COVER A long-exposure photograph shows the Clepsydra Geyser erupting on a moonlit night in Yellowstone National Park’s Lower Geyser Basin. Geysers and other hydrothermal features in Yellowstone National Park are fueled by heat from a magma reservoir in Earth’s upper crust. New seismic imaging of Yellowstone has provided further insight into the volume and distribution of magma in the subsurface. See pages 945 and 1001.

Photo: Paul D. Stewart/NPL/Minden Pictures

ONLINE COVER A smart dual hormone microneedle patch for blood glucose control. Insulin and glucagon secreted from the pancreas are essential for regulating blood glucose levels. Inspired by the dual-responsive mechanism, Yang et al. developed a glucose-responsive microneedle patch that enables closed-loop delivery of both insulin and glucagon for diabetes treatment. In a Type 1 diabetic pig model, the patch kept blood glucose levels within normal range and minimized the risk of hypoglycemia.

ONLINE COVER Rejuvenation Potential of T_RM Subsets. This month’s cover depicts a “fountain of youth” from which new tissue resident memory T cells (T_RM) are emerging after a secondary infectious challenge. Newly formed CD103^– T_RM (blue) and CD103⁺ T_RM (red) are derived from proliferating, CD103^– precursor cells and join the pool of preexisting CD103⁺ T_RM (gray) that lack this restorative capacity. This model for T_RM replenishment is supported by new fate-mapping mouse models described in separate papers by von Hoesslin and Kuhlmann et al. and Fung et al. that are discussed in a Focus by Jensen and Farber.

ONLINE COVER Bionics replicate biomechanics. Lower-limb prostheses are often heavy, consume a lot of energy, and do not replicate the natural gait. Tran et al. have developed a powered prosthesis for above-knee amputees that possesses knee, ankle, and toe joints to replicate the biomechanics of the leg. The device is lightweight and capable of regenerating energy during walking to extend its battery life. The researchers validated their design with preclinical studies on above-knee amputees showing the ability to ambulate on level ground and on stairs. This month’s cover is a photograph of the prosthesis device: Utah Bionic Leg.

ONLINE COVER This week, Rinschen et al. report that the lipid kinase VPS34 limits nutrient loss but enables viral entry in proximal tubule cells in the kidney. The image shows the reduced abundance of ACE2 (green), an entry receptor for the SARS-CoV-2 virus, in the renal cortex of a mouse deficient in VPS34 in proximal tubule cells.

ONLINE COVER Ketones Rekindle Megakaryocytes. The transmission electron microscopy micrograph shows a murine primary megakaryocyte, a bone marrow cell that produces platelets. Chemotherapy can reduce platelet counts in patients undergoing cancer treatment, which can in turn limit treatment efficacy. Xie et al. found that a ketogenic diet boosted platelet production in mouse models of thrombocytopenia by increasing circulating β-hydroxybutyrate, leading to altered histone acetylation of platelet biogenesis genes in megakaryocytes. A ketogenic diet modestly increased platelet counts in healthy volunteers, and, in a separate cohort of patients with cancer, a ketogenic lifestyle was associated with reduced chemotherapy-induced thrombocytopenia, suggesting this treatment strategy may translate to humans.