach in Xenopus laevis

The amphibian gastrointestinal tract is remodeled from a larval-type to an adult-type during metamorphosis. In the present study, we examined the products of subtractive hybridization between tadpole and frog stomach cDNAs of Xenopus laevis in order to identify genes expressed specifically in the larval stomach epithelium. A new gene homologous to xP1 was obtained and named xP1-L. In the genome database of Silurana tropicalis, we found a homologue of xP1-L and named it stP1-L. RT-PCR showed that the expression of xP1-L was detected in stage 41/42 tadpoles. In addition, in situ hybridization showed that xP1-L was localized to surface mucous cells of the larval stomach. The H +/K +-ATPase β subunit, a marker gene for manicotto gland cells in the tadpole stomach, was also detected at the same time. However, adult marker genes such as xP1 for surface mucous cells and pepsinogen C (PgC) for oxynticopeptic cells were not expressed in the tadpole stages. The expression of xP1-L gradually decreased towards the metamorphic climax and disappeared after stage 61 when larval-type gastric epithelium is replaced by adult-type. We found that xP1-L was never expressed in surface mucous cells of the adult-type stomach, and xP1, instead of xP1-L, was expressed. During T3-induced metamorphosis, xP1-L expression decreased in the same manner as during natural metamorphosis. Thus, xP1-L is a useful marker for larval surface mucous cells in tadpole stomach. This is the first demonstration of a marker gene specific for the surface mucous cells of the larval stomach.

Evidence that the genes encoding the melittin-related peptides in the skins of the Japanese frogs Rana sakuraii and Rana tagoi are not orthologous to bee venom melittin genes: Developmental- and tissue-dependent gene expression

The antimicrobial melittin-related peptides (MRPs) isolated from skin extracts of the Japanese frogs, Rana sakuraii and Rana tagoi, show amino acid sequence similarity with melittin from the venom of honeybees but the evolutionary relationship between the amphibian and insect peptides is unknown. cDNA clones encoding the MRP precursor (preproMRP) were obtained from R. sakuraii and R. tagoi skin total RNA. The nucleotide and deduced amino acid sequences of the clones indicated that the preproMRPs are organized like typical amphibian antimicrobial peptide precursors, with a highly conserved signal peptide, a more variable intervening sequence, and a hypervariable mature peptide region. This organization is markedly different from that of prepromelittin, in which the melittin sequence is flanked by multiple Xaa-Pro and Xaa-Ala dipeptides. The data indicate, therefore, that the genes encoding frog skin MRPs are not orthologous to the genes encoding melittins from bee venom. In adult R. sakuraii specimens, preproMRP gene transcripts were detected in total RNA from skeletal muscle as well as skin but not from heart, stomach, small intestine, or liver. In R. tagoi, preproMRP mRNA was not detected in skin prior to the onset of metamorphosis, but its level increased markedly during metamorphosis reaching a maximum at the stages of metamorphic climax.

Representation of particle motion in the developing midbrain

In underwater organisms such as tadpoles, both particle motion and pressure components provide important information about ambient sound sources. Using both anatomical and physiological techniques, we examined the representation of these components in the developing midbrain. Afferents from the lateral line and from the saccule both project to the medial vestibular nucleus in the medulla which, in turn, sends projections to the laminar nucleus of the torus semicircularis in the midbrain. Extracellular recordings from the laminar nucleus of tadpoles from early larval through metamorphic climax stages show robust responses to particle motion, but across a more constrained frequency range than observed in the medulla. Combined with earlier data showing good responses to the pressure component of sound sources, these data suggest that the laminar nucleus may be a site of integration of particle motion and pressure sensitivity.

Characterization of histone lysine‐specific demethylase in relation to thyroid hormone‐regulated anuran metamorphosis

The thyroid hormone receptor (THR) is a member of the nuclear transcription factor and plays a central role in regulating anuran metamorphosis. Previous studies with mammalian cells have suggested that THR is involved in chromatin remodeling through histone methylation. In the present study, we cloned cDNA of lysine-specific demethylase gene, xLSD1, from Xenopus laevis and examined its expression in relation to metamorphosis. Overexpression of xLSD1 in A6 cells, a Xenopus laevis cell line, resulted in the decrease of methylation status of lysine residues of histone H3, indicating that the protein of cloned xLSD1 was functionally active. The expression of LSD1 at mRNA levels was up-regulated in the body skin and the intestine during natural and thyroid hormone-induced metamorphosis. Larval epidermal basal cells and intestinal epithelial cells at the premetamorphic stage were identified as the xLSD1-expressing cells. At the metamorphic climax stage the progenitor cells of adult epidermal basal cells also expressed xLSD1, whereas those of the adult intestinal epithelial cells did not. We propose that LSD1 participates in the regulation of metamorphosis through THR- or another transcriptional factor-induced chromatin remodeling.