Microfilament-rich cells in the toad bladder epithelium.

Basal cells of the bladder epithelium of Bufo marinus have been found heterogenous and consist of microfilament-rich cells (MFR-cell) and undifferentiated cells (Un-cell). The MFR-cell, which represents approximately 20% of the epithelial cell population, lies between the epithelial layer lining the urinary space and the basement membrane; it extends under several epithelial cells by processes of varying widths and lengths which contact, via desmosomes, other MFR-cells, as well as cells in the superficial layer, i.e., granular and mitochondria-rich cells. The cytoplasm of MFR-cell is filled with intermediate filaments arranged in bundles which run parallel to the plane of the epithelium and no dense granules, typical of granular cells, have been detected. Strong immunofluorescence for actin is associated with cells which occupy the same basal position as MFR-cells. Undifferentiated cells have no contact via desmosomes with adjacent cells and their cytoplasm is filled with free ribosomes; they lack bundles of intermediate filaments and possess no specialized organelles. After a 4-hr pulse of 3H-thymidine, 1.5% of epithelial cells incorporate thymidine into nuclear DNA, out of which 3/4 are basally and 1/4 are apically located. Identification of cell types by electron microscopy reveals that approximately 10% of undifferentiated basal cells are labeled, whereas less than 0.1% of granular cells and no MFR-cells incorporate 3H-thymidine into DNA. When dissociated from the epithelium and separated by isopycnic centrifugation, MFR-cells possess a mean buoyant density of approximately 1.025, cosediment with mitochondria-rich cells and exhibit a strong immunofluorescence for actin. The function of MFR-cells remains unknown; however, they may play a role in cell coupling and responses to hormonal and physical factors.