Circadian changes in prolactin cell activity in the pituitary of the teleost Poecilia latipinna in freshwater

Summary Quantitative morphometric studies with the electron microscope were made on the prolactin cells of wild freshwater sailfin mollies taken in Florida in August at four different times of day. The results indicate a circadian rhythm in the prolactin cell, the period of highest synthetic activity being from midday to evening, as indicated by the condition of nucleus, Golgi apparatus and rough endoplasmic reticulum, and by the incidence of granule release profiles. No circadian changes were detected in the ACTH cells.Several distinct sites of prolactin granule release were recognised. However, there was no conclusive evidence of granule release by any mechanism other than classical exocytosis.We thank Mr. W. Thomson for technical assistance and Ann Grier for help in collecting the fish. T.F.C. Batten is in receipt of an S.R.C. Research Studentship.