The expression of fibroblast growth factors and their receptors in the embryonic and neonatal mouse inner ear

Four different fibroblast growth factor receptors (FGFR) are known, three o f which have splice variants (known as the b and c variants) in the FGF-bin ding domain, to give different patterns of sensitivity to the different FGF s. The expression of the b and c variants of the FGF receptors. together wi th the expression of the ligands FGF1. FGF2, FGF3, FGF7, FGF8b and FGF8c, w as determined by quantitative reverse transcription-polymerase chain reacti on in developing whole mouse inner ears, and in dissected components of the postnatal mouse inner ear. At embryonic age (E)10.5 days, when the otocyst is a simple closed sac, the receptor most heavily expressed was FGFR2b, re lative to the postnatal day 0 level. Over the period E10.5-E12.5. during wh ich the structures of the inner ear start to form, the expression of the di fferent FGF receptors increased 10(2)-10(4) fold per unit of tissue, and th ere was a gradual switch towards expression of the 'c' splice variants of F GFR2 and FGFR3 rather than the 'b' variants. At E10.5, the ligands most hea vily expressed, relative to the postnatal day 0 level, were FGF3, FGF8b and FGF8c. In the postnatal inner eat. the patterns of expression of receptors and ligands tended to be correlated, such that receptor variants were expr essed in the same regions as the ligands that are known to activate them ef fectively. The neural/sensory region expressed high levels of FGFR3c, and h igh levels of the ligand FGF8b. The same area also expressed high levels of FGFR1b and FGFR2b, and high levels of FGF3. The lateral wall of the cochle a (including the stria vascularis and the spiral ligament) expressed high l evels of FGFR1c and FGF1. 11 is suggested that the different FGF receptors and ligands are expressed in a spatially coordinated pattern to selectively program cochlear development. (C) 2001 Elsevier Science B.V. All rights re served.