Fertilization And Embryonic Development

Survival of organisms over the generations depends on successful reproduction. This process is critical for plants and animals including humans. Its aim is the production of viable progeny. In organisms affected by altered gravity, reproduction is not a self-evident fact, as shown by the following two examples:

a. Male rats mated 5 days after flight to non-space experienced females produced offspring with growth retardation and many abnormalities such as hydrocephaly, ectopic (out of place) kidneys, and enlargement of the bladder. Mating 2.5 to 3 months after the spaceflight produced healthy and viable offspring. Mating in hypergravity at 2 g revealed a reduction of pregnancy, while no successful pregnancy was reported after mating during exposure to 4.7 g (Tou et al. 2002).

b. Mating activity of orbited males of the parasitic wasp Habrobraeon was severely disrupted, hatchability decreased, and fecundity of eggs was enhanced (von Borstel et al. 1970).

Figure 5-02. This cartoon has been published in the New York Times (24 July 1994) during the IML-2 Spacelab mission. This flight carried out an experiment entitled "Mating behavior of the Medaka fish and development of their eggs in space ". Source Ijiri (1995).

Fertilization might also be affected by mobility changes in sperm. In fact, it is known that bull sperm swim with higher velocity in microgravity. This increased velocity is coupled to changes in phosphorilization of specific flagellar proteins (Tash and Bracho 1999).

Altered gravity changes mammalian male and female reproductive systems in a rather complex manner. For example, a transient but dramatic reduction in testis weight and testosterone has been reported in male rats.

However, the pituitary2 responded in a physiological manner to changes in plasma testosterone, indicating that the hypothalamic-pituitary-gonadal axis was not impaired by spaceflight. So, spermatogenesis was not reduced. Examination of the ovaries of postpartum3 rats flown in space during 9 to 20 days of gestation showed no effect on ovarian weight or number of preovulatory or atretic follicles4 (Tou et al. 2002). Observations about hormonal changes revealed reduced pituitary Luteinizing Hormone (LH), but no change in plasma LH. Pituitary Follicle Stimulating Hormone (FSH) was not affected while plasma FSH was elevated (Burden et al. 1997). Thus, the physiological mechanisms for reproduction are obviously intact in microgravity, despite of modifications of some components of the complete system.

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