Intense myocyte formation from cardiac stem cells in human cardiac hypertrophy

It is generally believed that increase in adult contractile cardiac mass can be accomplished only by hypertrophy of existing myocytes. Documentation of myocardial regeneration in acute stress has challenged this dogma and led to the proposition that myocyte renewal is fundamental to cardiac homeostasis. Here we report that in human aortic stenosis, increased cardiac mass results from a combination of myocyte hypertrophy and hyperplasia. Intense new myocyte formation results from the differentiation of stem-like cells committed to the myocyte lineage. These cells express stem cell markers and telomerase. Their number increased >13-fold in aortic stenosis. The finding of cell clusters with stem cells making the transition to cardiogenic and myocyte precursors, as well as very primitive myocytes that turn into terminally differentiated myocytes, provides a link between cardiac stem cells and myocyte differentiation. Growth and differentiation of these primitive cells was markedly enhanced in hypertrophy, consistent with activation of a restricted number of stem cells that, through symmetrical cell division, generate asynchronously differentiating progeny. These clusters strongly support the existence of cardiac stem cells that amplify and commit to the myocyte lineage in response to increased workload. Their presence is consistent with the notion that myocyte hyperplasia significantly contributes to cardiac hypertrophy and accounts for the subpopulation of cycling myocytes

modelling fluid-poroelastic media interaction

The interaction between a fluid and a poroelastic structure is a complex problem that couples the Navier-Stokes equations with the Biot system. The finite element approximation of this problem is involved due to the fact that both subproblems are indefinite. In this work, we first design residual-based stabilization techniques for the Biot system, motivated by the variational multiscale approach. Then, we state the monolithic Navier-Stokes/Biot system with the appropriate transmission conditions at the interface. For the solution of the coupled system, we adopt both monolithic solvers and heterogeneous domain decomposition strategies. Different domain decomposition methods are considered and their convergence 1 is analyzed for a simplified problem. We compare the efficiency of all the methods on a test problem that exhibits a large added-mass effect, as it happens in hemodynamics applications.

Fluid-structure interaction in blood flow capturing non-zero longitudinal structure displacement

We present a new model and a novel loosely coupled partitioned numerical scheme modeling fluid-structure interaction (FSI) in blood flow allowing non-zero longitudinal displacement. Arterial walls are modeled by a {linearly viscoelastic, cylindrical Koiter shell model capturing both radial and longitudinal displacement}. Fluid flow is modeled by the Navier-Stokes equations for an incompressible, viscous fluid. The two are fully coupled via kinematic and dynamic coupling conditions. Our numerical scheme is based on a new modified Lie operator splitting that decouples the fluid and structure sub-problems in a way that leads to a loosely coupled scheme which is {unconditionally} stable. This was achieved by a clever use of the kinematic coupling condition at the fluid and structure sub-problems, leading to an implicit coupling between the fluid and structure velocities. The proposed scheme is a modification of the recently introduced "kinematically coupled scheme" for which the newly proposed modified Lie splitting significantly increases the accuracy. The performance and accuracy of the scheme were studied on a couple of instructive examples including a comparison with a monolithic scheme. It was shown that the accuracy of our scheme was comparable to that of the monolithic scheme, while our scheme retains all the main advantages of partitioned schemes, such as modularity, simple implementation, and low computational costs

Instability of axion inflation in the regime of strong backreaction

openIn axion-U(1) inflation models, in the limit of strong coupling between the inflaton and the gauge field, one helicity mode of the gauge field undergoes exponential enhancement. This amplification, governed by the inflaton velocity, leads to a delayed backreaction of the gauge field on the inflaton motion, resulting in oscillations in the inflaton velocity. In this thesis we conduct an analytical examination of the evolution equation, relaxing certain assumptions typically found in the literature. Additionally, we explore the numerical effects of a time-delayed friction term within a single-field inflation model.In axion-U(1) inflation models, in the limit of strong coupling between the inflaton and the gauge field, one helicity mode of the gauge field undergoes exponential enhancement. This amplification, governed by the inflaton velocity, leads to a delayed backreaction of the gauge field on the inflaton motion, resulting in oscillations in the inflaton velocity. In this thesis we conduct an analytical examination of the evolution equation, relaxing certain assumptions typically found in the literature. Additionally, we explore the numerical effects of a time-delayed friction term within a single-field inflation model

High temperature behaviour of nuclear materials investigated by laser heating and fast pyrometry

LAUREA SPECIALISTICAL’obiettivo principale di questo lavoro è stato lo studio ad alta temperatura di alcuni ossidi alto fondenti di interesse nucleare, in particolare ThO2 e gli ossidi misti (U,Th)O2, che potrebbero essere impiegati come combustibile nucleare “sostenibile”. Sono stati studiati in parallelo altri ossidi (CaO, CeO2) per una migliore comprensione dell’effetto ottico di trasparenza, tipico del ThO2. Si sono affrontate diverse problematiche come l'elevata reattività chimica, i veloci processi di ossidazione/riduzione, la volatilizzazione e l’interazione dei campioni con il contenitore. Sono stati ottenuti nuovi dati sperimentali utilizzando un metodo sviluppato all’Istituto dei Transuranici (Karlsruhe, Germania), che si basa su riscaldamento laser accoppiato a pirometria veloce. I risultati sono in buon accordo e aggiungono nuovi importanti dati all’esistente letteratura. E’ stato studiato il comportamento ad alta temperatura del CaO, per la prima volta in atmosfera riducente e ossidante. Questo approccio ha evidenziato come la temperatura di fusione sia fortemente influenzata dall’atmosfera utilizzata. Sono stati ottenuti risultati simili per il CeO2. Si è inoltre caratterizzato il diagramma di fase ad alta temperatura del sistema UO2-ThO2. Le linee di solidus/liquidus, che definiscono il comportamento del combustibile nucleare a temperature prossime alla fusione, mostrano un minimo attorno a 3050 K per una composizione di 5 mol% di ThO2. Per composizioni prossime alla toria pura, la temperatura di fusione è stata riprodotta in accordo con i più recenti dati di letteratura (circa 3620 K). In aggiunta, il metodo sperimentale usato è stato validato mediante la misurazione della temperatura di radianza di fusione di cinque metalli refrattari, riferimenti secondari per ITS-90. Attraverso questa attività sono state inoltre individuate nuove fonti di errore. Infine, è stato prodotto un set di dati sperimentali per la validazione di un modello teorico per la simulazione di esperimenti di riscaldamento laser ed è stato misurato il profilo spaziale del raggio laser usato.The main goal of this work has been the study of the high temperature behavior of some high-melting oxides of interest for the nuclear industry. In particular, the investigated ThO2 and (U,Th)O2 mixed oxides can be employed as a “sustainable” nuclear fuel. Other oxides (CaO, CeO2) have been studied in parallel, to better understand the optical transparency effect, typical of ThO2, and their oxygen behavior. Many experimental difficulties have been faced (e.g., high chemical reactivity, fast oxidation/reduction processes, volatilization, interaction of the specimens with the container, temperature detection itself). New experimental data have been produced by using an innovative method, developed at the Institute for Transuranium Elements (Germany), and based on laser heating coupled with fast pyrometry. Current results are in fair agreement with literature, whereby they add new important data points to databases which are still limited and uncertain. In particular, the melting behavior of CaO has been studied under reducing and oxidizing conditions for the first time. This approach has shown that the large discrepancy found in literature is due to the paramount influence of the environment on its melting behaviour. Similar results have been obtained for CeO2. Moreover, the high-temperature phase diagram of the UO2-ThO2 system has been re-assessed. The solidus/liquidus lines, which define the melting behaviour of this nuclear fuel and constitute fundamental information for the reactor design, have been observed to present a minimum melting temperature around 3050 K for a composition of 5 mol% ThO2. For higher ThO2 contents, the most recent literature data on the melting point of pure ThO2 (around 3620 K) have been reproduced. Moreover, the current method has been validated through the measurements of the radiance temperature of melting of five refractory metals (secondary references points for the ITS-90). These tests have also served to identify some important error sources. Finally, a set of experimental data was produced to validate a theoretical model for the simulation of fast laser experiments, and the spatial beam profile of the employed laser was determined