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The development of embryonic and larval stages of the South African Toad Xenopus laevis D, was investigated in hyper-g up to 5 g (centrifuge), in simulated 0 g (fast-rotating clinostat), in alternating low g, hyper-g (parabolic flights) and in microgravity (Spacelab missions D1, D-2). The selected developmental stages are assumed to be very sensitive to environmental stimuli. The results showed that the developmental reaction processes run normal also in environments different to 1 g and that aberrations in behavior and morphology normalize after return to 1 g. Development, differentiation, and morphology of the gravity perceiving parts of the vestibular system (macula-organs) had not been affected by exposure to different g-levels.

Aboard the German-Spacelab-Mission D-2 the project "Gravity Perception and Neuronal Plasticity (STATEX II)" was performed. STATEX is for STATolith EXperiment. Objects were growing tadpoles of the South African Toad (Xenopus laevis D.) and a juvenile cichlid fish (Oreochromis mossambicus). The results give a broader base for the understanding of how environmental stimuli (e.g. linear accelerations) affect the development and function of the gravity perceiving systems in these two vertebrates. These systems are accepted as models for the human vestibulum. Results of experiments in hyper-g (up to 5 g), simulated weightlessness (Fast-rotating-clinostat) and parabolic flights are compared and discussed.

Cichlid fish larvae were reared from hatching to active free swimming under different gravity conditions: natural environment, increased acceleration in a centrifuge, simulated weightlessness in a clinostat and near weightlessness during space flight. Cytochrome oxidase activity was analyzed semiquantitatively on the ultrastructural level as a marker of regional neuronal activity in a primary, vestibular brainstem nucleus and in gravity receptive epithelia in the inner ear. Our results show, that gravity seems to be positively correlated with cytochrome oxidase activity in the magnocellular nucleus of developing fish brain. In the inner ear the energy metabolism is decreased under microgravity concerning utricle but not saccule. Hypergravity has no effect on cytochrome oxidase activity in sensory inner ear epithelia.

The effect of long-term (10 days) altered gravitational conditions upon succinate dehydrogenase (SDH) reactivity in total brains as well as in individual brain nuclei of developing cichlid fish larvae had been investigated by means of semiquantitative histochemical methods (densitometric grey value analysis). Increasing accelerations from near weightlessness (spaceflight) via 1g controls to 3g hyper gravity (centrifuge) resulted in slightly increasing "all over the brain" (total brain) SDH reactivity. When focusing on distinct neuronal integration centers within the same brains in order to find the anatomical substratum of the gross histochemical data, significant effects of altered gravity only within vestibulum related brain parts were obtained.

Preparing the German Spacelab Mission D-2 project "Gravity Perception and Neuronal Plasticity"--STATEX II--ground based experiments have been performed with larvae of the amphibian vertebrate Xenopus laevis Daud. to study the reactions to different levels of acceleration forces and profiles. The larvae have been exposed to accelerations of up to 5 g for different time periods using a modified laboratory centrifuge and the NIZEMI (Niedergeschwindigkeits-Zentrifugen-Mikroskop) which allows direct observation and video documentation. The results will be discussed and compared with those of the D1-Mission, parabolic flights, and simulated weightlessness.

Aquatic animals have almost no body weight related proprioception for spatial orientation. Xenopus larvae, like fish, maintain their attitude in water by continuous correction with their fin(s). For these reasons a special performance of the equilibrium system compared to terrestrial animals is necessary. Evidently fish therefore have more compact (dense) otoliths; Xenopus larvae have less dense otolith (membranes) similar to land vertebrates; but their sacculus-otoliths are vertically positioned, which also may lead to a higher g-sensitivity. For plausibility reasons gravity should influence the embryonic development of gravity receptors. Yet, evaluations of photographs taken from the surface of cut deep-frozen objects by incident light show no aberration of the shape of the whole vestibulum and of the shape, density, size and position of the otolith membrane in larvae developed under near-zero g (NEXPA-BW-STATEX in D-1-Mission). The further evaluation of the "weightless-larvae" revealed a probably not yet described statolith-like formation in the dorsal wall of the vestibulum. In the weightless larvae this formation outnumbers, also qualitatively, strongly the l-g controls. An extra result is the lack of striking effects of cosmic radiation on the embryonic development of the flown Xenopus eggs. The swimming behavior of the larvae which was observed about one hour after landing of the Space Shuttle showed a typical anomaly (loop swimming), which is known from larvae developed on the clinostat or from fish flown aboard Apollo capsules.

The vestibular apparatus of tadpoles (Rana temporaria) exposed to simulated weightlessness was examined by electron microscopy. Extended exposure to simulated weightlessness is followed by significant alterations in the sensory epithelia and also in the otolith membrane. Large vacuoles, filled with necrobiotic mitochondria and fragments of endoplasmic reticulum, were concentrated in the region where an otolith membrane covers the hair cells but were mostly absent in zones of the epithelia with undifferentiated cells. The number of otoconia in the otolith membrane was diminished. The results were compared with data from space flight experiments and some concordance was noted. The possible connection between some unusual behavior of the tadpoles after weightlessness simulation and the structural alterations in the gravitational sensors was discussed.

The paper describes an investigation of the influence of gravity on the early differentiation of gravity receptors in Xenopus embryos and larvae. There is evidence that the expression of crystals in the saccus endolymphaticus was statistically greater when the embryos developed in near weightlessness (hypogravity) than on earth. The function of these crystals is unknown but they may contribute to the functioning of the vestibular apparatus.