The time sequence of response of the prolactin cells of the freshwater teleost, Poecilia reticulata, to an altered environmental salinity.

The time sequence of the ultrastructural changes in the prolactin cells of the adult guppy, Poecilia reticulata, was studied in freshwater fish transferred to 1/3 seawater for 1, 4, 11 and 28 days. The morphological changes were slight and only detectable by quantitative (morphometric) procedures. The secretory granules showed a "biphasic' response to the altered environmental salinity, but the volume density of the Golgi apparatus progressively declined throughout the experiment. After an initial decrease in the volume density of the RER, that organelle regained its original prominence by 11 days in 1/3 seawater. The volume density of the nucleus was markedly higher in fish 28 days after transfer to 1/3 seawater than in any other group. Cell volume estimations showed that a shrinking of the cytoplasm rather than a swelling of the nucleus accounted for the high volume density figure. The changes in the volume density of the mitochondria closely paralleled those of the RER. Profiles of exocytosed granules were rarely found in any of the groups, but were least frequent in fish kept in 1/3 seawater for 28 days, when dense (lysosomal?) bodies were most abundant. The quantitative methods high-lighted some discrepancies in the rate and magnitude of the changes shown by some organelles and have led the authors to suggest that during adaptation of fish to dilute seawater, synthesis and release may not be coupled processes in the prolactin cell of P. reticulata.