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Vestibular Otolith Development In Spaceflight and Hypergravity.   J.D. Dickman¹, A. Lysakowski², D. Huss¹, and S. Price².  Dept. Anatomy & Neurobiology¹, Washington Univ., St. Louis, MO and Dept. Anatomy & Cell Biol.², Univ. Illinois - Chicago, Chicago, IL

   The goal of the present study was to determine how gravity affects the development of the vestibular otolith organs.  The receptors, otoconia, and afferent innervation, and regulatory genes of the otolith maculae were examined in embryonic quails raised in one of three gravity environments, including normal 1g, microgravity (0g), or hypergravity (2g).  Fertilized eggs were first arrested from development by cooling, then placed into one of three gravity conditions and allowed to develop for 12 days at 37deg C.  1G and 0G embryos were developed in low Earth-orbital spaceflight (STS-108) in a specially designed incubator (ADF - SHOT, Inc) that contained a microgravity carousel and a 1g centrifuge carousel.  2g embryos were developed during constant centrifugation in a laboratory incubator.  We found that the mean saccular epithelium area was smaller in 0g and larger in 2g embryos, than 1g controls.  Hair cells showed normal stereocilia polarizations for all gravity conditions.  We also found that no significant differences in otoconial formation nor the expression of Otop1 or Oc90 (two genes regulating otoconial growth) were present between 1g and 2g embryos.  However, neural tracings (HRP) of macular afferents revealed strong differences due to gravity exposure.  Zero g fibers had smaller axons, were less branched, and had fewer terminals as compared to 1g controls.  Two g afferents were significantly larger, contained more arborizations, larger terminals and more growth cones as compared to 1g controls.  Electron microscopic observations reveal that the ribbon synapses were more numerous for type I but not type II hair cells in both 1g and 2g maculae, as compared to 1g controls.  The results show that vestibular otolith development is dependent upon gravity exposure for synaptogenesis and afferent innervation.

   Supported in part by funds from NASA NNA04CC52G, NIH DC006913.