Martini Vahlii, profess. botan. Haun. membr. societ. lit. plur. Enumeratio plantarum :vel ab aliis, vel, ab ipso observatum, cum earum differentiis specificis, synonymis selectis et descriptionibus succinctis.

v.2

Patterns of oxygen consumption during establishment of cephalocaudal polarity in the early chick embryo.

Oxygen uptake was studied during the establishment of cephalocaudal polarity in the very early chick embryo, i.e., 10 hr before (stage VI) and at laying (stage X). Oxygen fluxes in minute regions of the intact blastoderms were measured in vitro by scanning microspectrophotometry in the presence or absence of glucose. The oxygen consumption of the whole blastoderm remained constant (6 nmol O2 X hr-1) throughout the period studied, although the number of cells increased more than twofold. The regional oxygen fluxes varied from 0.41 to 1.13 nmol O2 X hr-1 X mm-2 at stage VI and from 0.42 to 0.70 nmol O2 X hr-1 X mm-2 at stage X. At stage VI, the oxygen flux in the center of the blastoderm was significantly higher than that in its periphery. This pattern remained evident when the values were corrected for cell number or for cytoplasmic volume. At stage X, there was a tendency for the oxygen fluxes to decrease from the posterior to the anterior regions of the area pellucida. Thus the pattern of oxidative metabolism in the late uterine embryos seems to change from radial to bilateral. This change of symmetry probably reflects the process of formation of the embryonic axis. In addition, the fact that the oxygen uptake was similar in the presence or absence of glucose suggests that early chick embryos metabolize essentially intracellular stores.

Transient stimulation of glucose metabolism by insulin in the 1-day chick embryo.

Effects of insulin upon glucose metabolism were investigated in chick embryos explanted in vitro during the first 30 h of incubation. Insulin stimulated the glucose consumption of the chick gastrula (18 h) and neurula (24 h), but had no effect on the late blastula (0 h:laying) and on the stage of six to eight somites (30 h). The increase in glucose consumption concerned both the embryonic area pellucida (AP) and extraembryonic area opaca (AO). AP responded to a greater extent (50%) and at a lower range of concentrations (0.1-1.0 ng/ml) than AO (30%; 1-100 ng/ml). Insulin had no effect on the oxygen consumption of blastoderms, whereas it stimulated the aerobic lactate production (approximately 70% of the additional glucose consumption was converted to lactate). The nanomolar range of stimulating concentrations suggests that insulin has a specific effect in the chick embryo, and that it could modulate glucose metabolism in ovo as well. The transient sensitivity of the embryo to insulin is discussed in relation to behavior of mesodermal cells.

Proliferation of chick embryo neuroblasts grown in the presence of horse serum requires exogenous transferrin.

We have previously shown that neuroblasts from cerebral hemispheres of 6-day-old chick embryos are able to proliferate when grown in the presence of fetal calf serum. We report here that in the presence of horse serum alone the proliferative rate of neuroblasts is strongly reduced. A high proliferative rate is restored upon the addition of bovine transferrin and to a lesser extent with added FeSO4 or hemin. These findings suggest that the transferrin of horse serum cannot be used by chick neuroblasts in vitro, while bovine transferrin exogenously added is active in promoting cell proliferation. We propose that the stimulatory activity of the fetal calf serum is due to bovine transferrin, since when this serum is fractionated by gel filtration, the fractions that stimulate the proliferation of neuroblasts grown in the presence of horse serum are located in the molecular weight area of transferrin, and they do contain transferrin as seen by immunoblotting with a specific anti-transferrin antibody.

Hegel, Heidegger : pensée absolue, pensée abyssale (Heidegger lit Hegel)

Cette thèse de doctorat porte sur l'endurant travail d'interprétation auquel Martin Heidegger (1889-1976) s'est livré à propos de la pensée de Hegel (1770-1831), l'un des représentants de ce que l'on appelle « l'idéalisme allemand ». Ce travail d'interprétation résulte et prend place dans ce que Heidegger entend par le mot allemand Aus-einander-setzung : non pas la recherche de points communs entre deux pensées, par-delà des différences particulières qu'il s'agirait dès lors dépasser, mais au contraire la mise en évidence de leurs différends au sein même d'une appartenance commune à la pensée européo-occidentale. Notre présent travail s'est donné trois tâches : reconstruire l'explication de Heidegger avec Hegel qui se déroule sur près de quarante ans et qui se trouve éparpillée dans une multitude de textes d'époques et de statuts fort divers ; réfléchir et exposer le sens et les enjeux d'une telle explication ; indiquer et mettre à l'épreuve, par une étude immanente de plusieurs textes du corpus hégélien, l'apport de la phénoménologie herméneutique heideggerienne à l'intelligibilité actuelle de la pensée hégélienne. La pensée occidentale, qui a pour nom la philosophie, s'interroge toujours d'une manière ou d'une autre à propos de l'être. Et cela, à partir de Platon, dans un mouvement métaphysique qui va de ce qui est ou de l'étant à son être, ce dernier étant lui-même compris comme le genre commun de l'étant, soit comme « étantité (ousía, Seiendheit) ». Chez Hegel, cette manière bimillénaire de s'enquérir ainsi de l'être de l'étant parvient à son aboutissement, dans une compréhension dialectico-spéculative de l'être en tant que ce processus d'autocompréhension et d'autoproduction que Hegel nomme « l'esprit absolu ». À partir de cet aboutissement, de l'être, il n'en sera plus rien pour la pensée occidentale - « dernière fumée d'une réalité s'évaporant » dira Nietzsche. C'est ce phénomène, nommé par Heidegger « l'oubli de l'être (die Seinsvergessenheit) », qui constituera pour lui la motivation centrale de sa tentative de commencer autrement à penser l'être : dans un mouvement qui ne va plus de l'étant à l'être en tant qu'étantité, mais de l'être (Sein ou Seyn) à l'étant : où cela, qui d'ores et déjà se refuse (sich verweigert) à être un étant, ouvre par là même la possibilité à l'étant d'être ce qu'il est en propre. L'être est alors pensé en tant qu'Ereignis, cela qui advient en appropriant, c'est-à-dire en conduisant l'homme et l'étant à leur propre. Notre travail distingue, dans la lecture heideggerienne de Hegel, deux grandes périodes. Cellesci sont solidaires des deux manières non-métaphysiques d'élaborer la question de l'être qui rythment le chemin de pensée de Heidegger (la première ayant abouti à une impasse) : 1°) l'élaboration horizontaletranscendantale de la question de l'être, centrée sur le premier Hauptwerk de Heidegger de 1927, Être et temps, où la question de l'être est considérée primordialement à partir de ce phénomène qu'est l'entente qu'a l'homme de son être et de son destin qui est celui d'être l'espace ouvert, le là pour l'être en général (Dasein) ; 2°) l'élaboration destinale (seynsgeschichtliche) de la question de l'être, centrée sur le deuxième Hauptwerk de Heidegger que sont les Apports à la philosophie (Beiträge zur Philosophie) (1936- 1938), où la question de l'être est considérée cette fois à partir de l'être lui-même en tant qu'Ereignis. Dans l'interprétation horizontale-transcendantale de Hegel, Heidegger oppose à l'être hégélien en tant qu'esprit absolu, éternel et infini, la transcendance finie du rapport temporel de l'homme à l'être. L'explication se concentre ainsi sur les notions de finitude (chapitre II de notre thèse), de transcendance (chapitre III) et de temporalité (chapitre IV), dans laquelle Heidegger fait jouer son interprétation de Kant contre Hegel. Dans l'interprétation destinale de Hegel, qui est l'interprétation décisive, Hegel est compris, dans la perspective de l'histoire-destinée de l'estre (die Geschichte des Seyns), comme celui qui accomplit ce qui se trouve au coeur même de cette histoire-destinée, dès Héraclite et Parménide : le refus qu'a l'homme occidental de prendre en garde la nihilité (Nichthaftigkeit) de l'être. Hegel serait ainsi celui qui accomplit l'occidental refus du refus (Verweigerung) de l'être. Heidegger repère le site de cet accomplissement hégélien dans la conception dialectico-spéculative de la négativité, laquelle en tant que « négativité absolue (absolute Negativität) » constitue le coeur et l'âme de l'être qu'est le processus de l'esprit absolu (chapitre V). L'enjeu de l'interprétation heideggerienne de Hegel devient dès lors de montrer qu'une négativité plus originaire que la négativité absolue se trouve à l'oeuvre en l'être, une négativité non dialectique que nous avons nommé la « négativité abyssale » (chapitre VII). C'est de la compréhension et l'élaboration de cette dernière que dépend la possibilité pour la pensée occidentale de surmonter une bonne fois le nihilisme - gisant au coeur de la métaphysique - tel qu'il se déchaîne actuellement dans la civilisation technique devenue aujourd'hui planétaire (chapitre VI). Tels sont selon nous le sens et l'enjeu derniers de l'explication de Heidegger avec Hegel.

Early functional differentiation in the chick embryonic disc: interaction between mechanical activity and extracellular matrix

The mechanical behaviour of ectodermal cells in the area opaca and the supracellular organization of fibronectin in the adjacent extracellular matrix were studied in whole chick blastoderms developing in vitro. The pattern of spontaneous mechanical activity and its modification by immunoglobulins against fibronectin were determined using a real-time image-analysis system. The pattern of fibronectin was studied using immunocytochemical techniques. It was found that the ectodermal cells in the area opaca actively develop a radially oriented contraction, which leads to a distension of the area pellucida from which the embryo develops. Abnormally increased tension resulted in perturbations of gastrulation and neurulation. An optimized mechanical equilibrium within the blastoderm seems to be necessary for normal development. Anti-fibronectin antibodies applied to the basal side of the blastoderm led rapidly and reversibly to an increase of tension in the contracted cells. This observation indicates that modifications of the extracellular matrix can be transmitted to cytoskeletal elements within adjacent cells. The extracellular matrix of the area opaca contains fibronectin arranged in radially oriented fibrils. This orientation corresponds to the direction of migration of the mesodermal cells. Interestingly, the radial pattern of fibronectin is found in the regions where the ectodermal cells are contracted and develop radially oriented forces. This observation suggests that the supracellular assembly of the extracellular materials could be influenced by the mechanical activity of adjacent cells. Possible modulations of the supracellular organization of extracellular matrix by other factors, e.g. diffusible metabolites, is also discussed. The presence of characteristically organized extracellular matrix components, of spatially differentiated cell activities and of reciprocal interactions between them makes the young chick blastoderm an excellent system for physiological studies of the coordinated cellular activities that lead to changes in form, complexity and function.

Differential distribution of two microtubule-associated proteins, MAP2 and MAP5, during chick dorsal root ganglion development in situ and in culture.

Microtubule-associated proteins (MAPs) are essential components necessary for the early growth process of axons and dendrites, and for the structural organization within cells. Both MAP2 and MAP5 are involved in these events, MAP2 occupying a role predominantly in dendrites, and MAP5 being involved in both axonal and dendritic growth. In the chick dorsal root ganglia, pseudo-unipolar sensory neurons have a T-shaped axon and are devoid of any dendrites. Therefore, they offer an ideal model to study the differential expression of MAPs during DRG development, specifically during axonal growth. In this study we have analyzed the expression and localization of MAP2 and MAP5 isoforms during chick dorsal root ganglia development in vivo, and in cell culture. In DRG, both MAPs appeared as early as E5. MAP2 consists of the 3 isoforms MAP2a, b and c. On blots, no MAP2a could be found at any stage. MAP2b increased between E6 and E10 and thereafter diminished slowly in concentration, while MAP2c was found between stages E6 and E10 in DRG. By immunocytochemistry, MAP2 isoforms were mainly located in the neuronal perikarya and in the proximal portion of axons, but could not be localized to distal axonal segments, nor in sciatic nerve at any developmental stage. On blots, MAP5 was present in two isoforms, MAP5a and MAP5b. The concentration of MAP5a was highest at E6 and then decreased to a low level at E18. In contrast, MAP5b increased between E6 and E10, and rapidly decreased after E14. Only MAP5a was present in sciatic nerve up to E14. Immunocytochemistry revealed that MAP5 was localized mainly in axons, although neuronal perikarya exhibited a faint immunostaining. Strong staining of axons was observed between E10 and E14, at a time coincidental to a period of intense axonal outgrowth. After E14 immunolabeling of MAP5 decreased abruptly. In DRG culture, MAP2 was found exclusively in the neuronal perikarya and the most proximal neurite segment. In contrast, MAP5 was detected in the neuronal cell bodies and all along their neurites. In conclusion, MAP2 seems involved in the early establishment of the cytoarchitecture of cell bodies and the proximal axon segment of somatosensory neurons, while MAP5 is clearly related to axonal growth.

Immunocompetence and nestling survival in the house martin: the tasty chick hypothesis

In altricial birds post-fledging survival is usually positively related to nestling body mass. A large number of studies have shown that the latest hatched chick is the more likely to die, even if food is abundant. Here we suggest that ectoparasites may be a key factor in the evolution and the maintenance of the establishment of weight hierarchies within broods. We prepose the hypothesis that weight hierarchies within broods may be adaptive if the chick in poor condition is the one with the least efficient immune system within a nest. In this case parasites would preferentially feed on such a "tasty chick", because it would allow high reproductive rates for the parasites, without negatively affecting the survival of the other nestlings. This could prevent entire nest failure of the brood or allow the other chicks to grow more efficiently. This hypothesis was investigated in a colony of house martins Delichon urbica. We predicted that immunocompetence was positively correlated with body condition, and that nestlings dying before hedging should have lower immune responses when challenged with an antigen. T-cell immune response to an experimentally injected antigen was strongly positively related to body condition. Non-surviving chicks had low body condition and a weak immune response. The implications of these results are discussed in the context of the adaptive significance of hatching asynchrony.

Oxygen uptake during early cardiogenesis of the chick.

Oxidative metabolism of the isolated embryonic heart of the chick has been determined using a spectrophotometric technique allowing global as well as localized micromeasurements of the O2 uptake. Entire hearts, excised from embryos of 10 somites (primordia fused, stage 10 HH) and 40 somites (S shaped, stage 20 HH) were placed in a special chamber under controlled metabolic conditions where they continued to beat spontaneously and regularly. During the 32 h of development, the O2 consumption of the whole heart increased from 0.9 +/- 0.1 to 5.3 +/- 0.8 nmol O2/h. These values corrected for protein content were, however, comparable (0.45 nmol O2.h-1.micrograms-1). At stage 10-12, the O2 uptake varied along the cardiac tube (from 0.74 to 1.0 nmol O2.h-1.mm-2). From stage 10 to 20, the O2 uptake per unit area of ventricle wall increased from 0.7 +/- 0.2 to 1.8 +/- 0.2 nmol O2.h-1.mm-2, and the O2 uptake per myocardial volume during one cardiac cycle varied from 7 to 2.5 nmol O2/cm3. These results indicate that, despite an intense morphogenesis, the cardiac tissue has a rather low and stable oxidative metabolism, although the O2 requirement of the whole heart increases significantly. Moreover, the normalized suprabasal aerobic energy expenditure decreases throughout early cardiogenesis. The functional integrity of the isolated embryonic heart combined with the experimental possibilities of the microtechnique make the preparation appropriate for studying the changes in cardiac metabolism during development.

Pre-hatching maternal effects and the tasty chick hypothesis

Question: Are maternal effects (i.e. maternal transfer of immune components to their offspring via the placenta or the egg) specifically directed to the offspring on which ectoparasites predictably aggregate? Organisms: The barn owl (Tyto alba) because late-hatched offspring are the main target of the ectoparasitic fly Carnus hemapterus. Hypothesis: Pre-hatching maternal effects enhance parasite resistance of late- compared with early-hatched nestlings. Search method: To disentangle the effect of natal from rearing ranks on parasite intensity, we exchanged hatchlings between nests to allocate early- and late-hatched hatchlings randomly in the within-brood age hierarchy. Result: After controlling for rearing ranks, cross-fostered late-hatched nestlings were less parasitized but lighter than cross-fostered early-hatched nestlings. Conclusion: Pre-hatching maternal effects increase parasite resistance of late-hatched offspring at a growth cost.

Calbindin-immunoreactive sensory neurons in dissociated dorsal root ganglion cell cultures of chick embryo: role of culture conditions.

Immunoreactivity to calbindin D-28k, a vitamin D-dependent calcium-binding protein, is expressed by neuronal subpopulations of dorsal root ganglia (DRG) in the chick embryo. To determine whether the expression of this phenotypic characteristic is maintained in vitro and controlled by environmental factors, dissociated DRG cell cultures were performed under various conditions. Subpopulations of DRG cells cultured at embryonic day 10 displayed calbindin-immunoreactive cell bodies and neurites in both neuron-enriched or mixed DRG cell cultures. The number of calbindin-immunoreactive ganglion cells increased up to 7-10 days of culture independently of the changes occurring in the whole neuronal population. The presence of non-neuronal cells, which promotes the maturation of the sensory neurons, tended to reduce the percentage of calbindin-immunoreactive cell bodies. Addition of horse serum enhanced both the number of calbindin-positive neurons and the intensity of the immunostaining, but does not prevent the decline of the subpopulation of calbindin-immunoreactive neurons during the second week of culture; on the contrary, the addition of muscular extract to cultures at 10 days maintained the number of calbindin-expressing neurons. While calbindin-immunoreactive cell bodies grown in culture were small- or medium-sized, no correlation was found between cell size and immunostaining density. At the ultrastructural level, the calbindin immunoreaction was distributed throughout the neuroplasm. These results indicate that the expression of calbindin by sensory neurons grown in vitro may be modulated by horse serum-contained factors or interaction with non-neuronal cells. As distinct from horse serum, muscular extract is able to maintain the expression of calbindin by a subpopulation of DRG cells.

Differentiation of postmitotic neuroblasts into substance P-immunoreactive sensory neurons in dissociated cultures of chick dorsal root ganglion.

Counts performed on dissociated cell cultures of E10 chick embryo dorsal root ganglia (DRG) showed after 4-6 days of culture a pronounced decline of the neuronal population in neuron-enriched cultures and a net gain in the number of ganglion cells in mixed DRG cell cultures (containing both neurons and nonneuronal cells). In the latter case, the increase in the number of neurons was found to depend on NGF and to average 119% in defined medium or 129% in horse serum-supplemented medium after 6 days of culture. The lack of [3H]thymidine incorporation into the neuronal population indicated that the newly formed ganglion cells were not generated by proliferation. On the contrary, the differentiation of postmitotic neuroblasts present in the nonneuronal cell compartment was supported by sequential microphotographs of selected fields taken every hour for 48-55 hr after 3 days of culture. Apparently nonneuronal flat dark cells exhibited morphological changes and gradually evolved into neuronal ovoid and refringent cell bodies with expanding neurites. The ultrastructural organization of these evolving cells corresponded to that of primitive or intermediate neuroblasts. The neuronal nature of these rounding up cell bodies was indeed confirmed by the progressive expression of various neuronal cell markers (150 and 200-kDa neurofilament triplets, neuron specific enolase, and D2/N-CAM). Besides a constant lack of immunoreactivity for tyrosine hydroxylase, somatostatin, parvalbumin, and calbindin-D 28K and a lack of cytoenzymatic activity for carbonic anhydrase, all the newly produced neurons expressed three main phenotypic characteristics: a small cell body, a strong immunoreactivity to MAG, and substance P. Hence, ganglion cells newly differentiated in culture would meet characteristics ascribed to small B sensory neurons and more specifically to a subpopulation of ganglion cells containing substance P-immunoreactive material.