Oogenesis in Xenopus laevis (Daudin)

Summary This study was designed to explore the relationship of estrogen, human chorionic gonadotropin (HCG), and food availability to endocytosis in developing oocytes. When estrogen alone is administered to an animal, large amounts of vitellogenin are synthesized by the liver and secreted into the circulatory system, where it accumulates. Under these conditions there is no evidence of endocytosis at the surface of the oocytes. Other studies have shown that following HCG injection into estrogen-treated animals, vitellogenin is removed from the circulation and the oocyte surface is highly contoured and displays endocytotic activity. Food deprivation has much the same effect on oocyte endocytosis as does estrogen. When animals are given HCG and subsequently starved for 20 days, developing oocytes show little endocytotic activity. We conclude that HCG acts to promote or stimulate endocytosis in developing oocytes while estrogen and/or starvation inhibits this process.Research sponsored by the Energy Research and Development Adiministration under contract with Union Carbide Corporation.Predoctoral fellow supported by Grant GM 1974 from the National Institute of General Medical Sciences, National Institutes of Health.     

Oogenesis in Xenopus laevis (Daudin). V. Relationships between developing oocytes and their investing follicular tissues

The relationship of the cells and tissues which comprise the developing ovarian follicle in Xenopus laevis has been studied with scanning and transmission electron microscopy. The saclike ovary is covered on its coelomic side by a squamous epithelium. The cells of this epithelium are extensively interdigitated, and each bears a short, centrally positioned cilium. The lumenal surface of the ovary is covered with a layer of nonciliated squamous cells. The areas of cell-cell contact are characterized by desmosomes in both epithelia, and between the epithelia lies a connective tissue layer-the theca-which contains collagen fibers, blood vessels, nerves, smooth muscle cells and oogonia. Beneath the theca in each follicle lies a single layer of flat stellate follicle cells. Associations between adjacent follicle cells are intermittent, leaving wide spaces or channels. Junctional contacts between neighboring follicle cells are characterized by desmosomes. From the basal surface of each follicle cell extend long, broad macrovilli which penetrate the underlying acellular vitelline envelope and contact the surface of the oocyte. Evidence is presented which suggests that follicle cells may produce and release components which participate in the formation of the vitelline envelope which consists of a 3-dimensional lattice of ropey fibers. Passageways through the vitelline envelope allow the maintenance of contact between oocyte and follicle cells and also allow ready penetration of materials both to the oocyte (e.g., vitellogenin) and from it (e.g., cortical granule material) at different stages of its development.     

Oogenesis in Xenopus laevis (Daudin). II. The induction and subcellular localization of tyrosinase activity in developing oocytes

Tyrosinase activity is increased at specific stages of development in Xenopus laevis oocytes in mature females by an injection of 1000 units of human chorionic gonadtropin (HCG). Enzyme activity is stimulated slightly in stage II oocytes, greatly (5- to 6-fold) in stage III and early stage IV oocytes, slightly in late stage IV, and not at all in stage V oocytes. Tyrosinase activity has been localized cytochemically in oocytes by the DOPA-reaction. The DOPA-reaction product is found in the distal cisterna of the Golgi complex and in an anastomosing network of smooth-surfaced tubules associated with the Golgi complex. No reaction product is found in the clustered elements of smooth endoplasmic reticulum which gives rise to the premelanosomes. Substantial melanization of the premelanosomes does not occur until the DOPA-positive Golgi complexes move into proximity with the premelanosomes at the oocyte periphery. Biochemical assay of the isolated melanin granules shows that premelanosomes isolated from stage III and IV oocytes contain significant tyrosinase activity. This activity appears to decrease in the later stages of melanization. It is concluded that the metabolic activities leading to the formation of oocyte pigmentation are stimulated by gonadotropins and the degree of response to the stimulation is quantitatively regulated according to parameters typical of the specific stage of oocyte development.     

The lampbrush chromosomes of Xenopus laevis (Daudin)

A method for the preparation of Xenopus laevis lampbrush chromosomes is given and a map of the eighteen chromosomes is presented. The main structures and their variability have been described. At the observed stage, chiasmata are distributed distally. The shorter the chromosome, the more distally are the chiasmata. Centromere positions could not be localized.     

Skeletal development in Xenopus laevis (Anura: Pipidae).

Postembryonic skeletal development of the pipid frog Xenopus laevis is described from cleared-and-stained whole-mount specimens and sectioned material representing Nieuwkoop and Faber developmental Stages 46-65, plus postmetamorphic individuals up to 6 months old. An assessment of variation of skeletogenesis within a single population of larvae and comparison with earlier studies revealed that the timing, but not the sequence, of skeletal development in X. laevis is more variable than previously reported and poorly correlated with the development of external morphology. Examination of chondrocranial development indicates that the rostral cartilages of X. laevis are homologous with the suprarostral cartilages of non-pipoid anurans, and suggests that the peculiar chondrocranium of this taxon is derived from a more generalized pattern typical of non-pipoid frogs. Derived features of skeletal development not previously reported for X. laevis include 1) bipartite formation of the palatoquadrate; 2) precocious formation of the adult mandible; 3) origin of the angulosplenial from two centers of ossification; 4) complete erosion of the orbital cartilage during the later stages of metamorphosis; 5) development of the sphenethmoid as a membrane, rather than an endochondral bone; and 6) a pattern of timing of ossification that more closely coincides with that of the pelobatid frog Spea than that recorded for neobatrachian species.     

Ultrastructural observations on the parathyroid glands of Xenopus laevis Daudin

Summary The fine structure of the parathyroid glands of the anuran, Xenopus laevis Daudin is described during the metamorphosis of larvae and in young adults. Two distinct forms of epithelial cells are found viz.: dark and light cells and the significance of these cell conditions is considered and discussed. Parathyroid glands from young untreated toads are compared and contrasted with glands from toads maintained for prolonged periods in high concentrations, up to 1%, of calcium chloride in aqueous solution. The development of unusual membranous inclusions in the cytoplasm of the experimentally-treated toads is described.I am grateful to Messrs. R. L. Jones and Z. Podhorodecki for their expert technical assistance. I would also like to thank Professor N. Millott for his help and encouragement during the course of this work.     

Sublethal effect of a weak intermittent magnetic field on the development of Xenopus laevis (Daudin) tadpoles

In three repeated experiments with three different litters of Xenopus laevis (Daudin) tadpoles, three cohorts were reared in an aquarium under the "saw-tooth" magnetic field produced by a television set. Their maturation times are compared with those of three corresponding control cohorts grown in an unexposed aquarium. In the exposed aquarium, the magnetic field amplitude was less than 25 T and the frequency in the extremely-low-frequency and very-low-frequency wavebands. Neither the exposed nor the unexposed cohorts suffered significant mortality and malformations. However, the exposed tadpoles took about 5 days more than the unexposed ones to reach metamorphosis. The differences in mean maturation times between the exposed and control cohorts were extremely significant (P < 0.001). The results show that a biological population can suffer a sublethal effect when exposed to the magnetic field of a TV set for a long time in the course of juvenile life stages, and that this effect can consist of a delay in reaching the adult stage.     

Regulation mechanisms of intracellular pH of Xenopus laevis oocyte.

Intracellular pH values (pHi) of Xenopus oocytes were optically measured using a fluorescent dye, 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). The oocytes were loaded with dye by incubation with a membrane-permeable form (BCECF-AM). Mean pHi of the oocytes in pH 7.6 solution was 7.69. Increasing ambient pCO2 rapidly decreased pHi and estimated buffering power was 23.8 mM/pH unit. Changing ambient HCO3- from 5 to 30 mM did not alter pHi. After incubation in a Na(+)-free solution, Na+ addition to the bath rapidly increased pHi and this response was blocked by amiloride (ED50 2 microM). The addition of NH4Cl to the bath caused an initial transient increase in PHi followed by a secondary decrease. The secondary decrease was greatly inhibited by a histidine specific reagent, diethylpyrocarbonate. It was also slightly inhibited by ouabain, Ba2+ and furosemide, but not by amiloride. These data suggest that (1), fluorescence technique is applicable to PHi measurements of Xenopus oocytes; (2), Xenopus oocytes have an amiloride sensitive Na+/H(+)-exchange, and permeabilities to CO2, NH3, and NH+4. These observation may be useful in studying the relationship between pHi and oocytes development, and the expression of acid/base transporters in Xenopus oocytes.