The oral-aboral axis of the sea urchin embryo is specified conditionally via a regulated feedback circuit involving the signaling gene nodal and its antagonist lefty. In normal development nodal activity becomes localized to the prospective oral side of the blastula stage embryo, a process that requires lefty. In embryos of Strongylocentrotus purpuratus, a redox gradient established by asymmetrically distributed mitochondria provides an initial spatial input that positions the localized domain of nodal expression. This expression is perturbed by hypoxia, leading to development of radialized embryos lacking an oral-aboral axis. Here we show that this radialization is not caused by a failure to express nodal, but rather by a failure to localize nodal activity to one side of the embryo. This occurs even when embryos are removed from hypoxia at late cleavage stage when nodal is first expressed, indicating that the effect involves the initiation phase of nodal activity, rather than its positive feedback-driven amplification and maintenance. Quantitative fluorescence microscopy of MitoTracker Orange-labelled embryos expressing nodal-GFP reporter gene revealed that hypoxia abolishes the spatial correlation between mitochondrial distribution and nodal expression, suggesting that hypoxia eliminates the initial spatial bias in nodal activity normally established by the redox gradient. We propose that absent this bias, the initiation phase of nodal expression is spatially uniform, such that the ensuing Nodal-mediated community effect is not localized, and hence refractory to Lefty-mediated enforcement of localization.
The Lynx x-ray microcalorimeter (LXM) is an imaging spectrometer for the Lynx satellite mission, an x-ray telescope being considered by NASA to be a new flagship mission. Lynx will enable unique astrophysical observations into the x-ray universe due to its high angular resolution and large field of view. The LXM consists of an array of over 100,000 pixels and poses a significant technological challenge to achieve the high degree of multiplexing required to read out these sensors. We discuss the details of microwave superconducting quantum interference device (SQUID) multiplexing and describe why it is ideally suited to the needs of the LXM. This case is made by summarizing the current and predicted performance of microwave SQUID multiplexing and describing the steps needed to optimize designs for all the LXM arrays. Finally, we describe our plan to advance the technology readiness level (TRL) of microwave SQUID multiplexing of the LXM microcalorimeters to TRL-5 by 2024.
Transition-edge sensors (TESs) are thermal detectors in which a superconducting film that is electrically biased in the superconducting-to-normal transition is used as a thermometer. In most TESs, the film is a superconductor-normal metal bilayer where the two materials and their thicknesses are chosen to achieve various specifications including the transition temperature Tc
. Traditionally, the materials in the bilayer are deposited in sequence without breaking vacuum in order to achieve a clean, uniform bilayer interface at the wafer-scale. This approach leads to constraints in material properties, fabrication techniques and, ultimately, TES designs. To overcome these constraints, we have developed a bilayer fabrication process that allows the layers to be deposited and patterned separately with an exposure to atmosphere between the deposition steps. We demonstrate better than 6% transition-temperature uniformity across a 7.6 cm (3 in) substrate and present satisfactory spectra from TES x-ray detectors fabricated in this fashion. We show how the new hybrid additive-subtractive TES fabrication process creates new design possibilities, including broad tuning of Tc
across a substrate with a single bilayer thickness.
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