INVITED LECTURES

Combustion synthesis under microgravity conditions

Giacomo Cao
Dipartimento di Ingegneria Chimica e Materiali, Centro Studi sulle Reazioni Autopropaganti (CESRA) and Unità di Ricerca del Consorzio Interuniversitario Nazionale di Scienza e Tecnologia dei Materiali, Università di Cagliari, Piazza d'Armi, 09123 Cagliari, Italy

A combustion synthesis process is characterized by exothermic reactions in solid-solid and solid-gas systems, which are utilized to produce solid products. External heat sources allow one to heat the system up to a temperature at which the reaction starts and the subsequent sudden increase in temperature is brought about the substantial generation of heat of reaction only.
The combustion synthesis can be obtained either under the thermal explosion mode or under the self-propagating mode for which the acronym SHS (Self-Propagating High-Temperature Synthesis) has been coined (cf. Merzhanov and Borovinskaya, 1972). In the former one, the reaction takes place spontaneously everywhere in the reactive mixture once the ignition temperature is reached. In the latter mode, only localized zones of the system are preheated until the high-temperature front starts propagating through the sample, thus converting reactants into products, at a velocity, which varies from 0.1 to 25 cm/s. In the present work our attention is confined to the SHS mode, which is advantageous with solid-gas and solid-solid systems characterized by highly exothermic reactions. Over 600 compounds have been synthesized using the SHS technique. The exploited reactions range from simple elementary reactions, to more complex thermite-like reactions and have also included the synthesis of high-temperature superconducting materials. The main process parameters can be listed as particle size, use of diluents or inert reactants, green density and preheating of the reactants. Each of these parameters will influence the exothermicity of the reaction and therefore the combustion and ignition temperatures, thus affecting the microstructure and morphology of the products and the resulting properties of the synthesized materials.
SHS reactions and related structure formation mechanisms involve several stages, i.e. melting of reactants and products at, or ahead, of the reaction front, spreading of the melt, generation of gaseous species, droplet coalescence, diffusion and convection, buoyancy of solid particles and densification of liquid products, most of which are affected by gravity.
A summary of the relevant microgravity research in SHS will be given. The experimental results, which may be classified into two main groups, i.e. the synthesis of highly porous materials and the influence of gravity on the microstructure of final products, clearly demonstrate that gravity may significantly affect the process of structure formation during SHS. In particular, it was shown that products with finer and more uniform microstructure are obtained under low gravity conditions with respect to normal ones.

GRAVITY-DEPENDENT SKELETAL MUSCLE PLASTICITY IN MAMMALS: FROM STRUCTURAL PHENOMENA TO MECHANISMS

B.S. Shenkman, I.N. Belozerova, T.L. Nemirovskaya
Laboratory of Myology SRC Institute for Biomedical Problems, RAS
76A Khoroshevskoye Shosse, 12 3007 Moscow, Russia

Usually the muscle disuse (in other words, diminishing of its mechanical activities) is considered to be the most important cause for the numerous alterations in the structure and function of skeletal muscles under microgravity.
But less is known about the physiological nature of this disuse, differences in responses of tonic and phasic muscle systems and its functional and structural consequences in the time-course of the adaptation of the motor system to the microgravity conditions. According to our data during the first weeks of weightlessness the significant decline of the voluntary strength-velocity characteristics of ankle and knee extensors was accompanied with the profound drop of muscle stiffness and some structural changes of the muscle tissue. Some signs of muscle fiber atrophy became evident after 3 days of dry immersion. We revealed in these studies other structural changes in muscle as well (such as tendency to capillary loss, myofibrillar damages etc). At the same time these changes are much less profound than changes of functional characteristics during this functional period.
Later, during the next stage, the fiber atrophy in tonic and phasic muscles reaches significant values, but the levels of ST and FT-fiber atrophy in the same muscle remain closely similar. The atrophy development is accompanied with the capillary loss and decrease of muscle oxidative enzyme activities, fast myosin expression in slow-twitch fibers. The strength characteristics continue to decrease, the decreased endurance and increased physiological cost of the standard submaximal exercise are observed. We call this phase the stage of general atrophy.
During the next stage the atrophy development becomes the fiber-type-specific. According to our data, during this distant period, we see the predominant atrophy of ST-fibers in m.soleus, and FT fibers in m.vastus lateralis. The capillary number, the mitochondrial volume density and the physiological cost of the submaximal exercise are stabilized during this period. But the «substitution» of the fiber volume by the non-contractile elements becomes more profound during this stabilization stage. By the end of this stage, the muscle structural features seemed to meet the functional requirements at the diminished level of its contractile activities.
Evidently the main characteristics of muscle plasticity at each of these stages are determined by the action of some biomechanical triggers. The faster development of the atrophic processes under conditions of dry immersion as compared to head-down tilt bedrest and prevention from atrophy by means of

artificial support application allows to suppose that support deprivation may play a trigger role in the atrophy development in weightlessness. It is believed that the support stimulus on-ground maintaining the activity of postural muscles which is evidently necessary for maintaining in turn the volume of the fiber myofibrillar apparatus and consequently their size. The support deprivation leads to inactivation of the tonic system followed by the fiber atrophy development. The hypothetical cellular mechanisms involved in hypogravity-induced muscle fiber atrophy are worth discussing. It is supposed that documented loss of muscle stiffness may lead to a slack state of the sarcolemma and inhibition of the dystrophin cytoskeleton turnover, since even the short exposure to hypergravity induced the alterations of the dystrophin layer integrity. Dystrophin cytoskeleton changes are thought to be associated with the level of anabolic processes.
Thus in microgravity the changes in muscle contractile activities, biomechanically triggered, are thought to be followed by numerous alterations in muscle fiber elastic properties and by changes in intracellular signal transduction leading to changes in muscle structure and metabolic characteristics.

Transdifferentiation or the reversal of the differentiated state

Volker Schmid
University of Basel, Institute of Zoology, Biocenter/Pharmacenter
Klingelbergstrasse 50, CH- 4056 Basel, Switzerland

Transdifferentiation is defined as a reprogramming of differentiated cells. In most cases transdifferentiation occurs in regeneration, however, more recent reports indicate that it can also be part of ontogeny. The cloning experiments with sheep and other animals have demonstrated that nuclei of differentiated cells can be reprogrammed to follow any fate. Whereas in these experiments nuclei of differentiated cells are reprogrammed by the cytoplasm of the recipient egg, reprogramming in transdifferentiation occurs by cell external cues and the nucleus remains in its own cytoplasm. Almost unlimited is the transdifferentiation potential of isolated striated muscle of jellyfish where even functional organs can be build in-vitro autonomously from striated muscle tissue. In vertebrates, the potential is reduced. In some animals the lens of the eye can be formed from pigment cells of the iris and when new (amphibia) regenerate arms or legs, dedifferentiated striated muscle cells can contribute to form new cell types of the regenerate.
Although the molecular control of the reprogramming progress is still very little understood, recent progress with myocyte cultures demonstrate that the differentiated state can be reversed and commitment to new cell fates opened when specific genes are activated in the myocytes. This success with fully differentiated cells indicates that the still ongoing debate on ethical issues in connection with stem cells and embryos for organ cultures can be circumvented in the future by using the unlimited source of differentiated cells.


Recent Japanese Progress in Thermocapillary Flow in Liquid Bridge

Hisao Azuma
Osaka Prefecture University, Gakuen-cho, Sakai, J-593 Osaka, Japan

NASDA is developing the Fluid Physics Experiment Facility (FPEF) and the Advanced Furnace for microgravity Experiment with X-ray radiography (AFEX) which will be on-board the JEM of the ISS. For its effective and advanced use of them, Japan is concentrating efforts, with many Japanese researchers, on research themes related to these facilities. Japan has been conducting a systematic study on Marangoni flow of low to high Pr number to understand the transition behaviour from the steady to the oscillatory flow. It is performed by combining experiments and numerical simulations which are based on some models. The recent progress in this field in Japan is summarized and introduced, as well as other interesting research such as experimental work using melted Silicon and a drop tower.
I owe this report in great part to Dr. Yoda of NASDA


Activities of the Laboratoire Arago: Observation, Research, Teaching

Gilles Bœuf
University Pierre et Marie Curie/CNRS, Laboratoire Arago, BP 44,
66651 Banyuls-sur-mer cédex, France

The Laboratoire Arago (LA) is a pretty old marine station, created in 1882, at the extreme South of France, on the Mediterranean coast, close the boundary with Spain. It is under the administrative and scientific supervision of the University Pierre et Marie Curie (Paris 6, P6) and the CNRS (Centre National de la Recherche Scientifique). Presently, it is constituted by two Unités Mixtes de Recherche (UMR), the first one "Biological Oceanography" (P6/CNRS Universe Sciences) and the second one "Models in cellular Biology and Evolution" (P6/CNRS Life Sciences). One Unité Mixte de Service (UMS P6/CNRS Universe Sciences) allows the organisation of the common facilities (administration, library, lecture room, lessons rooms, hotel, restaurant, shop, study and public aquariums, diving center, ships, oceanographical observation team, informatics, electronic microscopy, security…). Today 120 people work in the LA in Banyuls (60 CNRS, 44 P6, 16 doc and post-doc).
The LA has three activities:
(1) Oceanographical Observation: the LA is one of the 10 French Universe Sciences Observatories, 4 being specifically dedicated to Oceanography (Banyuls, Marseille, Roscoff and Villefranche). Oceanographical observation is an old activity in Banyuls with the Système d'Observation du Laboratoire Arago (Sola), a fixed point at the sea (Bay of Banyuls), checked every week by our ship and where are permanently recorded environmental oceanographical data. Then they are used by technicians and scientists of the oceanographical team. They also are available for the public (on the web).
(2) Research: the scientists of the first team "Biological Oceanography" develop their studies on both benthic and pelagic oceanography. Studies are carried out in environmental conditions in using ships and diving facilities and in experimental conditions in the laboratory. Process and biological production are analysed at the interface water-sediment and specific species studied (i.e. Ditrupa arietina). Relationships with climatic and human influences are detected. Deep-sea fauna is experimented from the hydrothermal effluents (symbiosis mechanisms). Bacterial and phyto-planktons are studied and applied works developed ("cleaning" power of seawater, pollution, climatic influences, toxic seaweeds…). Cytometry is developed and molecular approaches used.
In the second team "Models in Cellular Biology and Evolution", are studied different topics. Today, an important role of the marine stations is to develop and to demonstrate new models for basic biology. Major works in Banyuls are dedicated to the regulation of the cellular cycle (in starfish and sea urchins, ascidians, microseaweeds, …), cancerogenesis, environmental control of development and growth in fish, including chronobiology. Ostreococcus tauri, nano phytoplanctonik species, is an important model (the smallest free eukaryotik cell known). Speciation and Evolution mechanisms are studied in Echinoderms of both the Bay of Banyuls and the Antarctic area. Slight activities are dedicated to spatial biology (low gravity and early embryogenesis in echinoderms) and to terrestrial fauna, micro-mammals and birds (influence of Mediterranean forest fires and parasitism).
(3) Teaching: 11 assistants and professors work in the LA and 1100 students come every year in the station. They receive lectures and practical formation in marine biology, oceanography, ecology and physiology of marine organisms, developmental biology, Mediterranean ecology and systematics of Mediterranean macro-seaweeds.