gms | German Medical Science

Birds, fish and amphibians replace lost sensory hair cells by asymmetric division of supporting cells which serve as bona fide stem cells resulting in high regenerative capacity of hair cell-bearing organs. In contrast, hair cell regeneration does not occur in the mammalian inner ear, but cells with proliferative capacity can be isolated from the postnatal organ of Corti. The progeny of these otic stem cells has the ability to differentiate into hair cell-like cells in vitro suggesting that these cells are a promising tool for proof-of-principle experiments aimed to replace lost hair cells in the organ of Corti. In serum-free, non-adherent culture conditions organ of Corti-derived stem cells display a distinct capacity to form clonal floating colonies, so-called spheres. In recent years, inner ear-derived spheres have been isolated by a number of laboratories and we noticed a considerable diversity in the reported morphology of spheres derived from the cochlear sensory epithelium. Here, we provide an in-depth characterization of different sphere morphologies that we classified as solid, transitional, and hollow. We show that these sphere types are not derived from different progenitor cell types, but represent the product of a single proliferating progenitor cell that initially grows into a solid compact sphere, which gradually converts into a buoyant hollow sphere via a transitional morphology. This process is associated with epithelial differentiation of sphere cells and a decline in the stem cell features of these cells including their capacity for self-renewing. Comparative analysis of the features of the disticint sphere types revealed that solid spheres contained significantly more rapidly cycling Pax-2-expressing presumptive otic progenitor cells than hollow spheres. Islet-1, which becomes upregulated in nascent sensory patches, was also more abundant in solid than in hollow spheres. Likewise, hair cell-like cells were found in significantly higher numbers in differentiated cell populations derived from solid spheres. Collectively, our results provide an explanation for the different sphere types that can be observed in cultures of cochlear sensory epithelial cells. They also suggest that solid spheres represent the most suitable sphere type for development of stem cell-based assays or cell transplantation experiments aimed to regenerate or replace lost inner ear hair cells.