Alteration assemblages in Martian meteorites: implications for near-surface processes

The SNC (Shergotty-Nakhla-Chassigny) meteorites have recorded interactions between martian crustal fluids and the parent igneous rocks. The resultant secondary minerals – which comprise up to 1 vol.% of the meteorites – provide information about the timing and nature of hydrous activity and atmospheric processes on Mars. We suggest that the most plausible models for secondary mineral formation involve the evaporation of low temperature (25 – 150 °C) brines. This is consistent with the simple mineralogy of these assemblages – Fe-Mg-Ca carbonates, anhydrite, gypsum, halite, clays – and the chemical fractionation of Ca-to Mg-rich carbonate in ALH84001 "rosettes". Longer-lived, and higher temperature, hydrothermal systems would have caused more silicate alteration than is seen and probably more complex mineral assemblages. Experimental and phase equilibria data on carbonate compositions similar to those present in the SNCs imply low temperatures of formation with cooling taking place over a short period of time (e.g. days). The ALH84001 carbonate also probably shows the effects of partial vapourisation and dehydration related to an impact event post-dating the initial precipitation. This shock event may have led to the formation of sulphide and some magnetite in the Fe-rich outer parts of the rosettes. Radiometric dating (K-Ar, Rb-Sr) of the secondary mineral assemblages in one of the nakhlites (Lafayette) suggests that they formed between 0 and 670 Myr, and certainly long after the crystallisation of the host igneous rocks. Crystallisation of ALH84001 carbonate took place 0.5 Gyr after the parent rock. These age ranges and the other research on these assemblages suggest that environmental conditions conducive to near-surface liquid water have been present on Mars periodically over the last 1 Gyr. This fluid activity cannot have been continuous over geological time because in that case much more silicate alteration would have taken place in the meteorite parent rocks and the soluble salts would probably not have been preserved. The secondary minerals could have been precipitated from brines with seawater-like composition, high bicarbonate contents and a weakly acidic nature. The co-existence of siderite (Fe-carbonate) and clays in the nakhlites suggests that the pCO2 level in equilibrium with the parent brine may have been 50 mbar or more. The brines could have originated as flood waters which percolated through the top few hundred meters of the crust, releasing cations from the surrounding parent rocks. The high sulphur and chlorine concentrations of the martian soil have most likely resulted from aeolian redistribution of such aqueously-deposited salts and from reaction of the martian surface with volcanic acid volatiles. The volume of carbonates in meteorites provides a minimum crustal abundance and is equivalent to 50–250 mbar of CO2 being trapped in the uppermost 200–1000 m of the martian crust. Large fractionations in 18O between igneous silicate in the meteorites and the secondary minerals (30) require formation of the latter below temperatures at which silicate-carbonate equilibration could have taken place (400°C) and have been taken to suggest low temperatures (e.g. 150°C) of precipitation from a hydrous fluid

Management of atherothrombosis with clopidogrel in high-risk patients with recent transient ischaemic attack or ischaemic stroke (MATCH): study design and baseline data.

BACKGROUND: The CAPRIE study showed the superiority of clopidogrel over acetylsalicylic acid (ASA) for reducing the combined risk of major atherothrombotic events in patients with recent myocardial infarction (MI), recent ischaemic stroke (IS) or established peripheral arterial disease. The benefit of clopidogrel over ASA is amplified in high-risk patients. Proof of concept for the benefit of clopidogrel in addition to ASA in patients with coronary manifestations of atherothrombosis was provided by the CURE trial. METHODS: MATCH is a randomized, double-blind, placebo-controlled trial that compares clopidogrel and ASA versus clopidogrel alone in high-risk patients with recently symptomatic cerebrovascular disease. Eligible patients have experienced a transient ischaemic attack (TIA) or IS within the last 3 months and have evidence of at least 1 additional risk factor within the last 3 years (prior IS, MI, stable or unstable angina pectoris, diabetes or symptomatic peripheral arterial disease). Patients were randomized to receive ASA 75 mg once daily or placebo, with both groups receiving clopidogrel 75 mg once daily as part of standard therapy. The primary end point is the composite of IS, MI, vascular death and rehospitalization for an acute ischaemic event. The duration of treatment and follow-up is 18 months for each patient. RESULTS: Enrollment was completed in April 2002, with 7,599 patients randomized to receive the study medication. The mean age at randomization was 66 years, and the qualifying event was IS in 78.9% of patients and TIA in 21.1%. The baseline features of the study cohort indicate a population that is at a high risk for atherothrombotic recurrence. CONCLUSION: MATCH is a major ongoing trial that will provide important data on the benefit of clopidogrel and ASA compared with clopidogrel alone for reduction of vascular ischaemic events in patients with recent TIA or IS who are at high risk of atherothrombotic event recurrence

The Role of Scgb1a1+ Clara Cells in the Long-Term Maintenance and Repair of Lung Airway, but Not Alveolar, Epithelium

SummaryTo directly test the contribution of Scgb1a1+ Clara cells to postnatal growth, homeostasis, and repair of lung epithelium, we generated a Scgb1a1-CreERTM “knockin” mouse for lineage-tracing these cells. Under all conditions tested, the majority of Clara cells in the bronchioles both self-renews and generates ciliated cells. In the trachea, Clara cells give rise to ciliated cells but do not self-renew extensively. Nevertheless, they can contribute to tracheal repair. In the postnatal mouse lung, it has been proposed that bronchioalveolar stem cells (BASCs) which coexpress Scgb1a1 (Secretoglobin1a1) and SftpC (Surfactant Protein C), contribute descendants to both bronchioles and alveoli. The putative BASCs were lineage labeled in our studies. However, we find no evidence for the function of a special BASC population during postnatal growth, adult homeostasis, or repair. Rather, our results support a model in which the trachea, bronchioles, and alveoli are maintained by distinct populations of epithelial progenitor cells

An exploratory study of the physiological correlates of neurodevelopmental delay in infants with sickle cell anaemia

This study aimed to investigate whether infants with sickle cell anaemia (SCA) are at risk of neurodevelopmental delay, and whether any delay is associated with SCA pathology. Twenty-eight infants (14 SCA; 14 age- and ethnic-similar controls) were assessed longitudinally with the Bayley Infant Neurodevelopmental Screener (BINS) at 3, 9 and 12 months. Transcranial Doppler (TCD) and pulse oximetry (SpO2) measures were recorded longitudinally in SCA infants, and a subgroup of controls. Haemoglobin values were obtained from SCA infants. At each age, SCA infants obtained BINS scores indicative of greater risk of neurodevelopmental delay compared with controls. The number of moderate–high BINS risk scores increased significantly between 3 and 9 months. At 9 months BINS raw scores correlated negatively with TCD velocity and positively with haemoglobin. This exploratory study suggests that SCA infants may be at greater risk of neurodevelopmental delay than previously considered, and may provide the impetus for further research into the very early precursors of cognitive impairment