Vertebrate development in the environment of space: models, mechanisms, and use of the medaka.

With the advent of space travel, it is of immediate interest and importance to study the effects of exposure to various aspects of the altered environment of space, including microgravity, on Earth-based life forms. Initial studies of space travel have focused primarily on the short-term effects of radiation and microgravity on adult organisms. However, with the potential for increased lengths of time in space, it is critical to now address the effects of space on all phases of an organism's life cycle, from embryogenesis to post-natal development to reproduction. It is already possible for certain species to undergo multiple generations within the confines of the Mir Space Station. The possibility now exists for scientists to consider the consequences of even potentially subtle defects in development through multiple phases of an organism's life cycle, or even through multiple generations. In this discussion, we highlight a few of the salient observations on the effects of the space environment on vertebrate development and reproductive function. We discuss some of the many unanswered questions, in particular, in the context of the choice of appropriate models in which to address these questions, as well as an assessment of the availability of hardware already existing or under development which would be useful in addressing these questions.

Gravitational effects on the rearrangement of cytoplasmic components during axial formation in amphibian development.

The spatial positioning of the dorsal-ventral axis in the amphibian, Xenopus laevis, can be experimentally manipulated either by tipping the embryo relative to Earth's gravitational force vector or by centrifugation. Experimental evidence suggests that certain cytoplasmic components are redistributed during the first cell cycle and that these components are, in part, responsible for the establishment of this axis. Further studies indicate that at least some of the cytoplasmic components responsible for establishing this axis may be RNA. Recombinant cDNA and PCR technology are utilized to isolate DNA clones for messenger RNA which becomes spatially localized to the dorsal side of the embryo. These clones are being used to study the mechanisms of spatial localization and the function of the localized RNA transcripts.

Relocation of mitochondria to the prospective dorsal marginal zone during Xenopus embryogenesis.

Dorsal-ventral axis formation in Xenopus laevis begins with a cytoplasmic rotation during the first cell cycle and culminates in a series of cell interactions and movements during gastrulation and neurulation that lead to the formation of dorsal-anterior structures. Evidence reported here indicates that mitochondria are differentially redistributed along the prospective dorsal-ventral axis as a consequence of the cortical-cytoplasmic rotation during the first cell cycle. This finding reinvigorates a possibility that has been considered for many years: asymmetries in cytoplasmic components and metabolic activities contribute to the development of morphological asymmetries.