The ultrastructure of the otolithic membrane and otolith in the juvenile mummichog, Fundulus heteroclitus

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Abstract
The sagitta otolithic membrane of F. heteroclitus consists of 2 different zones. A structured zone (gelatinous layer), which usually exhibits a reticulated or honeycomb-like architecture, is composed of tightly arranged fibrous material and covers only the sensory region of the macula. The gelatinous layer extends from the otolith surface to the tips of the sensory hairs; it probably functions primarily as a mechanoreceptor. The arrangement of this zone is closely associated with specific overlying structural features of the otolith surface and may influence the pattern of mineral deposition to some degree. A nonstructured zone (subcupular meshwork) consists of fibers in very loose networks and covers sensory and nonsensory regions of the macula. Over the sensory region some of this fibrous material extends from the epithelial surface through pores in the gelatinous layer to the surface of the overlying otolith. In the nonsensory region fibers of the subcupular meshwork are relatively more numerous and extend around the peripheral margin of the otolith. The fibrous material of the subcupular meshwork apparently is incorporated into the otolith as an organic matrix constituent. New aspects on the ultrastructure of the otolith are discussed.