Modeling and Forecasting the Yield Curve by an Extended Nelson‐Siegel Class of Models: A Quantile Autoregression Approach

This paper compares the in‐sample fitting and the out‐of‐sample forecasting performances of four distinct Nelson–Siegel class models: Nelson–Siegel, Bliss, Svensson, and a five‐factor model we propose in order to enhance the fitting flexibility. The introduction of the fifth factor resulted in superior adjustment to the data. For the forecasting exercise the paper contrasts the performances of the term structure models in association with the following econometric methods: quantile autoregression evaluated at the median, VAR, AR, and a random walk. As a pattern, the quantile procedure delivered the best results for longer forecasting horizons. Copyright © 2011 John Wiley & Sons, Ltd.     

The Philosopher's conception of Mathesis Universalis from Descartes to Leibniz

In Descartes, the concept of a ‘universal science’ differs from that of a ‘mathesis universalis’, in that the latter is simply a general theory of quantities and proportions. Mathesis universalis is closely linked with mathematical analysis; the theorem to be proved is taken as given, and the analyst seeks to discover that from which the theorem follows. Though the analytic method is followed in the Meditations, Descartes is not concerned with a mathematisation of method; mathematics merely provides him with examples. Leibniz, on the other hand, stressed the importance of a calculus as a way of representing and adding to what is known, and tried to construct a ‘universal calculus’ as part of his proposed universal symbolism, his ‘characteristica universalis’. The characteristica universalis was never completed—it proved impossible, for example, to list its basic terms, the ‘alphabet of human thoughts’—but parts of it did come to fruition, in the shape of Leibniz's infinitesimal calculus and his various logical calculi. By his construction of these calculi, Leibniz proved that it is possible to operate with concepts in a purely formal way.     

Hypothesis and experiment in the early development of Kekulé's Benzene theory

This article attempts a contextual study of the origin and early development of August Kekulé's theory of aromatic compounds. The terminus a quo is essentially August Hofmann's coining of the modern chemical denotation of ‘aromatic’ in 1855; the terminus ad quem is the first full codification of Kekulé's theory in the sixth fascicle of his Lehrbuch der organischen Chemie, published in the summer of 1866. Kekulé's theory is viewed in context with the earlier and concurrent experimental work of such chemists as Hermann Kolbe, Friedrich Beilstein, Rudolph Fittig, and Hugo Müller. The reception of the theory is briefly examined. Attention is paid to the role of Kekulé's molecular models and of his celebrated dream anecdote of the snake that seizes its own tail. The episode is used as a case study for the continuity of scientific progress, and to illustrate the close reciprocal interactions of hypothesis and experiment in the evolution of a scientific theory.     

L’émergence contrariée du chronographe imprimant dans les observatoires français (fin 19e – début 20e s.)

Les observatoires occidentaux se transforment en véritable usine scientifique à partir du milieu du 19^e siècle. L'astrométrie symbolise ce passage à une économie industrieuse des pratiques scientifiques. Le chronographe imprimant, qui permet de réduire les équations personnelles des observateurs, s'impose, d'abord aux Etats-Unis, puis en Angleterre, en instrument-emblème de cette transformation profonde. En France, les initiatives de l'astronome Liais restent prototypiques. Ce n'est qu'au début du 20^e siècle, par les voies détournées de l'observatoire d'Hendaye et de l'abbé Verschaffel, que le chronographe imprimant fait son retour et conquiert les espaces savants. La centralisation excessive de l'astronomie française, l'autoritarisme du directeur de l'Observatoire de Paris Urbain Le Verrier, et la faiblesse du marché des instruments expliquent pourquoi le chronographe imprimant n'a fait souche que très tardivement en France.     

Hormonal control of Sertoli cell metabolism regulates spermatogenesis

Hormonal regulation is essential to spermatogenesis. Sertoli cells (SCs) have functions that reach far beyond the physical support of germ cells, as they are responsible for creating the adequate ionic and metabolic environment for germ cell development. Thus, much attention has been given to the metabolic functioning of SCs. During spermatogenesis, germ cells are provided with suitable metabolic substrates, in a set of events mediated by SCs. Multiple signaling cascades regulate SC function and several of these signaling pathways are hormone-dependent and cell-specific. Within the seminiferous tubules, only SCs possess receptors for some hormones rendering them major targets for the hormonal signaling that regulates spermatogenesis. Although the mechanisms by which SCs fulfill their own and germ cells metabolic needs are mostly studied in vitro, SC metabolism is unquestionably a regulation point for germ cell development and the hormonal control of these processes is required for a normal spermatogenesis.     

Connexin 43 a check-point component of cell proliferation implicated in a wide range of human testis diseases

Gap junction channels link cytoplasms of adjacent cells. Connexins, their constitutive proteins, are essential in cell homeostasis and are implicated in numerous physiological processes. Spermatogenesis is a sophisticated model of germ cell proliferation, differentiation, survival, and apoptosis, in which a connexin isotype, connexin 43, plays a crucial role as evidenced by genomic approaches based on gene deletion. The balance between cell proliferation/differentiation/apoptosis is a prerequisite for maintaining levels of spermatozoa essential for fertility and for limiting anarchic cell proliferation, a major risk of testis tumor. The present review highlights the emerging role of connexins in testis pathogenesis, focusing specifically on two intimately interconnected human testicular diseases (azoospermia with impaired spermatogenesis and testicular germ cell tumors), whose incidence increased during the last decades. This work proposes connexin 43 as a potential cancer diagnostic and prognostic marker, as well as a promising therapeutic target for testicular diseases.