The postnatal development of F-actin in tension fibroblasts of the spiral ligament of the gerbil cochlea

The tension fibroblasts of the spiral ligament of the mammalian cochlea are thought to create radial tension on the basilar membrane. Their postnatal development was investigated in the gerbil (Meriones unguiculatus) with confocal fluorescence microscopy using phallotoxin as a specific marker for F-actin. In the adult cochlea, tension fibroblasts were restricted to the basal cochlear turn and were arranged in 2-4 rows in the marginal region of the spiral ligament. They contained intensely stained parallel bundles of F-actin. In upper cochlear turns, the marginal region of the spiral ligament was occupied by sparsely distributed, unobtrusively labeled fibrocytes, the bone lining cells. The spiral ligament of young postnatal stages (newborn--6 days after birth (DAB)) lacked F-actin labeling patterns that are characteristic for tension fibroblasts in the adult. Rather, the whole inner surface of the otic capsule throughout all cochlear turns was outlined by cell layers with distinct but diffuse cytoplasmic F-actin label. These cells may represent perichondrial fibrocytes. Around 9 DAB, the perichondrium revealed changes in morphology and F-actin patterns that indicate a further differentiation into tension fibroblasts (basal turn) or bone lining cells (more apical turns). At 12 DAB, around onset of hearing, adult-like bone lining cells were found in the marginal regions of the spiral ligament of upper cochlear turns. In the basal turn, tension fibroblasts were present, but their F-actin cytoskeleton was not fully developed. During the following days, F-actin label increased in tension fibroblasts and reached adult-like configuration at 17 DAB, coinciding with mature hearing characteristics. The role of tension fibroblasts in development of hearing characteristics is discussed.