Hair cell distribution and orientation in goldfish otolith organs.

Structurally diverse sensory regions occur in the otolith organs of the goldfish inner ear. Scanning electron microscopy reveals regional distinctions based on three criteria. (1) Hair cells have different sizes of apical bundles, based on thickness. In all three maculae, two central regions have hair cells with bundles significantly thicker than those in surrounding regions. (2) Hair cell population density varies, with regional aggregations present. The central regions with thick bundles have two to three times the density of surrounding regions with thin bundles, and contain 40-80% of the total hair cell number in each macula. (3) Hair cell orientation maps show that each macula has two oppositely oriented cell populations that can be separated completely, not by a zone of interspersion, but apparently by a single unbroken line. The lagena is like the utricle in having hair cells with the kinocilium on the side of the cell toward the opposition line, but in the saccule the kinocilia face away from the line, and the small macula neglecta consists of two completely separate, oppositely oriented patches. The opposition line does not divide each macula simply down its midline; instead, the line divides the regions with thich bundles into nearly equal opposing areas, except for a remarkably abrupt large loop in the line in the anterior part of the saccule. The regional structural diversity in these organs may relate to localized functional diversity of responses to tilt, vibration and sound.