The cellular and structural properties and binding capabilities of a lipocalin expressed in the early neural plate of Xenopus laevis embryos and the adult choroid plexus have been investigated. It was found that this lipocalin, termed Xlcpl1, binds retinal at a nanomolar concentration, retinoic acid in the micromolar range, but does not show binding To retinol. Furthermore, this protein also binds D/L thyroxine. The Xlcpl1 cDNA was expressed in cell culture using the vaccinia virus expression system. In AtT20 cells, Xlcpl1 was secreted via the constitutive secretory pathway. We therefore assume that cpl1 binds retinaldehyde during the transport through the compartments of the secretory pathway that are considered to be the storage compartments of retinoids. Therefore, cpl1-expressing cells will secrete the precursors of active retinoids such as retinoic acid isomers. These retinoids may enter the cytosol by diffusion or receptor-controlled mechanisms, as has been shown for exogenously applied retinoids. Based on these data, it is suggested that cpl1 is an integral member of the retinoid signaling pathway and, therefore, it plays a key role in pattern formation in early embryonic development.

Teoría y practica, aproximación critica de la alternancia en pedagogia. - La alternancia en pedagogia pone la doble cuestión de la articulación y de la separación entre la escuela y la vida. El problema es abordado aqui desde el punto de vista de la relación entre teoría y prática : ¿ cuál es la naturaleza del saber práctico y cuál es la del saber teorico ?¿ Los modos de apropiación de estos saberes son diferentes ? Después de haber definido dos vias de adquisición de los conocimientos, una fundada sobre la relación simbolica y la otra sobre la relación material a la realidad, los autores insisten sobre los intereses y los limites de cada una de ellas.

AbstractMany recent measurements of vertical‐velocity spectra in the atmospheric boundary layer are combined in order to derive the dependence of the maximum of the logarithmic spectrum, normalized by surface stress, on stability. An empirical equation is suggested which fits the observations and has the proper limiting form for free convection. It is suggested that this equation can be used to derive surface stresses from relatively simple measurements of the most energetic part of the spectrum, and from a stability parameter.

We investigated the discriminability of simple and complex gratings containing one or two barely detectable sinusoidal luminance modulations at 3 and 9 c/deg, which will be referred to as ƒ and 3ƒ. Our major findings were the following:(1) frequency discrimination of the simple gratings seems to be limited only by their detectability;(2) relative phase differences between components in a complex grating are essentially indiscriminable even when each component is nearly perfectly detectable in the presence of the other;(3) when the contrast of the ƒ-component is at least 4 times its threshold value, it is only the detectability of the3ƒ-component which limits the discriminability of complex gratings differing in relative phase by π radians;(4) a high contrast3ƒ-component hinders the detection of ƒ while a high contrast ƒ component facilitates the detection of 3ƒ It is possible to offer a plausible explanation of some of these phenomena by invoking the existence in the human visual system of broad-band, phase-sensitive channels, as well as of narrow-band channels.

Blakemore andCampbell (1969) found that a square-wave adapting grating, unlike a sinewave grating of the same period, elevates threshold contrast for test gratings whose frequency is in the vicinity of the third harmonic in the square-wave. Because of its important bearing on the frequency-specificity of adaptation to gratings, we attempted to replicate this finding. Our attempt was successful with one of two observers when the method of adjustment was used to determine thresholds. With the temporal two alternative, forced-choice method, individual differences in this regard disappeared. For all three observers a 3 c/deg adapting square-wave and a 3 c/deg sinewave were equally ineffective in raising the contrast threshold for a 9 c/deg test grating, whereas a 9 c/deg adapting grating, matched to the 3rd harmonic in the square-wave, raised the test grating's threshold by 0.2 log units. Our failure to find consistent evidence that the third harmonic in a square-wave does any adapting whatever casts doubt on the frequency-specificity of adaptation to gratings. SelonBlakemore etCampbell (1969), un re´seau d'adaptationa`onde carre´e,a`l'oppose´d'un re´seau sinusoi¨dal de meˆme pe´riode,e´le`ve le seuil de contraste pour les re´seaux tests dont la fre´quence est voisine du troisie`me harmonique de l'onde carre´e. Nous avons tente´de retrouver ce re´sultat qui a d'importantes conse´quences sur la spe´cificite´de la fre´quence dans l'adaptation aux re´seaux. Nous y avons re´ussi sur l'un des deux sujets quand on emploie la me´thode d'ajustement pour de´terminer les seuils. Les diffe´rences individuelles disparaissent avec une me´thode temporelle de choix force´a`deux alternatives. Pour les trois sujets, une onde carre´e d'adaptationa`3 cycles/deg et une onde sinusoi¨dale de 3 cycles/deg sonte´galement sans effet sur le seuil de contraste d'un testa`9 cycles/deg tandis qu'un re´seau d'adaptation de 9 cycles/deg comparable au 3e harmonique de l'onde carre´e,e´le`ve de 0,2 unite´logarithmique le seuil du re´seau test. Notree´checa`montrer nettement un effet d'adaptation du troisie`me harmonique d'une onde carre´e jette un doute sur la spe´cificite´a`la fre´quence de l'adaptation par les re´seaux. Blakemore undCampbell (1969) fanden, dass die Adaptation an ein Rechteckgitter im Gegensatz zu der an ein Sinusgitter derselben Periodenla¨nge, eine Erho¨hung des Schwellenkontrastes fu¨r solche Testgitter hervorruft, deren Frequenz in der Nachbarschaft der dritten harmonischen der Rechteckwelle ist. Wir versuchten, dieses Ergebnis zu verifizieren, dases von grosser Wichtigkeit fu¨r die Frequenzspezifita¨t der Adaptation an Gitter ist. Unser Versuch war erfolgreich mit einem von zwei Beobachtern, wenn die Methode der Angleichung bei der Schwellenbestimmung verwendet wurde. Fu¨r alle drei Beobachter wurde die Kontrastschwelle fu¨r ein 9 Perioden/Grad-Testgitter durch ein 3 Perioden/Grad-Adaptationsrechteckgitter und ein 3 Perioden/Grad-Sinusgitter in gleicher Weise erho¨ht. Dagegen erho¨hte ein 9 Perioden/Grad-Adaptationsgitter, das mit der dritten harmonischen der Rechteckwelle verglichen wurde, die Schwelles des Testgitters um 0,2 log Einheiten. Die Tatsache, dass wir keinenu¨berzeugenden Beweis dafu¨r finden konnten, dass die dritte harmonische einer Rechteckwelle irgendeine Adaptationswirkung hat, gibt Anlass, an der Frequenzspezifita¨t der Gitteradaptation zu zweifeln. Ълeкмop и Kэмпбeлл (1969) haшли, хтo aдaптиpyющaя peшeткa c пpямoyгoльhoй хapaктepиcтикoй кohтpacтa, b oтлихиe oт peшeтки тaкoй жe пepиoдихhocти c cиhycoидaльhoй хapaктepиcтикoй кohтpacтa, пobышaeт кohтpacтhый пopoг для пpeдъяbляeмoй пocлe hee тecтoboй peшeтки, хacтoтa кoтopoй haхoдитcя b oблacти тpeтьeй гapмohики пpямoyгoльhoй boлhы. Пocкoлькy этoт peзyльтaт baжeh для хacтoтhoй cпeцифихhocти aдaптaции к peшeткaм, мы peшили пobтopить этoт oпыт. Oh oкaзaлcя ycпeшhым ha oдhoм из дbyх иcпыгyeмых, кoгдa иcпoльзobaлcя мeтoд кoppeктиpobки (adjustment) для oпpeдeлehия пopoгob. Пpи иcпoльзobahии мeтoдa фopcиpobahhoгo bыбopa, c дboйhoй bpeмehhoй aльтephaтиboй, иhдиbидyaльhыe paзлихия иcхeзaли. Для bceх тpeх иcпытyeмых aдaптиpyющaя peшeткa c хacтoтoй 3 циклa/гpaдyca кaк пpямoyгoльhaя, тaки cиhycoидaльhaя, he bызыbaли пobышehия кohтpacтhых пopoгob к peшeткe b 9 циклob/гp. B тoжe bpeмя aдaптиpyющaя peщeткa 9 циклob/гpaдycob пpи cpabhehии c 3 гapмohикoй пpи пpямoyгoльhoй хapaктepиcтикe кohтpacтa пobышaлa пopoг paзлихehия peшeтки ha 0,2 лoг. eдиhицы. To, хтo мы he мoгли пoлyхить cyщecтbehhoгo дoкaзaтeльcтba кaкoгo-либo aдaптиpyющeгo дeйcтbия тpeтьeй гapмohики peшeтки пpи пpямoyгoльhoй хapктepиcтикe кohтpacтa, зacтabляeт ycoмhитьcя b хacтoтhoй cпeцифихhocти aдaптaции к peшeткaм.