The object of this experiment was to determine what role the gravitational field might have on the initial development of the gravity-sensing portions of the inner ear. Particular emphasis was placed on the formation of the otoliths, test masses on which gravitational and linear-acceleration forces act. If the growth of the otoliths is somehow regulated by their weight, their mass should be increased in reduced gravity. Prefertilized eggs of the Japanese red-bellied newt, some at developmental stages before any portion of the inner ear had formed and others just before the otoliths are formed were flown in the Aquatic Animal Experiment Unit. During the fifteen day flight, these eggs reached larval stages in which the inner ear neared its adult form. The otoliths of the saccule and utricle, the two gravity-sensing portions of the inner ear, were not of significantly different volumes when prepared for histologic study within the week after return of the shuttle. The endolymphatic duct and sac extend dorsally from the saccule and, in the adult newt, cover the brain stem and portions of the spinal cord. In the adult, these structures are filled with a different form of otoconia. This system develops earlier and is significantly larger in the flight-reared larvae, compared to ground-controls. At the stages the larvae reached at the end of the flight, endolymphatic otoconia begin to appear in the saccule and contribute to the saccular otolith. In X-ray microfocus studies performed by the Japanese collaborators, the saccular otolith and endolymphatic otoconia mass are seen to be significantly larger in one flight-reared larvae maintained in Japan for several months post-flight. Thus, the system of otoconia formed in the endolymphatic sac is greatly accelerated in the flight-reared animals and leads to a long-lasting increase in the saccular otolith in the one specimen maintained for months after landing.
Early development of a gravity-receptor organ in microgravity (Otoconia)
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