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John H Thomas

Publications and source records attributed to John H Thomas.

3 recordsLinked to original sources
PubMed2003

Evaluation of Xenopus tropicalis as an alternative test organism for frog embryo teratogenesis assay--Xenopus (FETAX).

As a formal recommendation from an Interagency Coordinating Committee for the Validation of Alternative Methods (ICCVAM) workshop review of the Frog Embryo Teratogenesis Assay--Xenopus (FETAX) developmental toxicity model, the use of Xenopus tropicalis as an alternative test species for this model was evaluated. Three test substances with varying developmental toxicity potentials were evaluated using FETAX modified to accommodate the use of X. tropicalis. Two separate definitive concentration-response tests were performed with isoniazid, methotrexate, and 6-aminonicotinamide. Historical FETAX results with X. laevis were compared to the results from FETAX assays with X. tropicalis. Test with X. tropicalis indicated that each of the compounds possessed teratogenic potential with varying degrees of potency: 6-aminonicotinamide > methotrexate > isoniazid. Based on overt teratogenicity, but not embryo-lethality, results from these studies indicated that these two species responded similarly to the test compounds. Malformation syndromes induced in both species were similar in X. tropicalis and X. laevis. These results suggested that X. tropicalis should be further evaluated as an alternative test organism for the FETAX model.

6-Aminonicotinamide↗
PubMed2002

Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae.

The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot.

Journal Article↗