Publications

nucleotide sequence through the 18s-28s intergene region of a vertebrate ribosomal transcription unit.	we have determined the nucleotide sequence of part of a cloned ribosomal transcription unit from xenopus laevis extending from the 3' region of the 18s gene through the 18s-28s intergene region into the start of the 28s gene. the 18s 3' region possess two tracts of high homology with the corresponding segments of other eukaryotic 18s genes (yeast and bombyx mori) separated by a tract of low homology which in x. laevis is rich in g plus c. the first internal transcribed spacer, between the 18s an ...	1980	6258158
characterization of na+/k+-atpase from xenopus laevis kidney. preliminary results.	in xenopus laevis, the renal na+/k+-dependent atpase is a very important enzyme involved in osmoregulatory processes and active transport. the enzyme was obtained from a microsome fraction purified by sucrose discontinuous gradient (10%, 15%, 29.4%) ultracentrifugation after sds treatment, and concentrated in the denser layer. the assayed biochemical parameters and their values are: 1) km (atp): 0.24 mm; 2) k1/2 (na+): 20.6 mm; 3) k1/2 (k+) 1.6 mm; 4) ki (ouabain): 0.025 micrometer; 5) optimum p ...	1980	6261778
expression of the herpes thymidine kinase gene in xenopus laevis oocytes: an assay for the study of deletion mutants constructed in vitro.	when xenopus laevis oocyte nuclei are injected with a recombinant plasmid containing the herpes simplex virus (hsv) thymidine kinase (tk) gene, a 100-fold increase in tk enzymatic activity is observed. three lines of evidence show that this increase in tk activity is a result of the expression of the hsv tk gene. first, the enzymatic activity is selectively inactivated by the igg fraction of antiserum raised against hsv tk protein. second, a polypeptide that comigrates with authentic hsv tk on p ...	1980	6258155
recombination of a eukaryotic dna in bacteria.	single, 824 bp repeating units of xenopus laevis oocyte-type 5s dna were inserted into the recombination vectors, lambda rva and lambda rvb. when the inserts had the same orientation with respect to the lambda chromosomes. spi- imm434 recombinants were recovered by selection on a p2, lambda double lysogenic host. because of the structure of the vectors, the crossover point in each recombinant must lie completely within the 5s dna insert. the physical characteristics of these recombinants were de ...	1980	6254845
isolation of two closely related vitellogenin genes, including their flanking regions, from a xenopus laevis gene library.	a gene library of xenopus laevis was constructed from embryonic dna partially digested with restriction endonucleases hae iii and alu i and joined to the phage lambda charon 4 cloning vector with ecori linkers. nucleotide sequences from three of the four related vitellogenin genes have been isolated. two of the genes (called a1 and a2) were isolated in their entirety together with long stretches of flanking sequences. these two closely related vitellogenin genes have lengths of about 21 and 16 k ...	1980	6445556
protein synthesis by hybrid ribosomes reconstructed from rabbit reticulocyte ribosomal core-particles and amphibian or fungal split-proteins.	it was shown that high-salt (2.75 m-nh4cl/69mm-mgcl2) shock treatment at 0 degrees c of the larger subparticles (l-subparticles) of rabbit, xenopus laevis and neurospora crassa cytoplasmic ribosomes yielded split-protein fractions that were not only functionally equivalent but also interchangeable. thus, although the remaining core-particles were inactive in both the puromycin reaction and in poly(u)-directed polyphenylalanine synthesis, activity was restored when they were combined with either ...	1980	6446904
maintenance of embryonically induced transplantation tolerance after short-term adult thymectomy in xenopus laevis.		1980	6447634
cupular secretion by xenopus laevis line organs: autoradiographic evidence for incorporation of 3h-glucose and 35s-sulfate.	autoradiographic evidence for incorporation of 3h-glucose and 35s-sulfate into the cupulae of xenopus laevis (african clawed toad) lateral line organs was obtained after injection into the dorsal lymph sacs of adult animals. time intervals of 15 minutes to 4 hours after administration of these labeled metabolic precursors were used to examine the time course of the apparent mechanism of growth of the cupulae. our results suggest that the two layers of accessory cells (the sustentacular cells and ...	1980	6447996
increased ornithine decarboxylase activity during meiotic maturation in xenopus laevis oocytes.		1980	6776957
the role of thyroxine in the transition of vitellogenin synthesis from noninducibility to inducibility during metamorphosis in xenopus laevis.		1980	6772502
the identification of messenger rna coding for immunoglobulin heavy and light chains of xenopus laevis.	total poly(a)-containing mrna isolated from xenopus spleens was translated in a rabbit reticulocyte lysate in vitro protein-synthesizing system. approx. 1% of the radioactivity incorporated into the protein was precipitated by an antibody directed against adult xenopus igm. the immunoprecipitated proteins were characterized as igm heavy and light chains by their molecular weight as determined by polyacrylamide-sodium dodecyl sulfate gel electrophoresis the sequence variability of the synthesized ...	1980	6772222
the determination of genome size in male and female germ cells of drosophila melanogaster by dna-feulgen cytophotometry.	the amounts of dna in haploid and diploid cells of drosophila melanogaster have been determined by dna-feulgen cytophotometry, using xenopus laevis erythrocyte nuclei as a reference standard. the haploid male genome is estimated to be 0.18 pg dna and the haploid female genome, 0.20 pg dna.	1980	6771237
intracellular ph in early xenopus embryos: its effect on current flow between blastomeres.	1. electrophysiological techniques were used to monitor the flow of electric current from one cell to the next in xenopus laevis embryos between the 4-cell and early blastula stages of development. intracellular ph and blastocoel ph were determined using ph-sensitive micro-electrodes. 2. the resting intracellular ph was 7.74+/-0.02 (s.e. of mean, n = 29); there were no systematic differences between developmental stages. blastocoel cavity ph was 8.4+/-0.06 (s.e. of mean, n = 10). the intracellul ...	1980	6770084
drosophila nuclei replicate in xenopus eggs.	nuclei isolated from a permanent cell line derived from drosophila melanogaster embryos have been injected, along with a radioactive dna precursor [3h]ttp, into xenopus laevis eggs. in culture, less than 7% of the cells were in s phase. after a 90 min incubation, following injection into eggs, 99% of the nuclei were shown by autoradiography to have synthesized dna. in a similar experiment, a density label brdutp was injected into eggs along with the nuclei. subsequent analysis on caesium chlorid ...	1980	6768826
sds-polyacrylamide gel electrophoresis of isolated cortices of xenopus laevis eggs.	outer cytoplasmic membranes of xenopus laevis eggs were isolated by manual dissection. cortices of unfertilized eggs and membranes of fertilized stages were subjected to sds-polyacrylamide gel electrophoresis. the coomassie-blue stained gels of membranes from unfertilized eggs and fertilized stages showed 21 and 17 major bands, respectively, covering a molecular weight range of 27000 - 275000 daltons. to demonstrate their purity, the preparations were examined by light and electron microscopy an ...	1980	6777051
transcription units of chicken ovalbumin gene observed after injection of cloned complete genes into xenopus oocyte nuclei.	the organization of transcription of a well-characterized protein encoding gene was studied by microinjection and electron microscopy. circular recombinant dna molecules containing the complete chicken ovalbumin sequences (7.7 kilobases, contained in 11.5 kilobases of chicken dna) were microinjected into germinal vesicles of living oocytes of the clawed toad xenopus laevis, and their transcription was studied in nuclear spread preparations. evaluation of spread chromatin showed a limited number ...	1980	6934527
lymphocyte reactivity to "t" and "b" cell mitogens in xenopus laevis: studies on thymus and spleen.		1980	6966237
a hormonal component in central vestibular compensation.	in developing clawed toads xenopus laevis, the compensation mechanism of behavioural defects caused by unilateral labyrinthectomy consists of two components, a vestibular one and a hormonal one. thyroxine accelerates this compensation.	1980	6971023
actin synthesis during the early development of xenopus laevis.	cytoskeletal beta and gamma-actin are amongst the most rapidly made proteins of oocytes, blastulae and late embryonic stages of xenopus laevis but, relative to other proteins, the rate of synthesis is low in the egg or cleaving embryo, although the messenger rna is present in comparable amounts at the different stages. actin synthesis therefore involves post-transcriptional regulation. alpha-actin, the actin class characteristics of straited muscle cells, is first detectable in late gastrulae an ...	1980	6893718
progesterone induces a rapid increase in [ca2+]in of xenopus laevis oocytes.	progesterone causes a rapid increase in the intracellular free calcium level in fully grown amphibian oocytes. when albino xenopus laevis oocytes were microinjected with the ca-specific photoprotein aequorin, the calcium-induced luminescence from this protein increased at 40-60 sec after the addition of progesterone and returned to the control level within 5-6 min. no further change in the aequorin glow could be detected through the remainder of the maturation period. this transient increase in ...	1980	6929506
photoreactivation of uv induced cell killing, chromosome aberrations, sister chromatid exchanges, mutations and pyrimidine dimers in xenopus laevis fibroblasts.	fibroblasts from xenopus laevis, which possess photoreactivating enzyme were used to study the influence of photoreactivating light on the frequency of pyrimidine dimers in dna, chromosomal aberrations, sister chromatid exchanges, cell killing and the induction of gene mutations (ouabain-resistance) induced by 254 nm ultraviolet irradiation. the frequency of all biological endpoints studied were reduced following exposure to photoreactivating light parallel to the reduction in the frequencies of ...	1980	6936598
responses of msh and prolactin cells to 5-hydroxytryptophan (5-htp) in amphibians and teleosts.	injection of 5-hydroxytryptophan (5-htp), a precursor of serotonin, induces dispersion of melanin in the amphibians, pleurodeles waltlii (urodela) and xenopus laevis (anura), in the goldfish, carassius auratus, and in the carp, cyprinus carpio. it is accompanied by a dispersion of erythrophore pigments. in the pituitaries of pleurodeles and goldfish, a stimulation of msh cells, characterized by a significant nuclear hypertrophy, is also observed; in carassius, msh cells may become degranulated. ...	1980	6967354
proteins iodinated by the chloramine-t method appear to be degraded at an abnormally rapid rate after endocytosis.	proteins labeled with either (3)h by reductive methylation or (125)i by the chloramine-t method were incubated with xenopus laevis oocytes; the incorporation and acid precipitability of the proteins were then studied. the uptake rates of both specifically incorporated (vitellogenin) and nonspecifically incorporated proteins (bovine serum albumin and x. laevis serum proteins lacking albumin) were not influenced by the method of labeling. however, (125)i-labeled proteins were apparently degraded a ...	1980	6929509
growth of anuran oocytes in serum-supplemented medium.	appropriately-sized oocytes from xenopus laevis can be grown in vitro in vitellogenin-containing serum for up to 2 weeks. the source of serum appears to be unimportant. rates of oocyte growth are somewhat better than those achieved in the presence of vitellogenin alone. rana pipiens oocytes grow about twice as fast as x. laevis oocytes under identical conditions. the oocyte culture conditions described appear to be applicable to amphibian oocytes in general.	1980	6984201
xenopus laevis larval thymocytes do not express surface immunoglobulin.		1980	6996841
development of the mitochondrial mass and accumulation of mtdna in previtellogenic stages of xenopus laevis oocytes.	the development of the mitochondrial mass of the previtellogenic oocytes of xenopus laevis has been analysed by a morphometric method and it has been correlated with the growth of the occyte itself. the study which was performed with oocytes from females of very different ages shows that the relative size of the mitochondrial mass in the smallest oocytes decreases gradually with the age of the females. cytophotometric analysis of the feulgen-stained mitochondrial mass enabled the mtdna content o ...	1980	6988447
initiation of ribosomal rna chains in homogenates of oocyte nuclei.	we have studied rna synthesis in homogenates of nuclei manually isolated from mature oocytes of xenopus laevis. transcription of the endogenous template is completely resistant to alpha-amanitin and represents primarily transcription of the amplified ribosomal genes. three lines of evidence demonstrate that rna polymerase i is reinitiating rna chains in this system. first, transcription continues in the homogenates for at least 8 h. during this time, 50 to 100 rna copies are synthesized per gene ...	1980	6995456
development of an amphibian neuromuscular junction in vivo and in culture.	during normal development presumptive synaptic sites in the myotomes of xenopus laevis begin to acquire a high density of ach receptors within as little as 2 h after the arrival of the nerve fibres. synaptic function also begins very shortly after the arrival of the nerve fibres. initially synaptic currents are some eight times longer lasting than at maturity and are not prolonged by anticholinesterase. during the first day after nerve-muscle contact is made there is a considerable decrease in s ...	1980	7009778
effects of n-methyl-n-nitrosourea on the lymphoid tissues and skin allograft response of xenopus laevis.		1980	6995191
further studies on acid proteolytic activity in the fore gut of xenopus laevis larvae.		1980	7011857
partial purification and characterization of the maturation-promoting factor from eggs of xenopus laevis.		1980	7000582
molecular cloning of cdna sequences coding for the major alpha- and beta-globin polypeptides of adult xenopus laevis.	this report describes the synthesis and cloning of almost complete dna copies of the mrnas encoding the major alpha-globin and major beta-globin of x. laevis. double-stranded globin cdna was inserted into the psti site of the plasmid pbr322 and two cloned recombinants (designated pxg6c1 and pxg8d2) were selected. these were shown to contain almost complete copies of x. laevis globin mrna. restriction enzyme maps were determined for each cdna sequence using the established method of partial diges ...	1980	7001356
the role of zinc and follicle cells in insulin-initiated meiotic maturation of xenopus laevis oocytes.		1980	7001631
[vestibular apparatus study of the toad, xenopus laevis, and rats under prolonged weightlessness].	fertilized eggs of the clawed toad xenopus laevis were placed aboard of orbital laboratory of "salut-6" spacecraft where they developed for 20 days at temperature 15 degrees c. the larvae were fixed in weightlessness. light and electronmicroscopic studies revealed undisturbed structure on the saccular and utricular maculae and otolith membranes. some ultrastructural abnormalities were found in the inner ear of adult rats after 20 days of weightlessness ("kosmos-936").	1980	6970471
an atlas of notochord and somite morphogenesis in several anuran and urodelean amphibians.	a scanning electron microscopic, comparative survey of notochord and somite formation including some details of change in cell morphology and arrangement, was made of selected stages of two species of anuran amphibians (xenopus laevis and rana pipiens) and two species of urodeles (ambystoma mexicanum and pleurodeles waltlii). the ectoderm or neural plate was removed from fixed embryos and the dorsal aspect of the developing notochord and somite mesoderm was photographed. micrographs of comparabl ...	1980	6971322
survey of the numbers and species of amphibia used in the united kingdom in 1977.	this survey suggests that about 115 000 amphibia, of which more than half were rana temporaria, were supplied for use in teaching and research in the uk during 1977. as only 27 000 were recorded as being used by universities, polytechnics and research institutes, probably mostly for teaching, it must be assumed that the majority are used for teaching purposes in schools. there has been an increase in the proportion of amphibia supplied by recognised suppliers, and also in that being 'purpose-bre ...	1980	6965743
investigation of the determinants of nuclear pore number.	to assess the functional significance of nuclear pore complexes, we have investigated whether the number of pores per nucleus is determined by such factors as the nuclear volume, nuclear surface area, dna content, or aspects of nuclear activity. comparisons were made between cell types chosen to permit observation of differences in nuclear pore number as a function of differences in the other qualities measured. the number of nuclear pores was determined by freeze-etching and measurements of nuc ...	1980	6966999
the spatio-temporal framework of melanogenic induction in pigmented retinal cells of xenopus laevis.	using methods involving transplantation of neuroectoderm and eye rudiments between ap/ap mutants and +/+ embryos of xenopus laevis, new aspects of melanogenic induction have been ascertained. the possibility of separate induction of a melanocyte cell type and of the product of terminal differentiation in it-melanosomes, has been demonstrated. melanogenic induction starts at the late gastrula stage. it has an irreversible character which has been shown using the technique of double transplantatio ...	1980	6975795
immunological identification and localization of the predominant nuclear protein of the amphibian oocyte nucleus.	nuclei of vitellogenic oocytes of the frog, xenopus laevis, contain a prominent protein, representing about 10% of nuclear protein, which is characterized by a polypeptide of m(r) 30,000. this protein is highly phosphorylated and acidic, displays several isoelectric variants with pi values ranging from 4.7 to 5.3, shows a high thermostability, is not stably complexed with other proteins, and is readily extracted in buffer solutions. guinea pig antibodies against this protein have allowed its spe ...	1980	6987661
more ribosomal spacer sequences from xenopus laevis.	the base sequence analysis of a xenopus laevis ribosomal dna repeat (7) has been extended to cover almost the entire non-transcribed and external transcribed spacer. a compilation of these sequences is presented. all the repetitive and non-repetitive sequence elements of the spacer are identified and their evolution discussed. comparison of the x.laevis and s.cerevisiae (25,26) ribosomal dnas shows about 80% sequence conservation in the 18s gene but no sequence conservation, from the available d ...	1980	7003549
rental projections in the adult xenopus laevis: a study with cobalt filling technique.	cobaltic-lysine complex was used to show retinal projections in adult xenopus laevis. retinal fibres partially decussate in the chiasma, and terminate on both sides in the nucleus ovoidalis, lateral geniculate complex, pretectal area, optic tectum, and in the basal optic nucleus. cobalt filled fibres were found in the preoptic area, dorsal hypothalamic area, and in the periventricular region close to the wall of the third ventricle.	1980	7008512
does h-y antigen induce the heterogametic ovary?	to determine whether phylogenetically conservative h-y antigen plays any part in gonadal differentiation among the nonmammalian vertebrates, we studied expression and binding of h-y in the frog, xenopus laevis. soluble h-y obtained from mouse testis and soluble h-w from chicken ovary bound specifically to cells of the zz testis from normal xenopus males. in addition, h-y (h-w) appeared selectively in the ovaries of zz genetic males that had been induced to become functional females by exposure t ...	1980	7191350
effects of temperature on sister chromatid exchanges.	the influence of temperature on sister chromatid exchanges was investigated, and the results are discussed in connection with factors possible involved in temperature-induced sce-formation. whereas the sce frequency increased with increasing growth temperature in a cell line of xenopus laevis (eax), which permits the examination of great temperature differences, a chinese hamster cell line (v-79) revealed a u-shaped temperature-response curve. in addition, it was found that cold treatment at 4 d ...	1980	7203467
the stability and translation of sea urchin histone messenger rna molecules injected into xenopus laevis eggs and developing embryos.		1980	7189491
the effect of a pesticide, gesagard 50, on regeneration of the endbrain of xenopus laevis (daud.) tadpoles.		1980	7215599
an autoradiographic study of the retinal projection in xenopus laevis with comparisons to rana.	the retinal projection to the brain in xenopus has been examined using autoradiographic techniques. the labelled moieties used were 3h-proline and 3h-fucose. there are three anterior thalamic optic terminal sites in xenopus. these are the nucleus of bellonci, the corpus geniculatum thalamicum and the rostral visual nucleus. the first two of these are similar to the homonymous nuclei in rana while the last, the rostral visual nucleus, appears to correspond to the rostral end of the posterior ento ...	1980	7351442
auditory and vocal nuclei in the frog brain concentrate sex hormones.	mate calling by south african clawed frogs, xenopus laevis, is under the control of androgens. autoradiographic studies demonstrate androgen-concentrating neurons in a motor nucleus that controls mate calling and a midbrain nucleus that is stimulated by sound. hormone concentration by laryngeal motor neurons suggests that steroids regulate the final common path for vocal behavior. modulation of auditory sensitivity by hormones could explain seasonal variations in behavioral responsiveness to con ...	1980	7352269
free calcium in xenopus embryos measured with ion-selective microelectrodes.	a regulatory role for intracellular free calcium has been suggested in cell division and intercellular communication via gap junctions. previous measurements of cytoplasmic free ca2+ during cell division and uncoupling have been made with the ca2+-sensitive photoprotein aequorin. we now report experiments using ca2+-sensitive microelectrodes to monitor free ca2+ in the cells and intercellular fluid of early embryos of the amphibian xenopus laevis. in addition to measuring basal levels, we have l ...	1980	7354852
nucleotide sequence of genes coding for trnaphe and trnatyr from a repeating unit of x. laevis dna.	a cloned 3.18 kb repeating unit of xenopus laevis dna, known to contain two trna1met genes, has been partially sequenced. the sequenced region could potentially code for two more trna species. one is a phenylalanine trna with the same primary sequence as that found in a variety of mammals. the other is a tyrosine trna with a 13 bp intervening sequence located immediately adjacent or very close to the 3' nucleotide of the anticodon. the two putative genes have opposite polarity and the sequences ...	1980	7357612
rohon-beard cells and other large neurons in xenopus embryos originate during gastrulation.	the time of origin (birthday) of rohon-beard cells in xenopus laevis was studied by 3h-thymidine autoradiography. rohon-beard cells were selected because they are a morphologically identifiable population of neurons in which the development of chemical and electrical excitability has been studied. a single injection of a radioactive dna precursor was given to animals in successive stages of development from blastula to late tail bud (nieuwkoop and faber stages 8--33/34). the label was available ...	1980	7364967
tadpole of the clawed frog, xenopus laevis, as an experimental intermediate host of angiostrongylus cantonensis.	frogs and toads, as paratenic hosts, are known to be sources of human angiostrongyliasis. the present investigation was carried out to examine the role of the tadpole as an experimental intermediate host. first-, second-, and third-stage larvae were found in tadpoles of xenopus laevis from the 1st, 14th, and 18th days, respectively, after exposure to the first-stage larva of angiostrongylus cantonensis. third-stage larvae collected from a frog which had metamorphosed from an exposed tadpole reac ...	1980	7369450
isolation and translation in vitro of four related vitellogenin mrnas of estrogen-stimulated xenopus laevis.	cloning of vitellogenin cdna of xenopus laevis revealed that vitellogenin is encoded in a small family of genes representing two distantly related main groups a and b, each comprising two more closely related subgroups a1, a2, and b1, b2 respectively. to characterize the proteins derived from these genes we have isolated the corresponding mrnas by hybridizing, under stringent conditions, cytoplasmic poly(a)-containing rna from the liver of estrogen-stimulated xenopus to filter-bound cdna clones ...	1980	7371639
specification of retinotectal connexions during development of the toad xenopus laevis.	the specification of central connexions of retinal ganglion cells was studied in xenopus laevis. in one series of experiments, the right eye primordium was rotated 180 degrees at embryonic stages 24-32. in the other series, the left eye was transplanted into the right orbit, and vice versa, with either 0 degrees or 180 degrees rotation. after metamorphosis the visual projections from the operated eye to the contralateral optic tectum were mapped electrophysiologically and compared with the norma ...	1980	7373205
trochlear nerve regeneration in xenopus laevis larvae.	the trochlear nerve of xenopus laevis larvae was sectioned in the orbit, and the nerve distal to the section was removed, so that it could not serve to guide the return of the regenerating nerve to the superior oblique muscle (som). in a second series, in addition to removing the distal nerve segment, the som was removed or damaged. the regeneration pattern of the trochlear nerve fibers was recorded in situ at 1 to 5-day intervals with the aid of methylene blue staining. the early growth or rege ...	1980	7373272
the effect of divalent and trivalent cations on the sodium permeability of myelinated nerve fibres of xenopus laevis.	the effect of external ca, mg, sr, ba and la concentration was studied in potential clamp experiments on single myelinated nerve fibres of the frog (xenopus laevis). an increase in cationic concentration caused a positive shift of the na permeability (pna) curve along the potential axis, a decrease in max. pna and a decrease in the slope of the pna curve. the effectiveness of mg, sr and ba was similar and about 0.6 compared to that of ca, whereas la was about 80 times more effective than ca.	1980	7376904
the use of xenopus laevis for studies of osmotic regulation.		1980	7379538
scanning electron microscopy of cells from hydroxyurea-arrested blastulae of xenopus laevis.	cells from xenopus embryos blocked at the blastula stage by treatment with hydroxyurea have been isolated and cultured in vitro. the morphology of these cells has been compared with that of cells from normal embryos using scanning electron microscopy. cells from such hydroxyurea-blocked embryos do not show the features, or changes in features, in culture shown by cells from normal embryos.	1980	7379924
ontogeny of the retina and optic nerve of xenopus laevis. iv. ultrastructural evidence of early ganglion cell differentiation.		1980	7380095
ontogeny of the retina and optic nerve in xenopus laevis. i. stages in the early development of the retina.	a staging series for retinal histogenesis is described for xenopus laevis. it is based on scanning em studies of intact eyes and serial section analysis of silver-stained preparations. retinal development is arbitrarily divided into twelve stages (i--xii) based on distinct morphological and histological changes. in general, four different phases of retinal development can be identified: an embryonic phase of rapid histogenesis (nieuwkoop-faber stages 21--39) and onset of visual function; a secon ...	1980	7381043
ontogeny of the retina and optic nerve in xenopus laevis. ii. ontogeny of the optic fiber pattern in the retina.	the ontogeny of optic fiber patterns within the xenopus laevis retina has been studied in silver-stained serial sections and whole mounts. ganglion cell axons ("pioneers") appear much earlier than previously recognized, at nieuwkoop-faber ('56) stage 28. fibers fasciculate into bundles as they arise in the retina and reach the chiasma by nf stage 32. because the retina grows in rings at its margins, ganglion cell axons differentiate at the periphery and join the nearest fiber bundles (fascicles) ...	1980	7381045
cytotoxic effects of sodium selenite on tadpoles (xenopus laevis).	the cytotoxic effects of sodium selenite on developing tadpoles (xenopus laevis) were examined by scanning, light, and electron microscopy. selenium exposure resulted in disorganization, vacuolization, and swelling of the outer layer of epithelial cells in the tadpole epidermis. examination of muscle cells in the somites revealed myofibril disorganization and cell degeneration. mitochondria in both epithelial and muscle cells were swollen and showed loss of cristae. it is likely that sublethal e ...	1980	7387186
construction of a cloned library of expressed embryonic gene sequences from xenopus laevis.		1980	7390012
use of a cloned library for the study of abundant poly(a)+rna during xenopus laevis development.		1980	7390013
messenger rna for ribosomal proteins in xenopus laevis oocytes.	xenopus laevis oocytes (stage ii--iii) incorporate [35s]methionine into ribosomal proteins identifiable by two-dimensional electrophoresis. poly(a)-rich rna prepared form this source has been translated in a wheat germ cell-free system. among the products synthesized in vitro 30 are clearly identifiable as ribosomal proteins. some are not individually resolved, whereas a few of them may lack methionine or may not be correctly processed after the synthesis in vitro. an enrichment of our mrna prep ...	1980	7398629
the persistence in xenopus laevis dna of o6-methylguanine produced by exposure to n-methyl-n-nitrosourea.		1980	7398719
chromatin organization within nuclear blebs in leukocytes of xenopus laevis.	the nuclei from leukocytes of peripheral blood, liver and spleen of an individual anaemic xenopus laevis have been found to possess numerous nuclear blebs or projections. there structures were found to be very variable in size and shape as viewed in electron micrographs, but commonly included an enclosed mass of cytoplasm bound on one side by a very thin section of nuclear material. such sections are membrane bounded on each side and frequently display an interesting ordered array of chromatin.	1980	7398859
ultrastructural analysis of ribosomal gene transcription in vitro.	xenopus laevis oocyte nuclei were manually isolated and incubated in vitro to synthesize rna. aliquots were removed before and during the reaction and prepared for electron microscopy, in order to observe the amplified ribosomal rna (rrna) genes. before in vitro incubation the rrna transcription units are covered with close-packed rna polymerase molecules and attached rrnp fibers, which are interpreted to have initiated in vivo. with incubation, these close-packed transcription complexes move do ...	1980	7400153
an interaction between dorsal and ventral regions of the marginal zone in early amphibian embryos.	when small explants from early gastrulae of xenopus laevis are allowed to develop in a buffered salt solution there is a considerable difference between the patterns of differentiation obtained from different dorsoventral levels of the marginal zone. these patterns of differentiation correspond to the fates of the different regions in the course of normal development. they are not altered if several explants of the same type are fused before culture. if a ventral marginal zone explant from xenop ...	1980	7400747
neutral carrier ion-selective microelectrodes for measurement of intracellular free calcium.	this paper describes the making and testing of calcium-selective microelectrodes for measurement of intracellular [free ca2+] levels. pipettes of tip outer diameter down to 0.4 micron were siliconized by a novel and easy method of vapor treatment. the tips were filled with a sensor mixture using the neutral ligand and solvent of oehme et al. (oehme, m., kessler, m. and simon, w. (1976) chimia (aarau) 30, 204-206) but with very hydrophobic cations replacing na+ in the salt component. this change ...	1980	7407109
synthesis of very high molecular weight thyroglobulin in three heterologous systems.	calf thyroid rna has been translated in the wheat germ and messenger-dependent reticulocyte lysate cell free systems and in xenopus laevis oocytes. peptides immunologically related to and comigrating with 330 000 molecular weight thyroglobulin as well as both larger and smaller immunopeptides have been synthesized. the smaller molecular weight peptides were due at least in part to degradation during incubation despite the presence of protease inhibitors. the larger molecular weight peptides coul ...	1980	7407682
lipoprotein crystals in the yolk platelet of a teleost, pelvicachromis pulcher (cichlidae).	yolk-platelt crystals in the oocytes of the teleost pelvicachromis pulcher (cichlidae) were shown, using electron diffraction and tilting of thin-sectioned specimens, to possess an orthorhombic lattice with unit-cells sides a = 8.3 nm, b = 16.6 nm and c = 18.0 nm. they thus closely resemble the crystals known for a newt (triturus sp.) and a frog (xenopus laevis).	1980	7407843
assembly of nucleosomes: the reaction involving x. laevis nucleoplasmin.	we analyze the nucleosome core assembly reaction which is mediated in vitro by a protein previously purified from xenopus laevis eggs, now named nucleoplasmin in reference to its occurrence in the soluble phase of the nucleus of a wide range of vertebrate cell types. nucleoplasmin is present in solution as a pentamer. we use nuclease digestion analysis to show that the protein assembles bona fide nucleosome cores in vitro from purified histones and dna. nucleoplasmin itself binds neither to dna ...	1980	7407918
biochemical research on oogenesis. composition of the 42-s storage particles of xenopus laevix oocytes.	previtellogenic oocytes of xenopus laevis contain far more 5-s rna and trna than 28-s + 18-s rna. trna and 5-s rna are storage products that will be used in later oogenesis for protein synthesis and ribosome assembly. stored trna and 5-s rna are not free in the cell sap but belong to nucleoprotein particles of various sizes. there are two prominent kinds of storage particles in previtellogenic oocytes of x. laevis. the smaller ones (7-s) contain about half of the cell's 5-s rna. the larger ones ...	1980	7408887
kinetics of synthesis and processing of precursor polypeptides of murine leukemia virus in frog oocytes following microinjection of viral rna.	microinjection of rauscher murine leukemia viral rna into living oocytes from xenopus laevis, in contrast to cell-free systems, allowed detailed studies on the processing of newly synthesized viral precursor polypeptides. the viral messenger appeared to be stable for at least 5 days. maximal rate of translation of 70-s virion rna was observed 10 h after injection. the predominant translation products after a 1-h labeling period were three precursor polypeptides of mr 77000, 75000 and 65000. foll ...	1980	7408891
effects of the carcinogens n-methyl-n-nitrosourea and n-dimethylnitrosamine on spleen cell antigen binding in xenopus laevis.		1980	7409292
determination of the dorsal-ventral axis in eggs of xenopus laevis: complete rescue of uv-impaired eggs by oblique orientation before first cleavage.		1980	7409319
xenopus laevis oocyte calmodulin in the process of meiotic maturation.	calcium ions are postulated to be involved in the process of meiotic maturation of amphibian oocytes. since several of the calcium effects in eukaryotic cells are mediated by calmodulin, the present study was undertaken to assess the presence and level of calmodulin in xenopus laevis oocytes before and after progesterone treatment. calmodulin was shown to be present at a concentration of approximately microm in control oocytes cytosol. this level remained stable for 2 h when the maturation promo ...	1980	7410431
electrical excitability of outgrowing neurites of embryonic neurones in cultures of dissociated neural plate of xenopus laevis.	1. i have studied the electrical excitability of outgrowing processes of individual neurones in cultures made from dissociated neural plates of embryos of xenopus laevis prior to the time of neurite outgrowth in vivo. 2. the electrical excitability of neurites was tested by stimulating them extracellularly and recording responses with an intracellular electrode in their cell bodies; neurites were excitable at all times examined. 3. the ionic basis of the excitability of neurites was tested by re ...	1980	7411426
modification of mitochondria-rich cells in different ionic conditions: changes in cell morphology and cell number in the skin of xenopus laevis.	xenopus laevis were kept in salt water (1.25% nacl), distilled water, or tapwater for a month. compared to the animals kept in tap water, the number of mitochondria-rich (mr) cells in the nacl-adapted animals was significantly reduced, while it was increased in those maintained in distilled water. in addition, the mr-cells of nacl-adapted animals lost their slender flask shape and developed large deposits of glycogen. the alteration of this cell type in conditions of high or low salinity may ref ...	1980	7416509
the morphology of cultured melanophores from tadpoles of xenopus laevis: scanning electron microscopical observations.	tail-fin melanophores of tadpoles of xenopus laevis (daudin) in primary culture were examined scanning electron microscopically in the aggregated and in the dispersed state. after isolation, the melanophores are spherical, but within 24 h they develop thin filopodia for attachment to the substratum. subsequently, cylinder-like as well as flat sheet-like processes are formed, which adhere to the substratum with terminal pseudopodia and filopodia. the processes of adjacent melanophores contact eac ...	1980	7417982
location of motoneurons supplying individual muscles in normal and grafted supernumerary limbs of xenopus laevis.	the purpose of this study was to investigate innervation of transplanted supernumerary hindlimbs in the frog (xenopus laevis). motoneurons innervating identified muscles in normal and supernumerary limbs were located by the method of retrograde transport of hrp after intramuscular injection. in the lumbar spinal cord of normal xenopus, motoneurons supplying medial hindlimb muscles, which are derived from the ventral muscle mass during development, are located at the medial end of the motor colum ...	1980	7419750
differential postendocytotic compartmentation in xenopus oocytes is mediated by a specifically bound ligand.	xenopus laevis oocytes were used as a model system to study the intracellular fate of proteins incorporated by endocytosis. we found that the intracellular stability and compartmentation of proteins incorporated by receptor-mediated endocytosis differed substantially from that of proteins incorporated by nonspecific endocytosis. after its uptake, the specifically sequestered yolk precursor protein vitellogenin was converted to the yolk proteins lipovitellin and phosvitin which were stable with t ...	1980	7428040
the effect of actinomycin d on the nucleolus and on pigment synthesis in pigment cells of xenopus laevis: an ultrastructural study.	the effect of amd on the nucleolus and on melanogenesis in differentiating pigment cells of xenopus laevis was investigated in cultured neural crest cells. cultures were treated with either 2 or 10 microgram/ml amd for 41/2 hours. following treatment the antibiotic was removed. observations of the cells were made with both scanning and transmission electron microscopes. actinomycin d almost entirely stopped pigment formation in neural crest cultures during treatment. the morphological sequence i ...	1980	7429969
membrane assembly in retinal photoreceptors i. freeze-fracture analysis of cytoplasmic vesicles in relationship to disc assembly.	to study precursor-product relationships between cytoplasmic membranes of the inner segment of photoreceptors and the continually renewed outer disc membrane, we have compared the density and size distribution of intramembrane particles (imp) in various membrane compartments of freeze-fractured photoreceptor inner and outer segments. both rod and cone outer segments of xenopus laevis are characterized by a relatively uniform distribution of approximately 4,400-4,700 imp/micron2 in p-face (pf) le ...	1980	7430251
disruption of optic fibre growth following eye rotation in xenopus laevis embryos.	eye rotation is an experimental paradigm used to study axial specification of the amphibian retina and its connections to the tectum. jacobson reported that the naso-temporal and dorso-ventral axes are determined sequentially and independently between nieuwkoop-faber (nf) stages 28 and 32 in xenopus. others claim however, that both retinal axes are determined much earlier and not independently in xenopus and rana. we now find that in xenopus this operation can disturb the exit of optic fibres fr ...	1980	7432498
cell movements in xenopus eye development.	the vertebrate eye develops morphological markers, such as the ventral choroid fissure, which define its anatomical polarity in relation to the body. also, retinal nerve fibres behave as though regionally differentiated by forming ordered, topographical maps of the visual field in the brain. it has long been held that these two features of eye polarity--its structure and the neural specificity of the ganglion cells--are labile in the early eye vesicle and become determined relative to body axes ...	1980	7432499
methylation map of xenopus laevis ribosomal rna.	one of the most enigmatic features of eukaryotic ribosomal rna is the presence of many methylated nucleotides. the numbers of rna methyl groups range from approximately 70 per ribosome in yeast to over 100 in vertebrates. here it is shown that the methylated nucleotides in xenopus laevis rrna are broadly but non-uniformly distributed. in 18s rrna 2'-o-methylations are partly concentrated in the 5' region and base methylations near the 3' end. in 28s rrna methyl groups are infrequent in the 5' re ...	1980	7432528
the nucleotide sequence of the major beta-globin mrna from xenopus laevis.	the nucleotide sequence of a cloned fragment containing an almost complete copy of the mrna encoding the major adult beta-globin polypeptide in xenopus laevis, the south african clawed toad, is presented. a procedure for strand separation by hybridization to complementary mrna was used to determine some of the sequence and this technique is described. the complete amino acid sequence of the polypeptide has been deduced and comparison with other vertebrate beta-globins reveals several highly cons ...	1980	7433108
multiple ribosomal gene sites revealed by in situ hybridization of xenopus rdna to triturus lampbrush chromosomes.	a variety of 3h-labelled ribosomal gene probes were hybridized in situ to the nascent transcripts of lampbrush chromosomes from the crested newt, triturus cristatus carnifex. the probes were from xenopus laevis and included rdna isolated by cscl gradient centrifugation, recombinant plasmids and purified restriction fragments of rdna. all the probes gave essentially the same result. about 10-15 loop pairs were distinctly labelled in each preparation, almost all of them located on the heteromorphi ...	1980	7438882
intracellular localization of dna polymerase activities within large oocytes of the frog, xenopus laevis.		1980	7439535
induction of estrogen receptor and reversal of the nuclear/cytoplasmic receptor ratio during vitellogenin synthesis and withdrawal in xenopus laevis.	the levels of cytoplasmic and nuclear estrogen receptor have been determined in livers of male xenopus laevis stimulated by estradiol-17 beta to synthesize vitellogenin mrna. estrogen receptor levels were also determined in unstimulated liver and following long term withdrawal of estrogen. in unstimulated liver cells, which do not contain detectable vitellogenin mrna, more than 80% of the estrogen receptor is located in the nucleus (550 high affinity estrogen binding sites/nucleus), while the cy ...	1980	7440543
similarities of membrane structure in freeze-fractured xenopus laevis kidney collecting tubule and urinary bladder.	the collecting tubule of xenopus laevis kidney is formed of two main types of cell: the socalled flask cells (or mitochondria-rich cells) and the remaining, more cuboidal epithelial cells. it has previously been shown that the flask-cell plasma membrane contains a population of elongated intramembrane particles similar to those found in mitochondria-rich cells of the toad bladder. it is now clear that the structure of the apical membrane of the remaining epithelial cells of the collecting duct i ...	1980	7440657
the development of lampbrush chromosome-type transcription in the early diplotene oocytes of xenopus laevis: an electron-microscope analysis.	oocytes of xenopus laevis in pachytene and early diplotene of meiosis have been studied using the miller spreading technique. transcription first appears in germinal vesicles 25-40 micrometers in diameter, when the oocyte is in early diplotene. transcription at this stage consists of arrays of short rnp transcripts, irregularly distributed along the dnp axis. linear regression analysis has shown that many of these arrays are transcription units (type i) with the transcripts having a common point ...	1980	7440660
re-evaluation of the 'contralateral' effects of sodium and potassium on sodium transport through toad skin.	1. changes in the concentration of na in the outer bathing solution, [na]o, or of k in the inner bathing solution, [k]i, alter the electrical responses of the isolated toad skin to changes in ionic concentrations in the contralateral solutions. the mechanism(s) of these apparently contralateral effects remain(s) unknown. 2. the phenomenon has been investigated here in the isolated abdominal skin of xenopus laevis. each skin was exposed to multiple levels of [na]o and [k]i, of between 5 and 112 m ...	1980	7441564
rabies mrna translation in xenopus laevis oocytes.	two rabies virus-specific mrna species were identified by analysis of their encoded proteins after translation of the partially purified species in xenopus laevis oocytes. one of these coded for the virion surface glycoprotein (g protein), and the other coded for the major structural protein of the virion nucleocapsid (n protein). the g-mrna sedimented in a sucrose density gradient at about 18s, and the n-mrna had a sedimentation coefficient of approximately 16s. their respective translation pro ...	1980	7441819
lipid metabolism during embryonic and early postembryonic development of xenopus laevis.	xenopus laevis embryos were analyzed for their lipid content from fertilization to feeding stage. the amounts of the major lipids did not change until after hatching, but at the feeding stage the amounts of phosphatidylcholine (pc), phosphatidylethanolamine (pe), and sterol esters (se) increased approximately twofold. prior to hatching, [14c]acetate label was incorporated primarily into the fatty acyl chains of pe. after hatching, increasing amounts of label were found in other phospholipids, in ...	1980	7448620
nucleolar localization of 'template-bound' rna polymerase i in nuclei of xenopus laevis blastulae.		1980	7449853
effect of triiodo-l-thyronine on amino acid transport by xenopus laevis embryos.		1980	7450293
developmental and ploidy characteristics of "senescent" unfertilized eggs of xenopus laevis.	unfertilized eggs of xenopus laevis have been stored in a high salt physiological saline (de boer's solution) over a period of about 24 hours. during this time, aliquots have been subjected to lowered salinity and fertilized. the stored eggs undergo a defined series of morphological changes, which are initially reversible upon fertilization. length of storage may have little or no effect on the capacity of the eggs to undergo cleavage, but the normal developmental capacity declines with increasi ...	1980	7452193
lens formation from cornea in the presence of the old lens in larval xenopus laevis.	three different types of experiments were carried out to investigate the role of the lens in lens-forming transformations of the outer cornea of xenopus laevis tadpoles (at stage 51, as defined by nieuwkoop and faber, '56): 1) simple lentectomy; 2) incision of outer and inner cornea in the region opposite the dorsal iris without lens removal; and 3) incision of outer and inner cornea and perforation of dorsal iris without lens removal. the results indicate that the outer cornea of xenopus laevis ...	1980	7452196