Three clonal types of urothelium with different capacities for replication.

Objectives: Similar to other epithelia, urothelium in vivo has a hierarchal organization of cells each with specific gradients of differentiation. While distinct cell types have been described as important in bladder cancer in vitro, clonal and proliferative capacities of normal urothelial cells have not been characterized.
Materials and Methods: Three cell types and colony types were identified from primary porcine urothelial culture. Proliferative activity, patterns of apoptosis and differentiation, colony forming efficiency and ability to change phenotype with passage were determined and compared.
Results: Small, T-I colonies with large flattened (type-1) cells had low levels of proliferation and high levels of apoptosis. Large T-III colonies had a central area of small (type-3) cells surrounded by type-1 and type-2 cells. Proliferation and apoptosis were asymmetrically distributed in the periphery of T-II and T-III colonies. T-III colonies proved to be significantly more clonogenic and proliferative. With appropriate induction, type-1 cells were able to proliferate upon passage and form type-3 cells, yet long-term culture demonstrated that progeny of type-1 cells appeared to have inherited a clonogenic handicap.
Conclusions: Type-3 cells in the centre of T-III colonies appear to harbour stem-like qualities with a relatively low proliferative and apoptotic index at homeostasis and the ability to become highly proliferative upon passage. This study demonstrates that distinct urothelial cell types with differing clonal capacities can be isolated from the bladder and these cells may have implications for tissue engineering and carcinogenesis.