Early Development of a Gravity-Receptor Organ in Microgravity

Neurophysiology

Cynops pyrrhogaster (newt)
Flight: 4 adults, 144 fertilized eggs Female/Male

Vivarium fertilized eggs: 144

Aquatic Animal Experiment Unit (provided by NASDA)

This experiment aimed to determine the effects, if any, of a microgravity environment on the initial development of the gravity-sensing portions of the inner ear. These organs contain sensory hair cells covered by a layer of dense calcium carbonate stones called otoconia. Particular emphasis was placed on the formation of otoliths, the 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. Furthermore, the otoconia are important for the proper functioning of many gravitactic reflexes in vertebrates and invertebrates. These reflexive circuits are established early in development and might be effected by exposure to microgravity.

Pre-fertilized eggs, some at developmental stages before the inner ear had formed and at the point just before the otoliths were formed, were launched in the Aquatic Animal Experiment Unit (AAEU). During flight, animals were video-recorded to observe progress of development. After flight, some larvae were fixed for sectioning. The sections were then analyzed with X-ray microimaging. The otolith volume and areas of sensory epithelia for various stages of development were calculated from three-dimensional reconstructions of the serial sections. Other larvae were tested to estimate the gain of the otolith-ocular reflex.

Approximately 62 out of 144 larvae survived the flight. According to morphological analysis of the videotape recording, both flight and ground controls developed at the same rates. Analysis of the three-dimensional reconstruction showed that flight-reared larvae have a larger mean endolymphatic sac (ES) and duct volume and a larger average volume of otoconia within the sac when compared to similarly staged ground controls. Furthermore, the appearance of otoconia in the ES was greatly accelerated in the larvae reared in microgravity. Otoconia from space-flown larvae also appeared more susceptible to degradation, and in a few cases, appeared to contain only longitudinal filaments, indicating alteration in processing in their assembly. These stones do not appear until after a return to normal gravity. More experiments with longer flight times must be performed to understand this phenomenon more clearly.

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