Cell type-specific reduction of β tubulin isotypes synthesized in the developing gerbil organ of Corti

There are seven isotypic forms of the microtubule protein β tubulin in mammals, but not all isotypes are synthesized in every cell type. In the adult organ of Corti, each of the five major cell types synthesizes a different subset of isotypes. Inner hair cells synthesize only β_I and β_II tubulin, while outer hair cells make β_I and β_IV tubulin. Only β_II and β_IV tubulin are found in inner and outer pillar cells, while β_I, β_II, and β_IV tubulin are present in Deiters cells, and β_I, β_II and β_III tubulin are found in organ of Corti dendrites. During post-natal organ of Corti development in the gerbil, microtubules are elaborated in an orderly temporal sequence beginning with hair cells, followed by pillar cells and Deiters cells. Using β tubulin isotype-specific antibodies, we show that, in the gerbil cochlea, the same three isotypes are present in each cell type at birth, and that a cell type-specific reduction in the isotypes synthesized occurs in hair cells and pillar cells at an unusually late stage in development. No β tubulin isotypes were detected in mature afferent dendrites, but we show that this is because few microtubules are present in mature dendrites. In addition, we show that primary cilia in inner hair cells, a feature of early development, persist much later than previously reported. The findings represent the first description of developmental cell type-specific reductions in tubulin isotypes in any system.