Presence of brush cells in the mouse gallbladder.

The brush cells (BC) are the second most frequent cellular component of the epithelium of the mouse gallbladder. They have a topographical distribution, being present in large numbers toward the neck and in the fundic regions of the organ and are scattered in the body. Serial section studies demonstrate that BC have a characteristic shape consisting of a narrow apical portion, bulky body and basal cytoplasmic projections. BC are located obliquely among the principal cells. Scanning electron microscopy demonstrates that the microvilli forming the prominent brush border, after which the cell was named, have a triangular arrangement. Due to their size and stiffness, the microvilli of BC have more similarity with stereocilia of sensory cells than with conventional microvilli. Furthermore freeze-fracture replicas demonstrate that, like stereocilia, the P face of the microvilli plasma membrane of BC is smoother than the E face but several intramembranous particles form small aggregates on the microvillus tip of both P and E faces. Numerous intramembranous particles are scattered on the lateral plasma membrane. An unusual, spatially organized cytoskeleton characterizes the apical cytoplasm of BC. The use of the appropriate fixative reveals that it consists of bundles of actin filaments originating from the axis of the apical microvilli and stretching continuously up to the supranuclear region of the cell. Microtubuli, also assembled in bundles, flank in alternating manner the actin filaments over their whole course. Due to the strong parallel arrangement of both cytoskeletal structures, the apical cytoplasm of the BC assumes a typical stiffness, observable in both thin sections and freeze-fracture replicas. A variable number of vesicles of different size are aligned between the bundles of actin filaments and microtubuli; their shape is highly influenced by the fixative used. Intraluminal injection of horseradish peroxidase demonstrates that these vesicles are not resorptive as they are not filled by the tracer. The BC possess a large number of lateral microvilli. These, whether single or in pairs, are rigid cytoplasmic protrusions that leave the lateral surface of the cell in all directions and penetrate deeply into the cytoplasm of the adjacent principal cells. The bundle of actin filaments emanating from each lateral microvillus extends at different angles into the cytoplasm. A conspicuous amount of bundles of 10 nm filaments is intertwined around the nucleus and extends toward the desmosomes of the lateral plasma membrane and into the basal cellular body. Arguments are considered in support of the view that interactions between the plasma membrane with its differentiations on the one hand and the cytoskeleton elements on the other hand, play a key role in the function of BC as a receptor (sensory) cell.