IIN
NTTR
RO
OD
DU
UC
CTTIIO
ON
N TTO
O SSPPA
AC
CEE D
DEEVVEELLO
OPPM
MEEN
NTTA
ALL B
BIIO
OLLO
OG
GYY
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.
If humanity is to transform itself into a multi-planet species it is imperative that
the life cycle, from egg to grave, can proceed in the new environments in which the
life is transplanted. In order to assure this, a great deal of research is necessary and
several questions and issues must be addressed. The two areas of investigation that
are most important to the development of terrestrial life forms off the Earth are gravity
and space radiation. Therefore, research focuses on the influences of variations in
gravity level as well as on the effects of radiation on biological development.
Altered Gravity Environments
The possibility of bringing human and other terrestrial life forms to off-Earth
environments raises the potential for a variety of situations vis-à-vis the gravity
environment and its effects on development. In the near future, exploration class
pre-colonial human missions to destinations such as Mars, the asteroid belt and
Jupiter’s moon Europa may expose astronauts and other terrestrial animals to
periods of several months to a few years of weightlessness aboard the interplanetary
spacecraft, interspersed with bouts of exposure to fractional g levels (perceived
gravity levels between 0 and 1 g) while on the surface of the destination planets or
within rotating intermittent artificial gravity rooms aboard the spacecraft. Because the
disadvantages of rotational artificial gravity increase with the selected gravity level, if
full time artificial gravity is employed by way of a rotating spacecraft, it may be
decided that full Earth gravity is not desirable and some fraction of terrestrial gravity
may be employed aboard the spacecraft in lieu of a full g. Additionally, if humans
colonize Mars, they and the other species that they bring with them will be full-time
inhabitants of an environment in which the ambient g level is approximately 0.38 that
of the terrestrial level in which they evolved.
While a centrifuge large enough to accommodate small mammals will soon fly
aboard the international space station (ISS), allowing for studies of the effects of
fractional g levels on development, studies have thus far addressed the effects of full
weightlessness as well as hypergravity (levels higher than 1 g) in order to gain insight
into the role of gravity and the effects of altering the level of gravity that the organism
perceives.
To date, a variety of studies aimed at providing insight into developmental
gravitational biology have been carried out on a variety of organisms.
To address the issue of human pregnancy in space the influence of the
gravity vector on fetal lay was investigated (Schenker, 1995) and these preliminary
studies suggest that, even in a relatively late stage of human pregnancy, gravity
plays a major role and that some gravitation level will likely be necessary for humans
to successfully populate off-Earth environments. This preliminary indication is
supported by studies involving earlier stages of human reproduction and well as
animal studies focussing on various stages. To begin, it is plausible that
weightlessness could affect the human hypothalmo-pituitary axis enough to
effectively render humans infertile so that pregnancy would not occur in
weightlessness. While it should be noted that these human studies are preliminary
and need to be controlled for factors such as stress and radiation, the general
implication is that a great deal of research is required before serious plans can be
made for space or interplanetary colonization.
Non-human studies can potentially offer a great deal of insight not only into
human development in future colonies but for the requirements connected with
raising multiple generations of animals as part of a closed life-support system during
pre-colonial exploration-class missions.
Various animal models are thus useful for study and are summed up here and
each is described in more detail in subsequent readings.
Drosophila:
Drosophila development has been extensively studied in terrestrial
environments and this species offers the advantage of having a genome that has
been completely sequenced. Therefore, a great deal of information about its
genetics, including developmental mutations is available. Additionally, its genetic
homology to vertebrates, short life cycle, external development, small size, large
number of offspring, large number of indicator lines and mutants, and ability to be
transported in cold storage to the desired environment (ie. altered gravity
environment) offer important advantages to space developmental biology .
C. elegans:
C. elegans development has been extensively studied in terrestrial environments
and this species offers the advantage of having a genome that has been completely
sequenced. A complete map of its development is available with tracking of each cell
division from egg to adult. Like drosophila, it shares extensive homology with
vertebrates at the molecular level, has a short life cycle, is small, develops externally,
can be placed into cold storage and a large data base of mutants is available. Also,
the developmental patterns of several genes are known and GFP-marker lines are
available.
Amphibia:
Like drosophila and C. elegans, amphibia 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, but comparable to
mouse in Xenopus. tropicalis. Amphibia 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. Amphibia
produce durable eggs and embryos in large numbers, fertilization and development
are external (so as in the 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 semitransparent.
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. 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 air breathing amphibian.
Fish:
Most data regarding fish come from zebrafish, but some are from studies
involving other species. As in the case of amphibia, 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 amphibia however,
zebrafish embryos are transparent. Additionally, significant flight data are available
for vestibular system development.
Mouse:
As in the case of amphibia and zebrafish, a genome project is proceeding for
mice, genetic information related to developmental mutations is available, there is a
great deal of homology developmental patterns of several genes are known, indicator
lines are available and they mice a short developmental cycle (21 days gestation)
and short life cycle (4 months). Additionally, since adults are small, habitats take less
space than those of other mammals.
Rat
Rats have a developmental cycle similar to mice and, as in the case of mice,
some flight data are available and a genome project has begun. Additionally, some
well-developed rat models for human disease and pathophysiology are available as
is a significant rat database of maternal fetal behavior.
Avians:
Avians share Extensive homology with mammals at the cell, tissue and
molecular level, developmental patterns of many genes are known, a genome project
is proceeding, some mutants are available, they have short developmental cycles (21
days for chick, 16 days for quail), though with relatively long life cycles, they can be
studied in large numbers and early embryos cane be stored at cool temperatures and
subsequently re-warmed to in order to restart development at desired times. Flight
data exist as well, indicating that there are some sensitive periods during which these
embryos do not do well in the flight environment.
Space Radiation
Least understood among the sources of radiation in space flight is the galactic
cosmic radiation (GCR) and this, along with other forms of radiation presents a
problem for space biology. For an overall discussion on space radiation biology, see
the module on radiation biology. In terms of biological development, space radiation
is a major factor that must be understood in order for humanity to move deeper into
space. Studies involving various animal models have been conducted and it has
been found that during space flight (in which there is exposure to radiation) mutations
occur in drosophila (Ikenaga et. Al. 1997, also see “Genetic effects of HZE and
cosmic radiation”
http://idb.exst.nasda.go.jp/ideadata/04012/199906E04012000/199906E04012000.ht
ml) and that the mutation rate in space for C. elegans is twice or three times as great
as what we would have anticipated on the ground (Hartman). A great deal of
research remains to be carried out in the area of the effects of space radiation on
development.