Supernova SN 2011fe from an exploding carbon-oxygen white dwarf star

Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor.     

‘Spuntar lo scoglio più duro’: did Galileo ever think the most beautiful thought experiment in the history of science?

Still today it remains unclear whether Galileo ever climbed the leaning tower of Pisa in order to drop bodies from its top. Some believe that he established the principle of equal speeds for falling bodies by means of an ingenious thought experiment. However, the reconstruction of that thought experiment circulating in the philosophical literature is no more than a cartoon. In this paper I will tell the story of the thought processes behind the cartoon.     

PML targeting eradicates quiescent leukaemia-initiating cells

The existence of a small population of 'cancer-initiating cells' responsible for tumour maintenance has been firmly demonstrated in leukaemia. This concept is currently being tested in solid tumours. Leukaemia-initiating cells, particularly those that are in a quiescent state, are thought to be resistant to chemotherapy and targeted therapies, resulting in disease relapse. Chronic myeloid leukaemia is a paradigmatic haematopoietic stem cell disease in which the leukaemia-initiating-cell pool is not eradicated by current therapy, leading to disease relapse on drug discontinuation. Here we define the critical role of the promyelocytic leukaemia protein (PML) tumour suppressor in haematopoietic stem cell maintenance, and present a new therapeutic approach for targeting quiescent leukaemia-initiating cells and possibly cancer-initiating cells by pharmacological inhibition of PML.     

Response to Maarten Van Dyck’s commentary

In response to Maarten Van Dyck’s commentary, I present a translation of Vailati’s original paper with a short introductory note.     

Chromatin remodelling and epigenetic features of germ cells

Germ cells have the unique capacity to start a new life upon fertilization. They are generated during a sex-specific differentiation programme called gametogenesis. Maturation of germ cells is characterized by an impressive degree of cellular restructuring and gene regulation that involves remarkable genomic reorganization. These events are finely tuned, but are also susceptible to the introduction of various types of error. Because stable genetic transmission to future generations is essential for life, understanding the control of these processes has far-reaching implications for human health and reproduction.     

Gli3 and Plzf cooperate in proximal limb patterning at early stages of limb development

The vertebrate limb initially develops as a bud of mesenchymal cells that subsequently aggregate in a proximal to distal (P-D) sequence to give rise to cartilage condensations that prefigure all limb skeletal components. Of the three cardinal limb axes, the mechanisms that lead to establishment and patterning of skeletal elements along the P-D axis are the least understood. Here we identify a genetic interaction between Gli3 (GLI-Kruppel family member 3) and Plzf (promyelocytic leukaemia zinc finger, also known as Zbtb16 and Zfp145), which is required specifically at very early stages of limb development for all proximal cartilage condensations in the hindlimb (femur, tibia, fibula). Notably, distal condensations comprising the foot are relatively unperturbed in Gli3(-/-);Plzf(-/-) mouse embryos. We demonstrate that the cooperative activity of Gli3 and Plzf establishes the correct temporal and spatial distribution of chondrocyte progenitors in the proximal limb-bud independently of known P-D patterning markers and overall limb-bud size. Moreover, the limb defects in Gli3(-/-);Plzf(-/-) embryos correlate with the transient death of a specific subset of proximal mesenchymal cells that express bone morphogenetic protein receptor, type 1B (Bmpr1b) at the onset of limb development. These findings suggest that the development of proximal and distal skeletal elements is distinctly regulated early during limb-bud formation. The initial division of the vertebrate limb into two distinct molecular domains is consistent with fossil evidence indicating that the upper and lower extremities of the limb have different evolutionary origins.