The avian embryo responding to microgravity of space flight

Central questions and paradigms of developmental biology have in recent decades evolved to stress gene regulation. (See Moore, 1987 for an excellent review). However, during fertilization and the pre-embryonic cleavage stages, cells of identical genome are responding to multiple microenvironmental factors, e.g., pH, spatial position, temperature, osmotic and barometric pressure, light, electrical field charge, and the extracellular concentration of substrates, cellular byproducts, gases, and electrolytes. These cells commonly retain their full potential until chance of spatial positioning alters their access to the internal or external environment. (The commercial exploitation of these techniques at the 8-cell stage is now well-developed in beef and dairy breeding.) Importantly, these factors are exerting their influence prior to the genome repression/activation, all of which leads to germ layer definition and basic tissue differentiation. Of all the many potential and real microenvironmental influences, only gravity would appear to have remained relatively constant and ubiquitous for developing organisms. Histo-and organogenesis as well as differential growth of the embryo and fetus may have evolved with a constant environmental factor of gravity (G=6.7 x 10(-8) c.g.s. units). Embryos of 2-d and 9-d stage of incubation were flown in an incubator on the Space Shuttle during a 5-d mission. Significant differences in embryo response to this microgravity environment were observed. This paper offers an analysis and suggests mechanisms which may contribute to these results.