Effect of Weightlessness on Development of Amphibian Eggs

Developmental biology

Xenopus laevis (frog)
Flight: 4 mothers, 70 tadpoles Female

Synchronous: 65 tadpoles

Ambient Temperature Recorder, Dissecting Microscope, Frog Embryology Unit and Kits, General Purpose Work Station, Refrigerator/Incubator Module

On Earth the animal-vegetal axis of amphibian eggs will rotate upon fertilization to align with the gravitational field. This rotation plays a role in determining the polarity of the embryonic axis and may also influence normal development. The objective of this experiment was to determine whether gravity is required for normal embryonic development.

Frogs were injected subcutaneously with human chorionic gonadotropin 18 hours into flight. Eggs were collected and fertilized with a sperm suspension prepared prior to flight and refrigerated until use inflight. The eggs were inserted into chambers filled with dilute Ringer's solution. Half were incubated in the FEU centrifuge at 1 G and half were incubated in the FEU at microgravity. Some embryos were fixed inflight and sectioned and examined postflight. Approximately 50 live embryos were received by the laboratory within 3.5 hours of landing and staged. Normality was assessed by several parameters, including visual observation of the intact embryo and histomorphometry. Optomotor behavior of the tadpoles was determined based on their tendency to track a moving stimulus.

Embryos at the two-cell stage showed a cleavage furrow in the normal position for both groups. There were no gross abnormalities in gastrulae, but embryos developing in microgravity had thicker blastocoel roofs. In addition, the blastopore lip formed at a slightly more vegetal latitude in the microgravity group than the 1 G group. Development to the neurula stage was unimpaired. All fixed neurula and tadpoles appeared normal. Live tadpoles, raised at microgravity and fixed shortly after landing, were found to have uninflated lungs. Although there were air bubbles in the egg chambers, the tadpoles were apparently unable to find the air-water interface and inflate their lungs. This result indicates that arrested lung development would have prevented the tadpoles from complete growth and metamorphosis in the absence of gravity. Flight tadpoles had stronger optomotor responses than control tadpoles. Because there were no gravitational clues, the tadpoles may have compensated with visual information, thus increasing the strength of their optomotor response. This difference disappeared by 9 days postflight.

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