The fate and function of oligodendrocyte progenitor cells after traumatic spinal cord injury

Oligodendrocyte progenitor cells (OPCs) are the most proliferative and dispersed population of progenitor cells in the adult central nervous system, which allows these cells to rapidly respond to damage. Oligodendrocytes and myelin are lost after traumatic spinal cord injury (SCI), compromising efficient conduction and, potentially, the long-term health of axons. In response, OPCs proliferate and then differentiate into new oligodendrocytes and Schwann cells to remyelinate axons. This culminates in highly efficient remyelination following experimental SCI in which nearly all intact demyelinated axons are remyelinated in rodent models. However, myelin regeneration comprises only one role of OPCs following SCI. OPCs contribute to scar formation after SCI and restrict the regeneration of injured axons. Moreover, OPCs alter their gene expression following demyelination, express cytokines and perpetuate the immune response. Here, we review the functional contribution of myelin regeneration and other recently uncovered roles of OPCs and their progeny to repair following SCI.     

Global silicate weathering flux overestimated because of sediment–water cation exchange

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO2 from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na⁺) from silicate minerals. The large exchange pool supplies Na⁺ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO2 consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth CO2 degassing fluxes need to be further investigated.

For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (1). Silicate mineral dissolution with carbonic acid converts atmospheric CO2 into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (2). There have been many attempts to quantify the silicate weathering flux (3), mostly assuming that riverine dissolved sodium (Na+) is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate Na+ to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate Na+ is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (4). Major and trace cations such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), and strontium (Sr2+) form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (4). The importance of the cation exchange pool is well recognized in soils and aquifers (4, 5), has significant implications for enhanced weathering (6), and has been proposed as an important mechanism for buffering the composition of river waters (7–9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (10, 11)], despite its significance in providing a source of elements that are immediately bioavailable (12), and their potential for biasing the quantification of silicate weathering (9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (13, 14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (15, 16). Important examples of cation exchange are the “swapping” of divalent cations Ca2+ and Mg2+ with Na+, in particular when there is a major change in water composition such as when fluvial clays reach the ocean,Caclay2++2Nawater+⇋2Naclay++Cawater2+.[1]As a result, marine sediments have an exchange pool that is dominated by Na+ (17). Subsequently, these marine sediments are uplifted and emplaced on the continents where Na+ in the exchange pool is released by cation exchange with Ca-rich fresh waters (9). This has major implications for estimates of silicate weathering fluxes and associated CO2 consumption, because they are calculated using the Na+ content of rivers (3). Cerling et al. (9) proposed that the Na+-rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g., Ca2+, which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric CO2 sink.     

COVID-19 and Acute Pulmonary Embolism: A Case Series and Brief Review

The SARS-CoV-2 virus, or COVID-19, is responsible for the current global pandemic and has resulted in the death of over 400,000 in the United States. Rates of venous thromboembolism have been noted to be much higher in those infected with COVID-19. Here we report a case-series of COVID-19 patients with diverse presentations of pulmonary embolism (PE). We also briefly describe the pathophysiology and mechanisms for pulmonary embolism in COVID-19. These cases indicate a need to maintain a high index of suspicion for PE in patients with COVID-19, as well as the need to consider occult COVID-19 infection in patients with PE in the right clinical circumstance.     

A pilot study of the relationship between Doppler‐estimated carotid and brachial artery flow and cardiac index

We measured carotid and brachial artery blood flow by Doppler ultrasound in 11 human volunteers, and related these to cardiac index and to each other. The median (IQR [range]) carotid arterial blood flow was 0.334 (0.223–0.381 [0.052–0.563]) l.min^?1 on the right and 0.315 (0.223–0.369 [0.061–0.690]) l.min^?1 on the left. The brachial arterial blood flow was 0.049 (0.033–0.062 [0.015–0.204]) l.min^?1 on the right and 0.039 (0.027–0.054 [0.011–0.116]) on the left. Cardiac index was 3.2 (2.8–3.5 [1.9–5.4]) l.min^?1.m^?2. There was a moderate to good correlation between right‐and left‐sided flows (brachial: ρ = 0.45; carotid: ρ = 0.567). Brachial and carotid flow had no or a negative correlation with cardiac index (right brachial: ρ = ?0.145, left brachial: ρ = ?0.349; right carotid: ρ = ?0.376, left carotid: ρ = ?0.285). In contrast to some previous studies, we found that Doppler‐estimated peripheral arterial blood flows only show a weak correlation with cardiac index and cannot be used to provide non‐invasive estimates of cardiac index in man.     

Baseline characteristics as 3-year predictors of tooth fracture and crack progression: Findings from The National Dental Practice-Based Research Network

BackgroundThe authors of this practice-based study estimated the risk of experiencing tooth fractures and crack progression over 3 years and correlated baseline patient-, tooth-, and crack-level characteristics with these outcomes.MethodsTwo-hundred-and-nine National Dental Practice-Based Research Network dentists enrolled a convenience sample of 2,601 participants with a cracked vital posterior tooth that had been examined for at least 1 recall visit over 3 years. Data were collected at the patient, tooth, and crack levels at baseline, annual follow-up visits, and any interim visits. Associations between these characteristics and the subsequent same-tooth fractures and crack progression were quantified.ResultsOf the 2,601 teeth with a crack or cracks at baseline, 78 (3.0%; 95% confidence interval, 2.4% to 3.7%) subsequently developed a fracture. Of the 1,889 patients untreated before year 1, 232 (12.3%; 95% confidence interval, 10.9% to 13.8%) had some type of crack progression. Baseline tooth-level characteristics associated with tooth fracture were the tooth was maxillary and had a wear facet through enamel and a crack was detectable with an explorer, on the facial surface, and in a horizontal direction. Crack progression was associated with males and teeth with multiple cracks at baseline; teeth with a baseline facial crack were less likely to show crack progression. There was no commonality between characteristics associated with tooth fracture and those associated with crack progression.ConclusionsDevelopment of tooth fractures and crack progression over 3 years were rare occurrences. Specific characteristics were associated with the development of tooth fracture and crack progression, although none were common to both.Practical ImplicationsThis information can aid dentists in assessing factors that place posterior cracked teeth at risk of experiencing adverse outcomes.     

Midfacial translocation, a variation of the approach to the rhinopharynx, clivus and upper odontoid process.

OBJECTIVE: A surgical variation of the technique of facial translocation procedure is proposed, which has been called midfacial translocation for approach to the entire medial and lateral region of the middle third of the face, including the rhinopharynx, sphenoid sinus, pterygomaxillary fossa, odontoid process, and clivus. PATIENTS AND METHODS: The medical records of five treated patients accordingly were reviewed for an analysis of the surgical technique, the disease, the topography of the lesion, and the complications. RESULTS: The approach permitted ventral decompression of the bulbomedullary junction with resection of the C1 arch and the odontoid process in four patients and resection of a chordoma of the clivus located along the midline and extending intradurally in the fifth patient. Only one patient presented with dehiscence of the posterior half of the soft palate, this being the only complication observed following surgery in these patients. Three months postoperatively, no patient presented any aesthetic alteration of the face. Functionally, there was only infraorbital hypoaesthesia on the side of flap rotation. CONCLUSION: The technique of midfacial translocation provides both good surgical approach and access to the rhinopharynx, pterygomaxillary fossa, high odontoid process and clivus, with few adverse sequelae for the patient.     

Persistent buccopharyngeal membrane in an adult

Persistence of the buccopharyngeal membrane (BPM) also called the oropharyngeal membrane is a rare congenital oropharyngeal anomaly. We report a case of an adult aboriginal male patient with a membrane that closed his oropharyngeal isthmus except for a 2 cm diameter central perforation. The patient had no symptoms related to this membrane and no other congenital anomalies were found. This finding has not previously been reported in an adult. The embryology and management of this rare condition is discussed.