PRESENCE OF BRUSH CELLS IN THE MOUSE GALLBLADDER

The blush cells (BC) are the second most frequent cellular component o f the epithelium of the mouse gallbladder. They have a topographical d istribution, being present in large numbers toward the neck and in the fundic regions of the organ and are scattered in the body. Serial sec tion studies demonstrate that BC have a characteristic shape consistin g of a narrow apical portion, bulky body and basal cytoplasmic project ions. BC are located obliquely among the principal cells. Scanning ele ctron microscopy demonstrates that the microvilli forming the prominen t brush border, after which the cell was named, have a triangular arra ngement. Due to their size and stiffness, the microvilli of BC have mo re similarity with stereocilia of sensory cells than with conventional microvilli. Furthermore freeze-fracture replicas demonstrate that, li ke stereocilia, the P face of the microvilli plasma membrane of BC is smoother than the E face but several intramembranous particles form sm all 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 apica l cytoplasm of BC. The use of the appropriate fixative reveals that it consists of bundles of actin filaments originating from the axis of t he apical microvilli and stretching continuously up to the supranuclea r region of the cell. Microtubuli, also assembled in bundles, flank in alternating manner the actin filaments over their whole course. Due t o 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 f ilaments and microtubuli; their shape is highly influenced by the fixa tive used. Intraluminal injection of horseradish peroxidase demonstrat es that these vesicles are not resorptive as they are not filled by th e tracer. The BC possess a large number of lateral microvilli. These, whether single or in pairs, are rigid cytoplasmic protrusions that lea ve the lateral surface of the cell in all directions and penetrate dee ply into the cytoplasm of the adjacent principal cells. The bundle of actin filaments emanating from each lateral microvillus extends at dif ferent angles into the cytoplasm. A conspicuous amount of bundles of 1 0 nm filaments is intertwined around the nucleus and extends toward th e desmosomes of the lateral plasma membrane and into the basal cellula r body. Arguments are considered in support of the view that interacti ons between the plasma membrane with its differentiations on the one h and and the cytoskeleton elements an the other hand, play a key role i n the function of BC as a receptor (sensory) cell. (C) 1997 Wiley-Liss , Inc.