Expression of mRNA encoding extracellular matrix glycoproteins SPARC and SC1 is temporally and spatially regulated in the developing cochlea of the rat inner ear

SPARC is a multifunctional extracellular matrix (ECM) glycoprotein that shares partial sequence homology with SCl/hevin. These ECM molecules exhibit calcium-binding properties and modulate cellular interactions. This study examines the expression of SCl and SPARC mRNA in the developing cochlea of the rat inner ear prior to and after the onset of hearing. At all ages examined, SCl mRNA is highly expressed in neurons of the spiral ganglion. In contrast, SPARC transcripts are not detected in the spiral ganglion but are enriched in the temporal bone and cartilaginous otic capsule surrounding the cochlea. Both SCl and SPARC mRNA are expressed in connective tissue elements involved in maintaining ionic homeostasis of cochlear fluids. SCI mRNA is localized to type III fibrocytes of the spiral ligament (slg) anti marginal cells of the stria vascularis, while SPARC mRNA is apparent in the spiral limbus and type I fibrocytes of the sig. At postnatal day 10, SPARC mRNA shows a dramatic change in expression. High levels of SPARC transcripts are induced in Deiters cells (dc) of the organ of Corti. Interestingly, this induction of SPARC mRNA correlates with the onset of hearing. This suggests that SPARC may play a role in calcium regulation in de when functional maturation of the cochlea is attained and rapid changes in calcium levels are required, (C) 2001 Elsevier Science B.V. All rights reserved.