Endolymphatic sac morphology after instillation of hyperosmolar hyaluronan in the round window niche

One hundred and sixty-eight mice were injected with hyaluronan (HA) of varying tonicity in the middle ear and compared with 81 control animals to study the effects of hypertonic solutions in the middle ear on inner ear fluid homeostasis and endolymphatic sac (ES) response. Six hours after 1.9% HA and 4, 6 and 8 h after 4% HA deposition there was a significant increase in the proportion of granule-containing light epithelial cells in the ES as compared with controls indicating an enhanced secretory activity from the epithelial lining. Six, 8 and 10 h after deposition of 4% HA, the ES volume was significantly reduced. The secretory activity and the ability of the ES to vary its volume could be specific responses to dehydration of the inner ear resulting from the high colloid osmotic pressure exerted by concentrated HA. The secretion of high-molecular substances into the ES may serve to keep the ES lumen patent and thus ensure a proper ES function and/or to regulate the intralabyrinthine volume and fluid pressure. Whether the findings may have clinical relevance for Ménière's patients remains to be elucidated.