GROWTH AND DIFFERENTIATION OF TRANSITIONAL EPITHELIUM INVITRO

The development of rat [urinary bladder] transitional epithelial cells grown on conventional non-permeable surfaces was compared with development on permeable collagen supports. On glass or plastic surfaces, cells grew as expanding monolayer sheets. Once confluent, growth continued with a bilayer being formed in most areas and apical cells being continuously sloughed off. Although most cells were interconnected by desmosomes, and junctional complexes were formed, no other indications of differentiation were observed. After 2-3 wk of growth, division stopped and cell death ensued. Single-cell suspensions plated on collagen-coated nylon disks reassociated into multicellular islands and commenced growth. Mitoses were confined to the basal cells in contact with the permeable substrate. The islands developed into epithelial trilayers tapering to monolayers along spreading edges. Once the islands were confluent, stratification was completed and appeared similar to that observed in vivo. Germinal cells formed a basal lamina, and the upper layer was composed of large, flattened cells with an unusually thick asymmetrical plasma membrane on the apical surface. EM and radioactive tracers demonstrated leaky zonulae occludentes with a restricted permeability to small molecules. The movement of urea was retarded in comparison to H2O. Unlike the slow turnover of adult epithelium in vivo, maturation and sloughing of apical cells were measurable. Transfer of cells was effected and growth maintained for up to 4 mo. These results may indicate the necessity of a nutrient-permeable growth surface for the polarized differentiation of adult transitional epithelium.