Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles

The processes that led to the formation of the planetary bodies in the Solar System are still not fully understood. Using the results obtained with the comprehensive suite of instruments on-board ESA’s Rosetta mission, we present evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle gravitational collapse of a bound clump of mm-sized dust aggregates (“pebbles”), intermixed with microscopic ice particles. This formation scenario leads to a cometary make-up that is simultaneously compatible with the global porosity, homogeneity, tensile strength, thermal inertia, vertical temperature profiles, sizes and porosities of emitted dust, and the steep increase in water-vapour production rate with decreasing heliocentric distance, measured by the instruments on-board the Rosetta spacecraft and the Philae lander. Our findings suggest that the pebbles observed to be abundant in protoplanetary discs around young stars provide the building material for comets and other minor bodies

The Dust Trail of Comet 67P/Churyumov-Gerasimenko

We report the detection of comet 67P/Churyumov-Gerasimenko's dust trail and nucleus in 24 micron Spitzer Space Telescope images taken February 2004. The dust trail is not found in optical Palomar images taken June 2003. Both the optical and infrared images show a distinct neck-line tail structure, offset from the projected orbit of the comet. We compare our observations to simulated images using a Monte Carlo approach and a dynamical model for comet dust. We estimate the trail to be at least one orbit old (6.6 years) and consist of particles of size >~100 micron. The neck-line is composed of similar sized particles, particles of size but younger in age. Together, our observations and simulations suggest grains 100 micron and larger in size dominate the total mass ejected from the comet. The radiometric effective radius of the nucleus is 1.87 +/- 0.08 km, derived from the Spitzer observation. The Rosetta spacecraft is expected to arrive at and orbit this comet in 2014. Assuming the trail is comprised solely of 1 mm radius grains, we compute a low probability (~10^-3) of a trail grain impacting with Rosetta during approach and orbit insertion.Comment: Accepted for publication in Icaru

The Refractory-to-Ice Mass Ratio in Comets

We review the complex relationship between the dust-to-gas mass ratio usually estimated in the material lost by comets, and the Refractory-to-Ice mass ratio inside the nucleus, which constrains the origin of comets. Such a relationship is dominated by the mass transfer from the perihelion erosion to fallout over most of the nucleus surface. This makes the Refractory-to-Ice mass ratio inside the nucleus up to ten times larger than the dust-to-gas mass ratio in the lost material, because the lost material is missing most of the refractories which were inside the pristine nucleus before the erosion. We review the Refractory-to-Ice mass ratios available for the comet nuclei visited by space missions, and for the Kuiper Belt Objects with well defined bulk density, finding the 1-σ lower limit of 3. Therefore, comets and KBOs may have less water than CI-chondrites, as predicted by models of comet formation by the gravitational collapse of cm-sized pebbles driven by streaming instabilities in the protoplanetary disc

Volatile exposures on the 67P/Churyumov-Gerasimenko nucleus

We present the most extensive catalog of exposures of volatiles on the 67P/Churyumov-Gerasimenko nucleus generated from observations acquired with the OSIRIS cameras on board the Rosetta mission. We identified more than 600 volatile exposures on the comet. Bright spots are found isolated on the nucleus or grouped in clusters, usually at the bottom of cliffs, and most of them are small, typically a few square meters or smaller. Several of them are clearly correlated with the cometary activity. We note a number of peculiar exposures of volatiles with negative spectral slope values in the high-resolution post-perihelion images, which we interpret as the presence of large ice grains ($>$ 1000 $\mu$m) or local frosts condensation. We observe a clear difference both in the spectral slope and in the area distributions of the bright spots pre- and post-perihelion, with these last having lower average spectral slope values and a smaller size, with a median surface of 0.7 m$^2$, even if the size difference is mainly due to the higher resolution achieved post-perihelion. The minimum duration of the bright spots shows three clusters: an area-independent cluster dominated by short-lifetime frosts; an area-independent cluster with lifetime of 0.5--2 days, probably associated with the seasonal fallout of dehydrated chunks; and an area-dependent cluster with lifetime longer than 2 days consistent with water-driven erosion of the nucleus. Even if numerous bright spots are detected, the total surface of exposed water ice is less than 0.1% of the total 67P nucleus surface, confirming that the 67P surface is dominated by refractory dark terrains, while exposed ice occupies only a tiny fraction. Moreover, the abundance of volatile exposures is six times less in the small lobe than in the big lobe, adding additional evidence to the hypothesis that comet 67P is composed of two distinct bodies.Comment: 24 pages, 19 Figures; paper accepted for publication in Astron. and Astrophysics on February 202

Tiny percutaneous needle biopsy: An efficient methodfor studying cellular and molecular aspectsof skeletal muscle in humans

Needle biopsy is widely used to obtain specimens for physiological, anatomical and biochemical studies of skeletal muscle (SM). We optimized a procedure which we termed tiny percutaneous needle biopsy (TPNB), to efficiently gather good numbers of human satellite cells and single dissociated fibers for the functional study of skeletal muscle; these samples permit isolation of high-quality RNA and sufficient amounts of proteins to allow molecular analysis. Moreover, TPNB showed a clear advantage in that the technique was easier than other procedures used on healthy volunteers in human trials. TPNB is a very safe minor surgical procedure. It is less traumatic than needle aspiration biopsy, and significant complications are improbable. TPNB should become established as an important tool in the investigation of SM and may be employed to study various physiological aspects of SM in human subjects. We suggest that TPNB should also be used in the study of muscle diseases and disorders including muscular dystrophy, congenital myopathy, and metabolic defects

Myomir dysregulation and reactive oxygen species in aged human satellite cells

Satellite cells that reside on the myofibre surface are crucial for the muscle homeostasis and regeneration. Aging goes along with a less effective regeneration of skeletal muscle tissue mainly due to the decreased myogenic capability of satellite cells. This phenomenon impedes proper maintenance and contributes to the age-associated decline in muscle mass, known as sarcopenia. The myogenic potential impairment does not depend on a reduced myogenic cell number, but mainly on their difficulty to complete a differentiation program. The unbalanced production of reactive oxygen species in elderly people could be responsible for skeletal muscle impairments. microRNAs are conserved post-transcriptional regulators implicated in numerous biological processes including adult myogenesis. Here, we measure the ROS level and analyze myomiR (miR-1, miR-133b and miR-206) expression in human myogenic precursors obtained from Vastus lateralis of elderly and young subjects to provide the molecular signature responsible for the differentiation impairment of elderly activated satellite cells. publisher: Elsevier articletitle: Myomir dysregulation and reactive oxygen species in aged human satellite cells journaltitle: Biochemical and Biophysical Research Communications articlelink: http://dx.doi.org/10.1016/j.bbrc.2016.03.030 content_type: article copyright: Copyright © 2016 The Authors. Published by Elsevier Inc. ispartof: Biochemical and Biophysical Research Communications vol:473 issue:2 pages:462-70 ispartof: location:United States status: publishe

Zeb2 regulates myogenic differentiation in pluripotent stem cells

Skeletal muscle differentiation is triggered by a unique family of myogenic basic helix-loop-helix transcription factors, including MyoD, MRF-4, Myf-5, and Myogenin. These transcription factors bind promoters and distant regulatory regions, including E-box elements, of genes whose expression is restricted to muscle cells. Other E-box binding zinc finger proteins target the same DNA response elements, however, their function in muscle development and regeneration is still unknown. Here, we show that the transcription factor zinc finger E-box-binding homeobox 2 (Zeb2, Sip-1, Zfhx1b) is present in skeletal muscle tissues. We investigate the role of Zeb2 in skeletal muscle differentiation using genetic tools and transgenic mouse embryonic stem cells, together with single-cell RNA-sequencing and in vivo muscle engraftment capability. We show that Zeb2 over-expression has a positive impact on skeletal muscle differentiation in pluripotent stem cells and adult myogenic progenitors. We therefore propose that Zeb2 is a novel myogenic regulator and a possible target for improving skeletal muscle regeneration. The non-neural roles of Zeb2 are poorly understood

Study on normal and otosclerotic bone cell cultures: an advance in understanding the pathogenesis of otosclerosis

The authors first reviewed the main theories concerning the pathogenesis of otosclerosis and studied the morphologic and functional characteristics of cell cultures derived from normal and otosclerotic bones. Light transmission and scanning electron microscopy did not permit definite identification of the cultured cells as predominantly osteoblasts, nor did these techniques show significant differences between cultured cells derived from normal and pathologic bone. Functional tests of the cell cultures proved more interesting. First, the bony nature of the cultured cells was demonstrated by studying the intracellular 45Ca++ uptake after stimulation with calcitonin and dybutryl-cAMP. Second, cell cultures derived from otosclerotic bone behaved differently from those derived from normal bone. Their peak uptake of calcium appeared later, and post-stimulatory values were higher, suggesting that cells derived from otosclerotic bone store a greater quantity of 45Ca++. Furthermore, after stimulation with calcitonin and propranolol, we observed an inhibition of the calcium uptake and decreased intracellular cAMP levels in normal bone cell cultures. In contrast, the cell cultures derived from otosclerotic bone exhibited an initial inhibition of calcium absorption followed by massive calcium penetration. The response of adenylate cyclase to the action of Mg++, Ca++, and F− ions was evaluated in cultures derived from normal bone, otosclerotic bone, and normal skin fibroblasts. The resulting data show that activation due to Mg++ is much lower in cultured cells derived from otosclerotic bone than in those from either normal bone or skin fibroblasts. No significant differences were found after Ca++ inhibition in any of the cell cultures. Moreover, in cell cultures derived from normal bone, F− ions induced a strong activation that was lower than the levels observed in cultures of otosclerotic bone or in normal fibroblasts. We hypothesize that an alteration at the calcitonin receptor site is responsible for the difference in calcium uptake and cAMP levels observed in the cells derived from otosclerotic bone as compared to those cultured from normal cells

Evidence of Fragmenting Dust Particles from Near-Simultaneous Optical and Near-IR Photometry and Polarimetry of Comet 73P/Schwassmann-Wachmann 3

We report imaging polarimetry of segments B and C of the Jupiter-family Comet 73P/Schwassmann-Wachmann 3 in the I and H bandpasses at solar phase angles of approximately 35 and 85deg. The level of polarization was typical for active comets, but larger than expected for a Jupiter-family comet. The polarimetric color was slightly red (dP/dL = +1.2 +/- 0.4) at a phase angle of ~ 35deg and either neutral or slightly blue at a phase angle of ~ 85deg. Observations during the closest approach from 2006 May 11-13 achieved a resolution of 35 km at the nucleus. Both segments clearly depart from a 1/rho surface brightness for the first 50 - 200 km from the nucleus. Simulations of radiation driven dust dynamics can reproduce some of the observed coma morphology, but only with a wide distribution of initial dust velocities (at least a factor of 10) for a given grain radius. Grain aggregate breakup and fragmentation are able to reproduce the observed profile perpendicular to the Sun-Comet axis, but fit the observations less well along this axis (into the tail). The required fragmentation is significant, with a reduction in the mean grain aggregate size by about a factor of 10. A combination of the two processes could possibly explain the surface brightness profile of the comet.Comment: 40 pages including 11 figure