• Prolactin;
  • Gill;
  • Chloride cells


Tilapia (Oreochromis niloticus), 21 g average body weight, were divided into two groups. A group was maintained in fresh water, whereas another group was adapted for 2 weeks to 20% salt water. Among the latter, fishes were injected every 2 days for a week with tilapia prolactin (ti-PRL I). Gills were prepared for electron microscopy in order to determine the types and surface areas of chloride cells in each experimental condition. Two types of chloride cells, the α and β cells were easily distinguished on the basis of their location and ultrastructural features in the gills of freshwater fishes, while only one type of cell, the saltwater α cells presumably derived from the transformation of the freshwater α cells, were encountered in saltwater adapted animals. After PRL injection ofsaltwater adapted fishes, small chloride cells, which displayed ultrastructural features similar to those of β cells in freshwater tilapia, reappeared in interlamellar regions of the gills. In the same experimental conditions, the voluminous saltwater α cells showed a tendency to resume ultrastructural features more characteristic of the freshwater α cells from which they were derived. These observations tend to indicate that prolactin behaves as a “freshwater adapting hormone” and that β cells are specifically involved in fish adaptation to freshwater living conditions. © 1993 Wiley-Liss, Inc.