Development of Intracellular Membrane Systems in the Columnar Epithelium of the Urinary Bladder of the Euryhaline Goby Gillichthys mirabilis urinary bladder/intracellular membranes/euryhaline teleost/prolactin/osmoregulation

The membrane systems in “columnar cells” of the goby urinary bladder were studied after staining with ferrocyanide‐reduced osmium tetroxide (Karnovsky). In addition to the endoplasmic reticulum, two distinct systems of membranes were observed: 1) a vesiculotubular system made up of vesicles and short tubules located between the Golgi area and the apical membrane and 2) well‐developed infoldings of the laterobasal plasma membrane which form either complete or fenestrated sheets. Adaptation to 5% seawater or prolactin exposure of seawater fish induces a proliferation of these membrane systems and, in particular, of the complete infoldings of the laterobasal plasma membrane. These observations suggest high activity of these bladder cells in osmoregulatory adjustments to hypotonic environments. The divergence between cytological and physiological indicators of activity is considered.