Una apertura hacia la noción de "modelo" siguiendo a Bachelard

Las ciencias naturales se constituyen en sistemas formales interpretados. Esto significa que dentro de un cuerpo de enunciados, que conforman los denominados términos teóricos, éstos adquieren un contenido al ser asociados indirectamente a observables de la realidad externa, o en otras palabras son interpretados mediante operadores en un modelo que pretende· ser la representación mental (teórica) de esa misma realidad (De la Torre, 1992; Adúriz Bravo y Galagosky, 1997). Se puede decir, entonces, que cada teoría posee modelos conexos (modelos teoréticos según Bunge, 1972), que articulan las representaciones que están asociadas con ella

Una forma divertida de experimentar y jugar con la estadistica

La Estadística tiene cada vez más influencia en la sociedad. Las razones para incluir su enseñanza en el nivel medio superior (16-18 años) se ha subrayado repetidamente durante los últimos 20 años. La Física emplea un tratamiento estadístico (Mecánica Estadística) cuando se trata de analizar sistemas que están compuestos por un gran número de partículas. El trabajo es un reporte de un taller para docente llevado a cabo en la Vigésimo Séptima Reunión Latinoamericana de Matemática Educativa. El mismo se fundamenta en la utilización de una actividad experimental basada en un modelo didáctico para ser implementada en el aula con el objetivo de que los alumnos accedan a los conocimientos estadísticos necesarios para interpretar el movimiento Browniano

La perspectiva filosófica de un concepto: una analogía de Bachelard

Es nuestra intención en este trabajo, construir las imágenes analógicas que implícitamente están bocetadas en "La filosofía del no" de Bachelard, tratando de mostrar a un Bachelard analógico al momento de exponer sus ideas, mostrándonos como la ciencia misma ha recurrido a imágenes que al destruirlas condujeron al pensamiento científico a descubrir sus leyes orgánicas. Así vemos como Bachelard si bien marca una ruptura entre las imágenes, metáforas y analogías de la experiencia cotidiana, nos muestra la necesidad de recurrir a ellas por parte de la ciencia, la cuál crea formas realistas y racionales de sus objetos que le otorgan una razón polémica. La historia de la imaginería científica conforma un plan estratégico pedagógico ineluctable para el crecimiento científico (Bachelard, 2003:115)

Meat processing and colon carcinogenesis: Cooked, nitrite-treated and oxidized high-heme cured meat promotes mucin depleted foci in rats

Processed meat intake is associated with colorectal cancer risk, but no experimental study supports the epidemiologic evidence. To study the effect of meat processing on carcinogenesis promotion, we first did a 14-day study with 16 models of cured meat. Studied factors, in a 2 × 2 × 2 × 2 design, were muscle color (a proxy for heme level), processing temperature, added nitrite, and packaging. Fischer 344 rats were fed these 16 diets, and we evaluated fecal and urinary fat oxidation and cytotoxicity, three biomarkers of heme-induced carcinogenesis promotion. A principal component analysis allowed for selection of four cured meats for inclusion into a promotion study. These selected diets were given for 100 days to rats pretreated with 1,2-dimethylhydrazine. Colons were scored for preneoplastic lesions: aberrant crypt foci (ACF) and mucin-depleted foci (MDF). Cured meat diets significantly increased the number of ACF/colon compared with a no-meat control diet (P = 0.002). Only the cooked nitrite-treated and oxidized high heme meat significantly increased the fecal level of apparent total N-nitroso compounds (ATNC) and the number of MDF per colon compared with the no-meat control diet (P < 0.05). This nitrite-treated and oxidized cured meat specifically increased the MDF number compared with similar non nitrite-treated meat (P = 0.03) and with similar non oxidized meat (P = 0.004). Thus, a model cured meat, similar to ham stored aerobically, increased the number of preneoplastic lesions, which suggests colon carcinogenesis promotion. Nitrite treatment and oxidation increased this promoting effect, which was linked with increased fecal ATNC level. This study could lead to process modifications to make non promoting processed meat

Exchange bias in Co/CoO core-shell nanowires: Role of the antiferromagnetic superparamagnetic fluctuations

The magnetic properties of Co (=15 nm, =130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.Comment: 10 pages, 10 figures, submitted to Phys. Rev.

Specific in vivo deletion of B-cell subpopulations expressing human immunoglobulins by the B-cell superantigen protein L

Some pathogens have evolved to produce proteins, called B-cell superantigens, that can interact with human immunoglobulin variable regions, independently of the combining site, and activate B lymphocytes that express the target immunoglobulins. However, the in vivo consequences of these interactions on human B-cell numbers and function are largely unknown. Using transgenic mice expressing fully human immunoglobulins, we studied the consequences of in vivo exposure of protein L of Peptostreptococcus magnus with human immunoglobulins. In the mature pool of B cells, protein L exposure resulted in a specific reduction of splenic marginal-zone B cells and peritoneal B-1 cells. Splenic B cells exhibited a skewed light-chain repertoire consistent with the capacity of protein L to bind specific kappa gene products. Remarkably, these two B-cell subsets are implicated in innate B-cell immunity, allowing rapid clearance of pathogens. Thus, the present study reveals a novel mechanism that may be used by some infectious agents to subvert a first line of the host's immune defense

Medición del índice de refracción del agua usando materiales sencillos

Palabras clave: índice de refracción; ley de Snell; comportamiento de la luz; experiencias físicas sencillas.Measuring the index of refraction of water using simple materialsKeywords: index of refraction; Snell’s law; behaviour of light; simple physical experiences