In vertebrates, only few experiments have been performed in microgravity to study the embryonic development from fertilization. To date, these concern only amphibian and fish. We report here a study on the embryonic development of Pleurodeles waltl (urodele amphibian) eggs oviposited in microgravity. The experiment was performed twice on board the Mir space station and the data obtained included video recording and morphological, histological and immunocytological analyses. The data confirm that the microgravity conditions have effects during the embryonic period, particularly during cleavage and neurulation, inducing irregular segmentation and abnormal closure of the neural tube. Moreover, we observed several abnormalities hither to undescribed corresponding to cortical cytoplasm movements, a decrease of cell adhesion and a loss of cells. These abnormalities were temporary and subsequently reversible. The young larvae that hatched during the flight displayed normal morphology and swimming behavior after landing. The results obtained in the urodele Pleurodeles waltl are in accordance with those observed earlier in the anuran Xenopus laevis and in the fish Oryzias latipes.
A major issue in radiation and space biology is whether gene expression levels are altered in cells exposed to gravity-changing stress. In the present study, genes up- or down-regulated in radiation-sensitive human RSa cells cultured under gravity-changing conditions, were identified using a PCR-based mRNA differential display method. Exposure of cells to gravity-changing stress was performed by free-fall with a drop-shaft facility or by an airplane-conducted parabolic flight. Among the candidates for gravity-changing stress-responsive genes obtained by the differential display analysis, the lactate dehydrogenase A gene (LDH-A) was confirmed by Northern blotting analysis to exhibit increased expression levels. The gravity-changing stress consisted of a combination of microgravity and hypergravity. However, exposure of the cells to hypergravity produced by centrifuge only slightly affected the LDH-A mRNA expression. Thus, LDH-A was found to be a candidate for the genes which play a role in the cellular response to gravity-changing stress, and mainly to microgravity.
The upside-down swimming catfish (Synodontis nigriventris) has unique behavior, i.e., it frequently shows a stable upside-down posture during swimming and resting. To examine whether the unique postural control in S. nigriventris results from the characteristics of the vestibular organ, we observed the morphological aspects of the otolith and the orientation of sensory hair cells in the utricle. Soft X-ray densitometry analysis showed that the transmittance of soft X-rays in the otolith of S. nigriventris was higher than that in a closely related species (Synodontis multipunctatus) belonging to Synodontis family, goldfish (Carassius auratus) or miniature catfish (Corydoras paleatus) which shows upside-up swimming. The higher transmittance of soft X-rays suggests that the density of the otolith in S. nigriventris is lower than that in S. multipunctatus, C. auratus or C. paleatus. It is possible that the low density of the otolith may have a relation to the control of the unique upside-down posture of S. nigriventris. The hair cells in S. nigriventris were present at the ventral to ventro-lateral site of the utricular epithelium, forming a single hair cell layer as in the other 3 species of fish. The orientation of the sensory hair cells does not appear to cause the unique postural control.