Space Developmental Biology- Review Reading
Eran Schenker, MD* and David M. Warmflash, M.D.#
*Israel Aerospace Medicine Institute and, Department of Obstetric & Gynecology, Hadassah University Hospitals Mt. Scopus Jerusalem Israel; #NASA Johnson Space Center, Houston, TX, 77058.
Introduction
The space environment impacts most of the systems of all species. The data concerning the effects of weightlessness on the reproductive function in most species and particularly mammalian are still limited and controversial. Human long duration space flight on the international space station (ISS), colonization of the Moon, exploration of Mars and other new space frontiers will ultimately depend on ability of plants, animals and humans to function and reproduce in the space environment.
It is commonly believed that some species make use of Earth's gravitational field as a positioning cue during early embryogenesis. Until now, there have been no studies that have looked at the entire process of reproduction in any animal species in space environment.
This type of investigation will be critical in understanding and preventing the problems that may affect reproduction in space. New approaches to experimental investigation in this area are needed. It is important to emphasize the need to study fertilization and the early stages of development during space flight, as understanding these processes should be the first step in understanding the entire process of reproduction. It is important to determine whether or not embryos can develop normally without gravitational cues, to determine if any stages of early embryogenesis are adversely affected by weightlessness and at which point the affected stages regulate back to producing normal embryos.
On 4 October 1957, the Soviets launched Sputnik 1 into Earth orbit. This heralded the beginning of the space age. A month later, the Soviets launched Sputnik 2, which carried a female dog named Laika. The cabin contained systems for air regeneration and thermal regulation. The correct functioning of the dog’s life support system (LSS) was monitored from Earth by telemetry. Vital signs such as heart rate, respiration, and blood pressure were also monitored throughout the 1-week flight, until the dog died from lack of oxygen. This flight provided the first evidence that a mammal, fairly similar physiologically to a human, could withstand the rigors of launch and remain alive in space.
Thereafter, a series of five Korabl Sputnik flights served as precursors to the first human mission. On 10 August 1960 Sputnik-S was launched, which carried two dogs, two rats, forty mice and a number of flies. This was the first time that living beings had been sent into space and subsequently returned safely to Earth. On 12 April 1961, 27 year-old Senior Lieutenant Yuri Gagarin completed the first human spaceflight in the spacecraft Vostok-l. His mission lasted only 108 minutes.
Although the Sputnik flights had indicated that normal bodily functions could be maintained during spaceflight, scientists were still unclear about the various effects of weightlessness upon the body. Consequently, Gagarin was very heavily instrumented, and medical monitoring included continuous measurement of heart rate, respiration, electroencephalogram (EEG), electrooculogram (EOG) electromyogram (EHG), electrocardiogram (ECG), thermography, and galvanic skin response. Gagarin wore a pressurized suit throughout the flight. He ejected from his capsule at an altitude of 22,000 feet to complete the final stages of his descent by parachute. His flight proved to the world that man could be sent into space and then returned safely to Earth.
The American program, initially called HISS and subsequently changed to Project Mercury, had in the meantime developed the capability to repeat Gagarin’s feat. On 5 May 1961, Alan Shepard completed a suborbital flight in the spacecraft Mercury MR-3, which lasted only 15 minutes.
From here onwards the duration of spaceflights have progressed steadily to those lasting more than 365 days. Today there are several Cosmonauts who have individually accumulated more than 500 man-days in space. Mankind’s insatiable curiosity and quest for knowledge will continue to grow. This quest will be carried out by future space explorers who will embark upon routine missions lasting several years.
Travel to and colonization of other habitable planets will eventually become inevitable. It is obvious that mankind’s exploration and colonization of the frontier of space will ultimately depend on men’s and women’s ability to live, work and reproduce in the space environment.
Much knowledge has been gained in the field of Space Medicine since the Vostok and Mercury flights. Weightlessness, radiation, altered pressures and breathing partial pressures, potential toxicological exposure, and exposure to harmful electromagnetic fields are some of the factors of the space environment that may limit our ability to reproduce. The various effects upon reproduction by these factors have been studied in animals, from microorganisms to mammals. Due to current operational constraints data on actual human reproductive physiology in the space environment are limited.
A 26-year-old ex-factory worker named Valentina Tereshkova was the first woman in space. On 16 June 1963 she piloted the Vostok 6 mission which lasted 70 hrs 50 min.. Later that year in the month of November she married Cosmonaut Andrian Nikolayev, the pilot of the 94hr 22min Vostok 3 mission. A year later Cosmonaut Valentina Tereshkova gave birth to a healthy baby girl. This provides us with the first evidence of post-flight normal pregnancy; at least as far as short duration space flights are concerned.
Currently, several women are flying as Mission/Payload specialists or pilots on board Space Shuttle flights. This includes.
Margaret Rhea Seddon (who first flew on the SLS-1 mission) is married to Navy Capt. Robert L. (“Hoot”) Gibson, the pilot commander of several space missions (STS 6-C and STS 27 of January 1986 and December 1988 respectively) including the 50th U.S. space shuttle mission, Spacelab J (12, 71). This mission included an astronaut couple, Dr. N. Jan Davis (37) and Lt. Col. Mark Lee (39) who are the first married couple to fly in space together.
It is noteworthy to add that like Valentina Tereshkova, Rhea Seddon conceived and gave birth to a healthy baby after being in space, which suggests that short duration space missions apparently cause no permanent harmful effects to the reproductive system in human beings. Despite all this, most of the knowledge gained so far has been implied from animal experiments.
Human gamete production, menstrual cycles, fertilization, fetal development, pregnancy and delivery are similar in many ways to those of other animals. So far, it has been impossible to study long term effects on human beings due to the present time limitations of space missions. Related issues, such as sexual intercourse and contraception, need to be evaluated as well. Key areas in which there will be limitations to men’s and women’s reproductive ability need to be identified. In the subsequent chapters of this review reading suitable recommendations with a view to future survivability of mankind as a species will be discussed.
There is little doubt that gravity provides a potential constraint on virtually all phases of reproduction. Similarly, shielding of radiation by the atmosphere prevents the occurrence of lethal genetic mutations on Earth. Even simple organisms such as the slime mold Dictyostelium display genetically distinct specified adaptations to gravity. Their stalks synthesize an extracellular matrix that allows the organism to exhibit a negative geotrophic growth pattern, which facilitates spore dispersal. Pollard speculated in 1965 that exposure to zero gravity might significantly impact nuclear and cellular division in both plants and animals.
Medaka Fish as a Model
The Life and Microgravity Spacelab (LMS) Space Tissue Loss-B (STL-B) hardware was developed to test the hypothesis that gravity is required for normal embryo development. Investigators are conducting systematic evaluation of vertebrate development and growth using the fish Medaka as a model (Crotty, DA).
The Medaka is particularly suited to this experiment 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. This provides more time to study each stage of vertebrate development and maximizes the effects of weightlessness on each stage. Also, the embryos are optically clear, which allows investigators to visually examine molecular markers and the development of the internal organ systems.
The STL-B hardware has flown experiments on the Shuttle on two separate occasions. The first, STS-59, was considered a hardware flight test. In addition, good video images of the developing embryos during space flight were obtained. The second flight, STS-70, provided the opportunity for histological examinations that the effects weightlessness might have on the development of the Medaka. These analyses are currently underway (Crotty, DA).
One of the direct molecular genetic studies was cloning of the Medaka homeobox-containing gene Hoxa-4. The Hoxa-4 gene as a marker of embryonic development for analyzing the effect of weightlessness stress on embryonic segmentation. The next series of experiments, in the near future on Medaka, will examine the Medaka Hoxa-4 gene which have showed several sequences conserved in mouse and chicken, suggesting a role in the regulation of Hoxa-4 pattern of expression (Wolgemuth, DJ).
Sea Urchins
During 1996, sea urchins were flown aboard a KC-135 and fertilization rates equivalent to those on Earth have been obtained in the first 24 seconds of weightlessness although sperm movement was somewhat slower (Chakrabarti, A).
Previous work has shown that ultraviolet (UV) space irradiation of fertilized frog eggs yields embryos that lack dorsal and anterior structures. 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, RP).
Drosophila Melanogaster
The results obtained during the last successful flight of the Challenger Shuttle, in early November 1985, indicate that oogenesis and embryonic development of Drosophila melanogaster are altered in the absence of gravity. Two hundred forty females and ninety males, wild type Oregon R Drosophila melanogaster flies were flown in the Space Shuttle. The results showed an increase in oocyte production and size, a significant decrease in the number of larvae hatched from the embryonic cuticles in weightlessness and alterations in the deposition of yolk (Vernos, I).
In connection with these results, at least 25% of the living embryos recovered from space failed to develop into adults. Studies of the larval cuticles and those of the late embryos indicate the existence of alterations in the anterior, head and thoracic regions of the animals. There was a delay in the development into adults of the embryos and larvae that had been subjected to weightlessness and recovered from the Space Shuttle at the end of the flight.
No significant accumulation of lethal mutations in any of the experimental conditions was detected as measured through the male to female ratio in the descendant generation. It seems that even Drosophila melanogaster flies are able to sense and respond to the absence of gravity, changing several developmental processes even during very short space flights. The results suggest that weightlessness interferes with the distribution and/or deposition of the maternal components involved in the specification of the anterioposterior axis of the embryo.
Xenopus Fertilized Eggs
The cytoplasm of Xenopus fertilized eggs appears to be organized into three major compartments based primarily on the uneven distribution of yolk platelets. There is a shift of these yolk compartments during the first cell cycle that is thought to be involved in the dorsal/ventral morphogenesis of the embryo. The involvement of gravity in Xenopus cytoplasmic organization and in compartment shifts was studied by Smith, RC.
The cytoplasmic organization into yolk compartments was found to be maintained, and the asymmetric movements of compartments still occurred in eggs that developed on the clinostat. Smith, RC suggests that the organization of Xenopus egg cytoplasm into discrete compartments relies on forces other than those involving gravity and that the compartment shifts that take place during the first cell cycle are active movements (Smith, RC).
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.
Huang, S. investigated the early changes in development caused by gravitational alterations at the cellular and molecular levels. Huang tried to define the periods and durations of exposure from which embryos can recover and this has lead to studies of which types of exposure embryos cannot tolerate. Defining this critical developmental window will contribute to NASA's research goals by providing basic information important to raising animals in space (Huang, S).
The unfertilized frog egg appears to be radially symmetric about its animalvegetal (AV) axis. Establishment of bilateral symmetry, dorsalventral (DV) axis specification, requires a 30 degree rotation of the vegetal yolk mass relative to the egg surface during the first cell cycle.
One wellknown external influence on frog eggs is gravity. When fertilized eggs are tilted from their usual orientation, gravitydriven internal rearrangements result in rotation directions different from those specified by the SEP. Endogenous cues may also be present in the unfertilized egg. For example, parthenogenetically activated eggs exhibit a normal rotation, even though they have not been fertilized. There are observations that eggs of the frog Xenopus laevis tilted 90 degrees off-axis during in vitro maturation do not have true radial symmetry.
There are at least two major controllers of development in the early stages of embryonic development that are of concern: gravity and the primitive steak. Gravity, probably acting through the distribution of yolk and its components, lays down the initial plans for polarity. The primitive streak, controls the orderly ingressions of the cells and imposes a pattern on the developing tissues (Bellairs, R).
To test whether gravity is required for normal amphibian development, Xenopus laevis females were induced to ovulate aboard the orbiting Space Shuttle (Souza, KA). 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, KA). On the Spacelab J mission the results where the same: “normal development can proceed in weightlessness” (Danilchik, MV).
Avian Development
Preliminary results indicate no adverse effects of vibration and g force (launch profile of the shuttle) on avian development. Only five quail embryos have survived weightlessness and only one of these embryos survived to the latter stages of development (15 days of incubation). Results of flight experiments indicate that gravity may be needed during the earliest stages of avian embryogenesis, but is not important for the latter stages of development.
An egg incubator within a centrifuge in space can allow us to determine if lack of gravity is the reason for the death of young avian embryos in space, as has occurred on one of the previous studies. The results indicate that chick embryos are more adversely affected by lack of turning than quail eggs and that they need to be turned at least 4 times daily or more for improved rate of hatch (Hester, PY).
On future space flights there are three objectives designed to determine the effect of weightlessness on embryonic development initiated after the launch; the fecundity of adult quail during orbit and the assessment of their hormones and reproductive tissues after orbit; and the regeneration potential of quail in weightlessness based on primordial germ cell migration and differentiation, gametogenesis, ovulation, fertilization, embryonic development, and hatching (Bernard, CW).
These experiments will provide substantial basic information about the effects of weightlessness on embryonic differentiation and development, as well as important information about adult quail endocrinology and physiology. Many aspects of the proposed work will focus on reproduction since this is the only path for successful animal bioregeneration in space.
On the Mir-19 flight there was a demonstrated adequate procedure for the delivery of fertile egg to the Mir Space Lab. Crew tasking for incubator monitoring, embryo fixation and return of the fixed eggs by either Soyuz or Shuttle was shown to be practical.
Additionally ground experiments are being conducted to determine at 1 g the effect of no turning verses turning eggs 1800 every four hours from embryogenesis through to hatching.
The results of the space flight consisted of early embryonic death with only two embryos living to the 16th day of incubation. Interpretations of the results were made more difficult by the fact the synchronous control showed a similar lack of viability. Retrospective analysis of onboard flight recording data suggests that the incubator temperature control malfunctioned and the eggs were being incubated at 42 C instead of the programmed 37.5 C.