Effects of prolactin on α and β chloride cells in the gill epithelium of the saltwater adapted tilapia “Oreochromis niloticus”

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.