Amphibians were the first vertebrates to come on land and to resist gravity by supporting their weight. Frogs and toads typically start life in water, and become terrestrial, arboreal, or even remain aquatic after metamorphosis. They have various morphological, physiological, and behavioral adaptations to the diverse environments in which they are found.

Like Drosophila and C. elegans, amphibians have short developmental periods, using only a few days to proceed from fertilization to larva and this is an advantage when studying this period in altered gravitational environments. The life cycle through adulthood, however, is relatively long in Xenopus laevis, an aquatic frog, but comparable to mouse in Xenopus tropicalis (Figure 2-11).

Figure 2-11. Eleven weeks after the egg was laid, a fully developed frog with lungs, legs, and no tail emerges from the water. From bottom to top: After its 21-day development period, the embryo leaves its jelly shell and becomes a tadpole. After about five weeks, the tadpole begins to change. It .starts to grow hind legs. Behind their heads bulges appear their front. Their tails become smaller. Lungs begin to develop, preparing the frog for its life on land.

Amphibians also share homology to mammals at the molecular level and for the mechanisms of tissue induction, developmental patterns of several genes are known and some GFP-marker lines are available. Amphibians produce durable eggs and embryos in large numbers, fertilization and development are external (so as in the case of C. elegans and Drosophila, development can be videotaped), a genome-mapping project has begun and some mutations are available. While embryos are opaque, tadpoles of some species are semi-transparent.

Preliminary flight data have been collected from several amphibian species showing that eggs are stratified based on the gravity vector and that cytoplasmic localization of maternal factors (necessary for formation of the germ line and initial axes) are potentially affected by gravity (see Chapter 5). The unfertilized frog egg appears to be radially symmetric about its animal

vegetal axis. Establishment of bilateral symmetry, dorsal-ventral axis specification, requires a 30-deg rotation of the vegetal yolk mass relative to the egg surface during the first cell cycle. One well-known external influence on frog eggs is gravity. There are observations that eggs of the frog Xenopus laevis tilted 90 deg off-axis during in vitro maturation do not have true radial symmetry (Smith and Neff 1986).

To test whether gravity is required for normal amphibian development, Xenopus laevis females were induced to ovulate on board the orbiting Space Shuttle. Eggs were fertilized in vitro, and although early embryonic stages showed some abnormalities, the embryos were able to regulate and produce nearly normal larvae. These results demonstrate that a vertebrate can ovulate in the virtual absence of gravity and that the eggs can develop to a free-living stage (Souza et al. 1995, Danilchik and Savage 1994). However, recent intriguing work suggests that there are subtle developmental changes in the Xenopus laevis embryos subjected to novel gravitational fields. These changes include the position of the third cleavage plane, the dorsal lip of the blastopore and the size of the head and eyes. The eggs fail to undergo the cortical/cytoplasmic rotation that specifies dorsoventral polarity, and they lack an array of parallel microtubules associated with the rotation (Elinson andPasceri 1989).

Additionally, it has been shown that Xenopus laevis larvae fail to inflate their lungs in a weightless environment suggesting that a complete life cycle in weightlessness would not be possible for such an airbreathing amphibian (Wassersug 2001).

4.2 Fish

Most space data regarding fish come from zebrafish, but some are from studies involving other species. As in the case of amphibians, a genome project is proceeding for zebrafish, genetic information related to developmental mutations is available, there is a great deal of homology to mammals at the molecular level, developmental patterns of several genes are known, indicator lines are being developed, they have a short life cycle, fertilization and development are external and eggs and embryos are hardy and produced in large numbers. As opposed to amphibians however, zebrafish embryos are transparent. Additionally, significant flight data are available for vestibular system development.

The fish Medaka is particularly suited for systematic evaluation of vertebrate development and growth since it is a hardy fish, whose embryos tolerate reduced temperatures well, allowing researchers to subject the embryos to low temperatures and slow embryonic development (Figure 2-12). This provides more time to study each stage of vertebrate development and maximizes the effects of weightlessness on each stage. The Medaka fish are one of the shortest life-cycle vertebrate animals: eggs become sexually mature fish to lay eggs (egg-to-egg) within 3 months. Also, the embryos are optically clear, which allows investigators to visually examine molecular markers and the development of the internal organ systems Fixations of embryos can occur at different stages so that all phases of growth and development can be compared and studied. Rates of development for some key organs, such as the eyes and the heart, can be established before the mission, and molecular probes can be used to establish relationships between specific pattern-regulating genes and the development of specific organs (Crotty et al. 1995). One of the direct molecular genetic studies consists in cloning of the Medaka homeobox-containing gene Hoxa-4. The Hoxa-4 gene is a marker of embryonic development for analyzing the effect of weightlessness stress on embryonic segmentation.

Figure 2-12. In the Summer of 1994, four Japanese killifish (Medaka) flew for 15 days on board the Space Shuttle Columbia, IML-2 STS-65). These fish mated in space for the first time among vertebrate animals (A) and laid eggs (B), which developed normally and hatched as fry (C). Adapted from Ijiri (1995).

Figure 2-12. In the Summer of 1994, four Japanese killifish (Medaka) flew for 15 days on board the Space Shuttle Columbia, IML-2 STS-65). These fish mated in space for the first time among vertebrate animals (A) and laid eggs (B), which developed normally and hatched as fry (C). Adapted from Ijiri (1995).

Pregnancy Guide

Pregnancy Guide

A Beginner's Guide to Healthy Pregnancy. If you suspect, or know, that you are pregnant, we ho pe you have already visited your doctor. Presuming that you have confirmed your suspicions and that this is your first child, or that you wish to take better care of yourself d uring pregnancy than you did during your other pregnancies; you have come to the right place.

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