The cause for the development of endophytic growth (nodular downgrowth) in rat urinary bladder epithelia cultured in protein-free medium was traced to a deficiency of Ca2+ in the culture medium. Endophytic growth, which is a characteristic in vivo histological feature of the preneoplastic phase of experimental bladder cancer, was induced in vitro by 0.3 mm Ca2+ but was suppressed by 1.8 mm Ca2+. Magnesium at 1.8 mm had no effect on endophytic growth.

Ca2+ deficiency was also found to be a significant cause of hyperplasia. Ca2+ concentrations above 0.9 mm resulted in a substantial inhibition of hyperplasia in the bladder epithelium; a greater than 50% inhibition was observed with 1.8 mm Ca2+. Conditions were found, however, which caused the suppression of endophytic growth without an apparent suppression of hyperplasia.

When the Ca2+ concentration was decreased to 0.075 mm and below, epithelial cells lost cohesiveness and infiltrated into the stroma. Epithelial cells were observed individually and in clusters within the lamina propria. Evidence of lytic activity was also observed in the lamina propria. Epithelial cells that infiltrated into the stroma displayed occasional mitotic figures and frequently contained abnormally appearing nucleoli.

The ability of low Ca2+ concentration in the culture medium to induce epithelia to mimic some of the growth patterns that appear during the development of experimental bladder cancer in vivo, suggests that there may be an early change in some aspect of cellular Ca2+ in epithelia and/or stroma that have been exposed to bladder carcinogens.

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