Filtros selectivos de dinámicas traslacionales en microestructuras de materia blanca con MRI

La resonancia magnética nuclear (NMR) es, al día de hoy, uno de los métodos más utilizados a la hora de obtener imágenes médicas de forma no invasiva. Sin embargo, los límites de resolución de las secuencias convencionales no permiten acceder a los procesos que suceden a escalas micro y sub-micrométricas, los cuales representan información de vital importancia tanto en el ámbito clínico como en el de investigación. Trabajos recientes sugieren que la secuencia NOGSE (Non-uniform Oscillating-Gradient Spin-Echo) es capaz de obtener información cuantitativa en estas escalas por medio de la difusión molecular, utilizando a los espines nucleares como sensores cuánticos que permiten extraer información sobre su entorno. Sin embargo, los modelos actuales para su respuesta no contemplan la variedad de procesos difusivos que suceden en tejidos biológicos como la materia blanca, lo que limita su capacidad de predicción cuantitativa. Por otra parte, trabajos anteriores han demostrado de forma teórica y experimental que es posible utilizar la secuencia NOGSE para filtrar la señal proveniente de procesos que suceden en escalas espaciales especificas. A partir de esto, en este trabajo implementamos la secuencia de modulación de gradiente NOGSE para filtrar selectivamente la señal proveniente de moléculas confinadas en microestructuras de tamaños específicos, con el objetivo de extraer información estructural de tejidos biológicos aún en presencia de dinámicas moleculares complejas. Para esto, se analizaron los datos correspondientes a experimentos de DWI realizados sobre un fantoma construido con el fin de emular los regímenes difusivos que tienen lugar en los espacios extra-axonales de la materia blanca. En una primer etapa de caracterización, se encontró evidencia consistente de la presencia de una dinámica traslacional compleja, conformada por un proceso de difusión confinada a tiempos cortos y un límite de tortuosidad a tiempos largos. Se observó que la secuencia es capaz de filtrar la señal y observar selectivamente la proveniente de moléculas confinadas a las restricciones de interés en un proceso que denominamos tortuosidad microscópica. A través de un modelo teórico de la señal, se demostró esta observación experimental y posibilitó la determinación de las formas óptimas de configuración de la secuencia para observar esta información cuantitativa. Estos resultados permitieron optimizar protocolos para generar imágenes cuantitativas de información microestructural en cerebro humano para el diseño de nuevas herramientas de diagnóstico médico

Maximum weight cycle packing in directed graphs, with application to kidney exchange programs

Centralized matching programs have been established in several countries to organize kidney exchanges between incompatible patient-donor pairs. At the heart of these programs are algorithms to solve kidney exchange problems, which can be modelled as cycle packing problems in a directed graph, involving cycles of length 2, 3, or even longer. Usually, the goal is to maximize the number of transplants, but sometimes the total benefit is maximized by considering the differences between suitable kidneys. These problems correspond to computing cycle packings of maximum size or maximum weight in directed graphs. Here we prove the APX-completeness of the problem of finding a maximum size exchange involving only 2-cycles and 3-cycles. We also present an approximation algorithm and an exact algorithm for the problem of finding a maximum weight exchange involving cycles of bounded length. The exact algorithm has been used to provide optimal solutions to real kidney exchange problems arising from the National Matching Scheme for Paired Donation run by NHS Blood and Transplant, and we describe practical experience based on this collaboration

Mating system parameters in species of genus Prosopis (Leguminosae)

The section Algarobia of genus Prosopis involves important natural resources in arid and semiarid regions of the world. Their rationale use requires a better knowledge of their biology, genetics and mating system. There are contradictory information about their mating system. Some authors claim they are protogynous and obligate outcrosser. However, some evidence have been shown indicating that they might not be protogynous and that they might be somewhat self-fertile. The current paper analyses genetic structure and mating system parameters in populations of seven species of this section from South and North America based on isozyme data. In all species a significant homozygote excess was found in the offspring population but not in mother plant genotypes. Multilocus and mean single locus outcrossing rates (tm, ts) indicated that about 15 % selfing can occur in the studied populations. The heterogeneity between pollen and ovule allele frequencies was low suggesting population structuration, in agreement with the estimates of correlation of tm within progeny (rt) and correlation of outcrossed paternity (rp). The difference of F(IS) estimates between offspring and mother plants suggest some selection favouring heterozygotes between seedling and adult stages.Fil:Bessega, C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Ferreyra, L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Saidman, B. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Vilardi, J.C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Utilizing List Exchange and Non-directed Donation through “Chain” Paired Kidney Donations

In a list exchange (LE), the intended recipient in an incompatible pair receives priority on the deceased donor waitlist (DD-waitlist) after the paired incompatible donor donates a kidney to a DD-waitlist candidate. A non-directed donor’s (ND-D) kidney is usually transplanted directly to a DD-waitlist candidate. These two established practices would help even more transplant candidates if they were integrated with kidney paired donation (KPD). We consider a scenario in which the donor of an LE intended recipient (LE-IR) donates to a compatible KPD intended recipient (KPD-IR), and the KPD donor (KPD-D) donates to the waitlist (an LE-chain). We consider a similar scenario in which an ND-D donates to a KPD-IR and the KPD-D donates to the DD-waitlist (an ND-chain). Using data derived from the New England Program for Kidney Exchange (NEPKE) and from OPTN/SRTR recipient-donor distributions, simulations are presented to evaluate the potential impact of chain exchanges coordinated with KPD. LE donors (LE-D) and ND-D who are ABO-O result in the highest number of additional transplants, while results for ABO-A and B donors are similar to each other. We recommend that both LE and ND donations be utilized through chain exchanges

Increasing the Opportunity of Live Kidney Donation by Matching for Two and Three Way Exchanges

Background: To expand the opportunity for paired live donor kidney transplantation, computerized matching algorithms have been designed to identify maximal sets of compatible donor/recipient pairs from a registry of incompatible pairs submitted as candidates for transplantation. Methods: Demographic data of patients who had been evaluated for live donor kidney transplantation but found to be incompatible with their potential donor (because of ABO blood group or positive crossmatch) were submitted for computer analysis and matching. Data included ABO and HLA types of donor and recipient, %PRA and specificity of recipient alloantibody, donor/recipient relationship, and the reason the donor was incompatible. The data set used for the initial simulation included 29 patients with one donor each and 16 patients with multiple donors for a total of 45 patients and 68 donor/patient pairs. In addition, a simulation based on OPTN/SRTR data was used to further assess the practical importance of multiple exchange combinations. Results: If only exchanges involving two patient-donor pairs were allowed, a maximum of 8 patient-donor pairs in the data set could exchange kidneys. If 3-way exchanges were also allowed, a maximum of 11 pairs could exchange kidneys. Simulations with OPTN/SRTR data demonstrate that the increase in the number of potential transplants if 3-way exchanges are allowed is robust, and does not depend on the particular patients in our sample. Conclusions: A computerized matching protocol can be used to identify donor/recipient pairs from a registry of incompatible pairs who can potentially enter into donor exchanges that otherwise would not readily occur

Increasing the Opportunity of Live Kidney Donation by Matching for Two and Three Way Exchanges

Background: To expand the opportunity for paired live donor kidney transplantation, computerized matching algorithms have been designed to identify maximal sets of compatible donor/recipient pairs from a registry of incompatible pairs submitted as candidates for transplantation. Methods: Demographic data of patients who had been evaluated for live donor kidney transplantation but found to be incompatible with their potential donor (because of ABO blood group or positive crossmatch) were submitted for computer analysis and matching. Data included ABO and HLA types of donor and recipient, %PRA and specificity of recipient alloantibody, donor/recipient relationship, and the reason the donor was incompatible. The data set used for the initial simulation included 29 patients with one donor each and 16 patients with multiple donors for a total of 45 patients and 68 donor/patient pairs. In addition, a simulation based on OPTN/SRTR data was used to further assess the practical importance of multiple exchange combinations. Results: If only exchanges involving two patient-donor pairs were allowed, a maximum of 8 patient-donor pairs in the data set could exchange kidneys. If 3-way exchanges were also allowed, a maximum of 11 pairs could exchange kidneys. Simulations with OPTN/SRTR data demonstrate that the increase in the number of potential transplants if 3-way exchanges are allowed is robust, and does not depend on the particular patients in our sample. Conclusions: A computerized matching protocol can be used to identify donor/recipient pairs from a registry of incompatible pairs who can potentially enter into donor exchanges that otherwise would not readily occur

Combined liver-kidney transplantation and the effect of preformed lymphocytotoxic antibodies

Thirty-eight sequentially placed liver and kidney allografts were evaluated with respect to patient and graft survival, and the influence of preformed lymphocytotoxic antibodies was analysed. The results suggest that the survival rate of combined liver and kidney transplantation is similar to the survival rate of liver transplantation alone. Sequentially placed kidney allografts may be protected from hyperacute rejection in the presence of donor specific lymphocytotoxic antibodies, but not in all instances. Both patient and kidney allograft survival was lower in positive crossmatch patients (33% and 17% respectively) than in negative crossmatch patients (78% and 75%). High levels of panel reactive antibodies (>10%) also appeared to have a deleterious effect on survival, although the majority of the patients who failed also had a positive crossmatch. Although preformed lymphocytotoxic antibodies are not an absolute contraindication to combined liver-kidney transplantation, they do appear to have a deleterious effect on long-term graft survival. However, more correlation with clinical parameters is needed. © 1994

Utilizing List Exchange and Non-directed Donation through “Chain” Paired Kidney Donations

In a list exchange (LE), the intended recipient in an incompatible pair receives priority on the deceased donor waitlist (DD-waitlist) after the paired incompatible donor donates a kidney to a DD-waitlist candidate. A non-directed donor’s (ND-D) kidney is usually transplanted directly to a DD-waitlist candidate. These two established practices would help even more transplant candidates if they were integrated with kidney paired donation (KPD). We consider a scenario in which the donor of an LE intended recipient (LE-IR) donates to a compatible KPD intended recipient (KPD-IR), and the KPD donor (KPD-D) donates to the waitlist (an LE-chain). We consider a similar scenario in which an ND-D donates to a KPD-IR and the KPD-D donates to the DD-waitlist (an ND-chain). Using data derived from the New England Program for Kidney Exchange (NEPKE) and from OPTN/SRTR recipient-donor distributions, simulations are presented to evaluate the potential impact of chain exchanges coordinated with KPD. LE donors (LE-D) and ND-D who are ABO-O result in the highest number of additional transplants, while results for ABO-A and B donors are similar to each other. We recommend that both LE and ND donations be utilized through chain exchanges