Laevis Larvae Research Articles

Whole-body composition of Xenopus laevis larvae: implications for lean body mass during development.

Body composition in developing animals has been extensively investigated in fish larvae and bird embryos. However, no studies to date have attempted to determine whole-animal body composition or lean body mass (MLB) in developing amphibians. The present study investigates how body composition changes during development in Xenopus laevis and the potential implications of MLB for substrate turnover, energy stores, oxygen consumption and other physiological measures. Whole-animal composition was determined during development from eggs (NF stage 1) to 2 weeks post-feeding (NF 50-51), which represents two-thirds of the developmental period. Wet and dry masses were found to be highly correlated, with water content remaining constant at 93 % of wet mass. Whole-animal nucleic acid content was linearly correlated with both wet and dry masses, and declined relative to mass as development progressed. Similarly, total protein content was linearly correlated with wet and dry masses; however, total protein content increased with developmental stage. Amounts of individual neutral lipids were variable although, overall, total neutral lipid content declined progressively with development. The stoichiometric energy balance paralleled the changes seen in mass-specific .MO2, with the energy primarily from lipids fueling respiration up to NF 44-45. Quantification of total body composition revealed that lipid stores greatly influenced the calculations of MLB and therefore had profound underestimating effects on the mass-specific expression of numerous physiological measures through development.

A role for slow NMDA receptor-mediated, intrinsic neuronal oscillations in the control of fast fictive swimming in Xenopus laevis larvae.

In larvae of the amphibian, Xenopus laevis, spinal neurons which are active during fictive swimming also display tetrodotoxin-resistant membrane potential oscillations following the coactivation of N-methyl-DL-aspartate (NMDA) and 5-hydroxytryptamine (serotonin or 5-HT) receptors (Scrymgeour-Wedderburn et al., 1997; Eur. J. Neurosci., 9, 1473-1482). The oscillations are slow (approximately 0.5 Hz) compared with swimming (approximately 7-35 Hz) raising doubt over their contribution to the cycle by cycle depolarizations occurring during swimming. We investigated an alternative: that the intrinsic oscillations modulate swimming activity over many consecutive cycles. Bath application of NMDA induced continuous fictive swimming that differed between embryonic and larval preparations. In 81% of larval preparations (n = 36), there was a slow (approximately every 2 s) rhythmic modulation of ventral root activity in which burst durations and intensities increased as cycle periods decreased. This pattern of activity was enhanced rather than abolished following blockade of glycine and gamma-aminobutyric acid (GABA) A receptors and presumably therefore resulted from a periodic increase in the excitation of motor neurons. To determine whether this slow rhythm resulted from intrinsic, 5-HT-dependent membrane potential oscillations, larvae were spinalized to prevent the release of 5-HT from brainstem raphe neurons. The resulting pattern of NMDA-induced activity lacked any slow modulation. The slow modulation could also be enhanced by the bath application of a 5-HT receptor agonist (5-carboxamidotryptamine) and abolished either by the addition of an antagonist (pindobind-5-HT1A) or by removal of magnesium ions, providing more direct evidence for a contribution of intrinsic oscillations. Thus, the 5-HT-dependent intrinsic oscillations modulate NMDA-induced swimming activity over several consecutive cycles.

EFFECTS OF PHOTOINDUCED TOXICITY OF FLUORANTHENE ON AMPHIBIAN EMBRYOS AND LARVAE

Embryos and newly hatched larvae of three amphibian species, the spotted salamander (Ambystoma maculatum), the northern leopard frog (Rana pipiens), and the African clawed frog (Xenopus laevis), were exposed to fluoranthene and ultraviolet (UV) light in two scenarios. Embryos were exposed in a laboratory setting from an early developmental stage through hatching under artificial UV light, and newly hatched larvae were exposed outdoors in varying sunlight intensity levels. Outdoor exposures indicated greater sensitivity in the toxic response than did laboratory exposures. In the laboratory, mortality and malformation of X. laevis were the most sensitive indicators of exposure. Xenopus laevis was also the most sensitive species tested to the effects of UV light alone. Hatching success of R. pipiens was monitored outdoors and was not a useful predictive endpoint in the determination of photoinduced toxicity; however, newly hatched larvae were sensitive to the effects of photoinduced toxicity. Amybstoma maculatum and X. laevis larvae were affected by low (μg/L) concentrations of fluoranthene in sunlight. These findings suggest that low levels of polycyclic aromatic hydrocarbons could be acting synergistically with environmental factors such as UV light to place young amphibians at risk.

Lens formation from cornea implanted into amputated hindlimbs of Xenopus laevis larvae requires innervation or proliferating cell populations in the stump.

The capacity of amputated early and late limbs of larval Xenopus laevis to promote lens-forming transformations of corneal implants in the absence of a limb regeneration blastema has been tested by implanting outer cornea fragments from donor larvae at stage 48 (according to Nieuwkoop and Faber 1956), into limb stumps of larvae at stage 52 and 57. Blastema formation has been prevented either by covering the amputation surface with the skin or by reconnecting the amputated part to the limb stump. Results show that stage 52 non-regenerating limbs could promote lens formation from corneal implants not only when innervated but also when denervated. A similar result was observed in stage 57 limbs where blastema formation was prevented by reconnecting the amputated part to the stump. In this case, relevant tissue dedifferentiation was observed in the boundary region between the stump and the autografted part of the limb. However, stage 57 limbs, where blastema formation was prevented by covering the amputation surface with skin, could promote lens formation from the outer cornea only when innervated. In this case, no relevant dedifferentiation of the stump tissues was observed. These results indicate that blastema formation is not a prerequisite for lens-forming transformations of corneal fragments implanted into amputated hindlimbs of larval X. laevis and that lens formation can be promoted by factors delivered by the nerve fibres or produced by populations of undifferentiated or dedifferentiated limb cells.

The identification and repair of DNA adducts induced by waterborne benzo[a]pyrene in developing Xenopus laevis larvae

We report on the formation and subsequent repair of benzo[a]pyrene-induced DNA adducts in Xenopus laevis larvae in vivo, as monitored by 32P-post labelling. In vivo benzo[a]pyrene is metabolized by the cytochrome P450 family of enzymes to metabolites, of which the 7,8-diol-9,10-epoxides have been implicated as causing potentially tumourigenic lesions. Larvae were exposed to waterborne benzo[a]pyrene (0.01, 0.05 and 0.1 mg/l) for 24 h at stages 38, 45 and 50 of development (24 h, 5 days and 2 weeks post-hatching, respectively) and allowed to recover for up to 6 days. A wide range of adduct lesions were observed at stage 50, three of which were observed at all stages investigated. Adduct repair was biphasic, with an initial rapid repair over the first 24 h post exposure, followed by a much slower decline, resulting in persistence of adducts for at least 6 days post exposure. The individual lesions were repaired at different rates, with some being almost completely repaired after 6 days recovery, whereas one of the main adducts showed restricted repair at stage 50 and another no repair at all. Identification of some adducts has been achieved, by the inclusion of isomeric standards of (+)- or (-)-anti-benzo[a]pyrene diol epoxide reacted with deoxyguanosine and adenosine 3'-monophosphates prepared in vitro. The non-repairable lesion at stage 50 has been shown to be the (+)-trans-anti-benzo[a]pyrene diol epoxide-N2-guanine adduct. This adduct was observed at all stages, but was only maximally repaired at stages 38 and 45.

Effect of weightlessness on the development of the nervous system and peripheral analyzers inXenopus laevis larvae

The study was carried out within the framework of a Russian-Canadian experiment aboard the Bion-10 satellite. The volume and surface area of the gray and white matter and ventricles of the brain, retina, olfactory placodes, the VIII nerve ganglia, and vascular plexus were measured inXenopus laevis which had been in a state of weight-lessness for 2 weeks since their hatching. Zero gravity was found to stimulate the growth of nerve processes, to increase the surface of the vascular plexus, and to impede the development of the retina, olfactory placodes, and VIII nerve ganglia.

Xenopus laevis tadpole limb regeneration in vivo and in vitro: thyroxine directly promotes blastemal cell proliferation and morphogenesis.

Regeneration in hindlimbs of Xenopus laevis larvae which were amputated at stage 53 and 55 through the tarsalia region is promoted by thyroxine (T4), while propyl-thiouracil (PTU) inhibits regeneration when compared to controls. In this paper, by in vivo and in vitro experiments, we demonstrate that the promoting effect of T4 on the regenerative processes of larval X. laevis hindlimbs is a direct effect of this hormone on the blastemal cells. By contrast, the inhibitory effect of PTU on the regenerative process is not due to a direct effect on blastemal cells or to a general toxic effect on the treated larvae, but is related to hypothyroidism induced by the drug. We find that: (i) an increase in blastemal cell proliferation is observed not only in blastemata of T4-treated larvae, but also in blastemata cultured in vitro in a medium supplemented with T4; (ii) the renegerative process is accelerated not only in larvae reared in T4 but also in larvae submitted to a combined treatment of T4 and PTU; (iii) inhibition of cell proliferation is observed in blastemata of PTU-reared larvae but not in blastemata cultured in vitro in a medium supplemented with PTU. Experiments on thyroidless larvae (which were submitted to transplantation of hindlimbs from larvae at stages 53 and 55 followed by amputation of their own right hindlimb and the transplanted limbs) have shown that without thyroid hormone the regenerative process is arrested at cone stage and the promoting effect of T4 treatment is dependent on limb stage and amputation level.

Immunohistochemical studies on the development of TSH cells in the pituitary of Xenopus laevis larvae.

Since thyroid-stimulating hormone (TSH) producing cells are thought to play an essential role during metamorphosis, their development was immunohistochemically examined to reveal the appearance and topographical changes in the distribution in the hypophysis of Xenopus laevis tadpole from hatching to the end of metamorphosis. TSH-immunoreactive cells initially appeared at stage 49 (just the beginning of development of the pars nervosa in the hypophysis) as small clusters in the middle part of the pars distalis. They showed conspicuous changes in number during late premetamorphosis: their number reached a peak at stage 51, suddenly decreased at stage 52 (just before completion of the hypophysial histogenesis) followed by gradual increase until the end of metamorphosis. At later stages, they were restricted to the posterior half of the pars distalis.

Larval Cardiorespiratory Ontogeny and Allometry in Xenopus laevis

Very little is known about the early development of cardiorespiratory regulatory mechanisms in newly hatched amphibian larvae. We tested whether early cardiovascular responses to hypoxia reflect local-flow regulation in tissues and whether regulation of ventilation would improve during larval development. Cardiac output was calculated from heart rate and stroke volume, and buccal pumping rate was measured at 19° -21° C for Xenopus laevis larvae between Nieuwkoop and Faber stages 44 (just after hatching) and 57 (4-1,102 mg) denied access to air at a range of ambient aquatic Po₂ from normoxia (150-155 mmHg) to severe hypoxia (27-45 mmHg). Cardiac output decreased in severe hypoxia in stage 44-49.5 larvae, but not in stage 51-54 larvae, because heart rate decreased significantly in the early larvae, probably a direct effect of O₂ limitation on cardiac metabolism. Stroke volume did not change significantly in hypoxia in either early- or late-stage larvae. Thus there was no evidence of a tissue-mediated increase in cardiac output in hypoxia. Buccal pumping increased by about 50% over normoxic rates in moderate hypoxia in all larvae but sharply decreased in severe hypoxia, decreasing more in younger larvae than older. Younger larvae show significantly more variability in buccal pumping than older larvae, which suggests that regulatory mechanisms are not yet fully developed in early larvae. Cardiac output scales to body mass with a allometric coefficient of 1.15 ± 0.15 (95% confidence limits), significantly higher than literature values for O₂ uptake (0.83), implying that cardiovascular gas transport may be less important (compared to direct diffusion) in very small early-stage larvae than in larger, late-stage larvae.

Effects of thyroxine and propyl-thiouracil on hindlimb regeneration of larvalXenopus laevis.

Xenopus laevis larvae at stage 53 and 55 (according to Nieuwkoop and Faber 1956) were subjected to amputation of one or both hindlimbs and reared either in thyroxine (T4) 2.5 to 10 μg/l or in propyl-thiouracil (PTU) 0.01%. Results have shown that when the limb was amputated through a nearly undifferentiated region (tarsalia level, at stage 53) or through a differentiating region (tarsalia level, at stage 55), T4 accelerated the regenerative process and enhanced the mitotic and labelling indices of blastemal cells, when compared with controls. However, PTU delayed the regenerative process and lowered the mitotic and labelling indices. When the limb was amputated through an almost differentiated region (mid-thigh level, at stage 55), T4 inhibited the conic blastema formation, while PTU did not significatively influence limb regeneration. T4 did not modify the morphogenetic properties of the regenerative blastemata, which are characteristic of the developmental stage and the degree of differentiation of the limb tissues at the amputation level. On the whole, the data show that T4, besides being indirectly responsible for the decline of the limb regenerative capacity in a proximodistal direction by promoting limb differentiation, also exerts a direct effect on the regenerative process.

Epidermal Growth Factor-like Immunoreactivity in the Buccopharyngeal Mucous Glands of Xenopus laevis Tadpoles

We report epidermal growth factor (EGF)-like and epidermal growth factor receptor (EGFR)-like immunoreactivity in the buccopharyngeal mucous glands of Xenopus laevis larvae. EGF-like immunostaining was heaviest at the apices of the secretory cells of these glands. Imumnostaining for EGFR-like protein was also observed in the mucosal lining of the alimentary tract. This staining was heaviest in cells lining the foregut but was almost absent in cells from the hindgut. A potential role for an orally secreted EGF homologue in anuran amphibians is discussed. Wassersug (1986) hypothesized the existence of a metamorphic inhibitory agent, produced by larval buccopharyngeal mucous glands, which could indirectly link food ingestion to the endocrine control of metamorphosis. The presence of EGF-like immunoreactivity in the oral mucous glands of X. laevis larvae, as well as the presence of EGFR-like immunoreactivity in the gut wall of this tadpole, satisfies many of the criteria for Wassersug's regulatory agent. It remains to be shown that EGF (or an anuran EGF homologue) has a direct inhibitory effect on anuran metamorphosis when administered via an orogastric route.

Recoverin in pineal organs and retinae of various vertebrate species including man

Recoverin is a recently discovered 26 kDa calcium-binding protein, which activates guanylate cyclase in retinal photoreceptors when the intracellular concentration of free calcium drops upon photoexcitation. In this study we examined the distribution of recoverin in retinae and pineal organs of Xenopus laevis larvae, 1-day-old chicken, adult pigeon, albino rat, sheep and man by means of immunocytochemistry. Recoverin immunoreaction was found in all species investigated except for the chicken. In the retina, recoverin immunoreaction was restricted to photoreceptors; all other cell types were immunonegative. In the pineal organ, the recoverin immunoreaction labeled ‘pinealocytes of the sensory line’, i.e. classical pineal photoreceptors of Xenopus laevis larvae, modified pineal photoreceptors of pigeon, and pinealocytes of mammals. The number of recoverin immunoreactive pinealocytes varied considerably among species of mammals; very few cells were stained in the rat pineal organ, whereas in rabbit, sheep and man, numerous pinealocytes were found to be recoverin-immunoreactive. No immunocytochemical staining was observed after preabsorption of the recoverin antibody with the recombinant protein. Immunoblotting experiments showed that the immunoreaction is due to a protein of 26 kDa in both retina and pineal tissue. Thus, recoverin appears to belong to the family of proteins which are expressed in both retina and pineal organ and are highly conserved in the course of phylogeny. Recoverin may be involved in phototransduction in the directly light-sensitive pineal organs of poikilothermic vertebrates and birds. However, the functional role of recoverin in the mammalian pineal organ, which is not photosensitive, remains unknown.

Functional response of suspension feeding anuran larvae to different particle sizes at low concentrations (Amphibia)

The influence of particle size, initial particle concentration and larval stage on the ingestion rate, ‘retention efficiency’, and filtering rate of anuran larvae with varying filter apparatus anatomy and different life histories was investigated for four species. Larvae of premetamorphic Stages 28 and 32 and prometamorphic Stage 40 were selected for filtering experiments on the basis of their different growth rates. Three different sizes of silica gel particles were offered as mock food. Particle concentration was measured photometrically. The Michaelis-Menten model was used to describe the dependency of ingestion rate, filtering rate, and ‘retention efficiency’ upon initial particle concentration, and to calculate maximum ingestion rate, threshold concentration, and the half-saturation constant. (1) The highest ingestion rates, filtering rates and ‘retention efficiencies’ were achieved by Xenopus laevis larvae, followed by Bufo calamita larvae. Bufo bufo larvae lay at the opposite end of the scale. Rana temporaria larvae were placed between B. calamita and B. bufo larvae. This order is attributed to differences in life histories, especially the different breeding environments in which these larvae occur. (2) The larger the particle size and the older the stage, the greater the tendency toward saturation of the ingestion rate, filtering rate and ‘retention efficiency’. These filtration parameters are graded according to particle size. The ingestion rate (number of particles), filtration rate and ‘retention efficiency’ are greatest for PS3. Ingestion volume is greatest for PS 1. The difference between PS3 and PS2 on the one hand, and PS1 on the other, is often great; for Stage 28 X. laevis it is very great. This shows that larvae ingest large particles more effectively, and that the most effective ingestion takes place at Stages 28 and 32, owing to the growth function of these stages. The ability of larvae to ingest large particles effectively is possibly a very basic phylogenetic characteristic. (3) The threshold concentration is lowest when the particles are at their largest. In accordance with conclusions drawn by other authors, threshold feeding is attributed to regulation by buccal pumping and mucus production. Considerable importance is attributed to threshold feeding with respect to larval adaptation to oligotrophic environments.

Nerve-independent DNA synthesis and mitosis in regenerating hindlimbs of larval Xenopus laevis.

Xenopus laevis larvae at stage 52-53 (according to Nieuwkoop and Faber 1956) were subjected to amputation of both limbs at the thigh level as well as to repeated denervations of the right limb. Results obtained in larvae sacrificed during wound healing (1 after amputation), blastema formation (3 days) and blastema growth (5 and 7 days) showed that denervated right limbs have undergone the same histological modifications observed in innervated left limbs and have formed a regeneration blastema consisting of mesenchymal cells with a pattern of DNA synthesis and mitosis very similar to that in presence of nerves. Also, the patterns of cellular density in regenerating right and left limbs were very similar. On the whole, the data here reported show a highly remarkable degree of nerve-independence for regeneration in hindlimbs of larval Xenopus laevis at stage 52-53 and lend some substance to the hypothesis that, in early limbs, there would exist trophic factors capable of replacing those released by nerves, promoting DNA synthesis and mitosis in blastemal cells.

The development of swimming rhythmicity in post-embryonic Xenopus laevis

The post-embryonic development of 'fictive' swimming in immobilized Xenopus laevis tadpoles has been examined during the first day of larval life. In Xenopus embryos (stage 37-38; Nieuwkoop & Faber 1956), the rhythmic ventral root activity underlying swimming occurs as single brief (ca. 7 ms) compound impulses on each cycle. However, by stage 42 (about 24 h after hatching), ventral root discharge consists of bursts lasting around 20 ms per cycle. In addition to increased burst duration in each cycle of larval swimming, the range of cycle periods within an episode increases, although mean period values (ca. 70-80 ms) remain similar to those of the younger animal. Consequently, motoneurons at developmental stage 42 are active during swimming for a greater percentage (ca. 25%) of cycle time than at stage 37-38 (ca. 10%). Developmental stage 40 (ca. 12 h post-hatching) is an intermediate stage in rhythm development. Ventral root discharge varies from bursts of 10-20 ms at the start of an episode to embryonic (ca. 7 ms) spikes at the end of an episode. Furthermore, discharge varies from bursts of activity in rostral segments of stage 40 larvae to 7 ms spikes more caudally, as in embryos. The data thus suggest that Xenopus swimming rhythmicity develops relatively rapidly, along a rostrocaudal gradient, and may involve acquisition of multiple spiking in spinal neurons.

Occurrence of nonlymphoid leukocytes that are not derived from blood islands in Xenopus laevis larvae

Previous immunohistochemical observations using the monoclonal antibody (XL-1) which recognizes all types of leukocytes in Xenopus laevis revealed the occurrence of XL-1 + cells in the mesenchyme throughout the early larval body, before the appearance of any lymphocytes. The present experiments were performed to determine whether these leukocytes originate, like lymphocytes and red blood cells (RBCs), in the ventral blood islands (VBI) or the dorsolateral plate (DLP). For tracing the derivation of cells, a specific staining by quinacrine to nuclei of X. laevis and Xenopus borealis hybrid ( LB) cells was used to distinguish them from X. laevis ( LL) cells. Orthotopic graftings of VBI tissue from st.22–23 LB embryos to the stage-matched LL embryos and examinations at st.44–45 before differentiation of the lymphocytes showed that the proportion of XL-1 + LB cells was always significantly lower than that of RBCs with the same marker in all experimental larvae. The head ( LB)-body ( LL) chimeras from st.22–23 embryos and culture of the head-portions as VBI- and DLP-free explants from st.14–23 embryos both demonstrated that a significant number of XL-1 + cells which had originated in the head portions had begun to differentiate by st.42–43. These results indicate that there is a significant population of larval nonlymphoid leukocytes (mostly macrophages) that do not originate from either the VBI or DLP region, and are distributed in the mesenchyme throughout the body.

Effect of denervation on hindlimb regeneration in Xenopus laevis larvae

Abstract Xenopus laevis larvae at stages 51–57, according to Nieuwkoop and Faber [18], were subjected to amputation of the right hindlimb or of both limbs at the thigh or the tarsal level, as well as to somatic denervation of the right limb. Larvae at the same stage having undergone amputation of the right limb or of both limbs and sham denervation of the right limb were used as controls. In experimental series I a single denervation of the right limb was performed at the time of amputation. In experimental series II repeated denervations were performed (before, during and after amputation). Results show that in larvae at stages 51–53 subjected to limb amputation at the proximal level (thigh) even repeated denervation of the right limb did not prevent regeneration, although giving rise to various degrees of hypotrophy. In stage-55 larvae partial inhibition of the regenerative process in the right limb was clearly visible only after repeated denervations and amputation at the proximal level. After amputation at the distal level (tarsalia) the regenerative process in the right limb underwent no significant delay with respect to the controls, although the regenerated right limb was hypotrophic. In stage-57 larvae even a single denervation at the time of amputation was enough to inhibit regeneration of the right limb after either proximal or distal amputation. Therefore, in Xenopus laevis larvae, nerve-dependence for hindlimb regeneration takes place proximodistally as the nerve fibers grow in the limb and it gradually undergoes a process of proximodistal differentiation. Comparison of the regenerative processes occurring in regenerating innervated and denervated limbs amputated at various larval stages has revealed that the evolution of the regenerative process and the morphological complexity of the regenerate depend on the stage of development reached by the limb at the time of amputation rather than on extent of innervation. Therefore, in Xenopus laevis larvae, the decrease in the regenerative capacity of the developing hindlimb cannot be explaned solely in terms of extrinsic changes, i.e. through a reduction in the innervation of the limb, but could be dependent upon intrinsic changes occurring in limb cells during development.

Amphibian albumins as members of the albumin, alpha-fetoprotein, vitamin D-binding protein multigene family

The Xenopus laevis 68-kd and 74-kd albumin amino acid sequences are examined with respect to their relationship to the other known members of the albumin/alpha-fetoprotein/vitamin D-binding protein gene family. Each of the three members of this family presents a unique pattern of conserved regions indicating a differential selective pressure related to specific functional characteristics. Furthermore, an evolutionary tree of these genes was deduced from the divergence times calculated from direct nucleotide sequence comparisons of individual gene pairs. These calculations indicate that the vitamin D-binding protein/albumin separation occurred 560-600 million years (Myr) ago and the albumin/alpha-fetoprotein divergence 280 Myr ago. This observation leads to the hypothesis according to which the albumin/alpha-fetoprotein gene duplication occurred shortly after the amphibian/reptile separation. Consequently, and unlike mammals, amphibians and fishes should lack an alpha-fetoprotein in their serum at larval stages, which is consistent with a recent analysis of serum proteins in Xenopus laevis larvae. This hypothesis now will have to be tested further in additional lower vertebrates.

A monoclonal antibody specific for an epidermal cell antigen of Xenopus laevis: electron microscopic observations using a gold-labeling method.

A monoclonal antibody (EPI-1), raised against the supernatant of a homogenate of Xenopus laevis larvae at the tailbud stage (stage 36/37), interacts specifically with a 250 KD epidermal antigen of Xenopus. An immunocytochemical gold-labeling technique was used to investigate changes in antigen distribution during epidermal development of Xenopus laevis. Specific immunolabeling was initially detected over the endoplasmic reticulum in the outer epithelial cells of the late gastrula stage (stage 12.5). After the early neurula stage (stage 13), immunolabeling appeared over moderately electron-dense bodies (these bodies disappear after stage 29), and also over the apical cell surface and adjacent cytoplasm of all the outer epithelial cells. During metamorphosis, labeling decreased and disappeared after stage 62, as the superficial layer had peeled off. These data suggest that the antigen is useful as a marker of general differentiation in studies of epidermal development during the embryonic and larval stages of Xenopus laevis.

The effects of heat shock on the morphology and protein synthesis of the epidermis of Xenopus laevis larvae.

By scanning electron microscopy, we have observed that a 20-min heat shock at 37 degrees C, although not lethal, causes extensive damage to the epidermis of 30-h and 2-d (post-fertilization) Xenopus laevis larvae. The primary effects of heat shock are the apical swelling of the epidermal cells, giving the epidermis a "cobblestone" appearance, and the selective shedding of the ciliated cells. The shed cells may be cell fragments, however, because some of them are anucleate. Shed cells also exhibit the enriched synthesis of a group of heat shock proteins of 62,000 D molecular weight, suggesting that these proteins are specific to the shed cells. Prolonged heat shock of these larvae (i.e., 30 min at 37 degrees C) results in the complete disintegration of the epidermis, followed by larval death. At later stages of development (3-d and 4-d post-fertilization), the epidermis becomes more resistant to heat-induced damage inflicted by a 20-min heat shock. This increase in resistance coincides with the development of large secretory cells and the loss of ciliated cells in the epidermis and thus parallels a change in the state of histological differentiation.