Experimental studies on cysts in the prolactin zone of the pituitary in the 9-spined stickleback, Pungitius pungitius L.

The incidence of pituitary cysts in the prolactin zone of the stickleback, P. pungitius, cannot be reduced, either compared with laboratory controls or with wild fish, by keeping animals in conditions of minimal osmotic stress (1/3 strength SW). As there were no signs of cyst repair in any animal and as previous attempts to reduce cyst incidence also failed, it is likely that animals with cysts cannot lose them. The study also showed that keeping fish in captivity and handling them each day can induce cysts to appear in the pituitary. Records of fish mortality and the number of cysts per gland and their size and locularity suggest that animals with large cysts (greater than 70 micron in diameter) survive better in 1/3 strength SW and that the factors causing cysts to appear in the pituitary may not be those that induce them to enlarge.