Seasonal variation in the structure and ultrastructure of the pituitary of the marine form (Trachurus) of the threespine stickleback, Gasterosteus aculeatus L. I. Rostral pars distalis.

Abstract

The rostral pars distalis of the anadromous marine stickleback Gasterosteus aculeatus L., form trachurus is composed of three distinct cell types, eta (prolactin-secreting), epsilon (corticotroph) and chromophobic cells. The epsilon and eta cells appear minimally active in early-winter fish collected in the sea and maximally active in spring fish collected in fresh water. Similarly, the chromophobes are least developed in the winter stages and maximally developed in spawning fish. Two possible mechanisms of hormone release were observed in the eta cells of spring fish collected in fresh water, but these were absent from the stages collected in or transferred to sea water. The mode of regulation of eta cell activity is discussed.