James N. Dumont

Bio: James N. Dumont is an academic researcher from Oak Ridge National Laboratory. The author has contributed to research in topics: Oocyte & Xenopus. The author has an hindex of 31, co-authored 56 publications receiving 4669 citations. Previous affiliations of James N. Dumont include City University of New York & University of Texas–Pan American.

Oogenesis in Xenopus laevis (Daudin). I. Stages of oocyte development in laboratory maintained animals.

Abstract: Oogenesis in the anuran Xenopus laevis can be divided into six stages based on the anatomy of the developing oocyte. Stage I consists of small (50 to 100 μ) colorless oocytes whose cytoplasm is transparent. Their large nuclei and mitochondrial masses are clearly visible in the intact oocyte. Stage II oocytes range up to 450 μ in diameter, and appear white and opaque. Stage I and II are both previtellogenic. Pigment synthesis and yolk accumulation (vitellogenesis) begins during Stage III. Vitellogenesis continues through Stage IV (600 to 1000 μ), the oocytes grow rapidly, and the animal and vegetal hemispheres become differentiated. By Stage V (1000 to 1200 μ) the oocytes have nearly reached their maximum size and yolk accumulation gradually ceases. Stage VI oocytes are characterized by the appearance of an essentially unpigmented equatorial band. They range in size from 1200 to 1300 μ, are postivtellogenic and ready for ovulation. These stages of oocyte development have been correlated with physiological and biochemical data related to oogenesis in Xenopus.

Protein incorporation by isolated amphibian oocytes III. Optimum incubation conditions

Abstract: Available evidence indicates that protein incorporation by amphibian oocytes is mediated through a process of micropinocytosis. Oocytes incubated under our standard conditions sequester protein at a constant rate for at least 24 hours and appear to maintain approximately the same level of micropinocytosis as that observed in situ. Protein uptake is markedly sensitive to temperature and the ionic composition of the medium, with calcium being a particularly important component. Magnesium, phosphate, and additional energy sources are of only minor significance. Based on these observations, several slightly improved reformulations of the standard incubation medium are provided.

Oogenesis in Xenopus laevis (Daudin)

Abstract: 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.

Frog Embryo Teratogenesis Assay: Xenopus (FETAX) — A Short-Term Assay Applicable to Complex Environmental Mixtures

Abstract: There is no question that a rapid and inexpensive screening tool is needed to assess potential teratogenicity. The now classical Ames test (Ames, 1975) and other tests screen for potential mutagenicity and carcinogenicity. Although a number of relatively rapid bioassays, ranging from the invertebrates through lower vertebrates to mammals, as well as cell, organ, and embryo culture systems, have been used to examine teratogenicity, none has been adequately validated or widely accepted for routine use. The tier-testing approach, which uses rapid screens to detect potential hazard and indicate need for further testing, has been useful and expedient. Comparable tests, however, are not yet applicable for teratogenesis testing despite the fact that there are a number of methods available. Among these is one which we submit may be a valid model for preliminary assessment of potential teratogens. This model, referred to as FETAX (Frog Embryo Teratogenesis Assay: Xenopus), has been applied to examine the relative teratogenic risk of a variety of chemicals and complex mixtures. The mixtures we have tested come from coal-conversion and shale-oil technologies and their effects have been compared to those of similar materials derived from natural petroleums.

Oogenesis in Xenopus laevis (Daudin). I. Stages of oocyte development in laboratory maintained animals.

Abstract: Oogenesis in the anuran Xenopus laevis can be divided into six stages based on the anatomy of the developing oocyte. Stage I consists of small (50 to 100 μ) colorless oocytes whose cytoplasm is transparent. Their large nuclei and mitochondrial masses are clearly visible in the intact oocyte. Stage II oocytes range up to 450 μ in diameter, and appear white and opaque. Stage I and II are both previtellogenic. Pigment synthesis and yolk accumulation (vitellogenesis) begins during Stage III. Vitellogenesis continues through Stage IV (600 to 1000 μ), the oocytes grow rapidly, and the animal and vegetal hemispheres become differentiated. By Stage V (1000 to 1200 μ) the oocytes have nearly reached their maximum size and yolk accumulation gradually ceases. Stage VI oocytes are characterized by the appearance of an essentially unpigmented equatorial band. They range in size from 1200 to 1300 μ, are postivtellogenic and ready for ovulation. These stages of oocyte development have been correlated with physiological and biochemical data related to oogenesis in Xenopus.

Positional cloning of zebrafish ferroportin1 identifies a conserved vertebrate iron exporter

Abstract: Defects in iron absorption and utilization lead to iron deficiency and overload disorders. Adult mammals absorb iron through the duodenum, whereas embryos obtain iron through placental transport. Iron uptake from the intestinal lumen through the apical surface of polarized duodenal enterocytes is mediated by the divalent metal transporter, DMT1 (refs 1,2,3). A second transporter has been postulated to export iron across the basolateral surface to the circulation. Here we have used positional cloning to identify the gene responsible for the hypochromic anaemia of the zebrafish mutant weissherbst. The gene, ferroportin1, encodes a multiple-transmembrane domain protein, expressed in the yolk sac, that is a candidate for the elusive iron exporter. Zebrafish ferroportin1 is required for the transport of iron from maternally derived yolk stores to the circulation and functions as an iron exporter when expressed in Xenopus oocytes. Human Ferroportin1 is found at the basal surface of placental syncytiotrophoblasts, suggesting that it also transports iron from mother to embryo. Mammalian Ferroportin1 is expressed at the basolateral surface of duodenal enterocytes and could export cellular iron into the circulation. We propose that Ferroportin1 function may be perturbed in mammalian disorders of iron deficiency or overload.

Molecular basis of mechanotransduction in living cells

Abstract: The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lip...