Formation and release of secretory granules during mitosis in the anterior pituitary gland.

Electron microscopic images indicating exocytosis of secretory granules were obtained in rat prolactin cells undergoing mitosis at the estrus in the female reproductive cycle. Immature secretory granules looking like small cored vesicles were found among chromosomes and microtubules of the mitotic spindle. Golgi apparatus was fragmented and its small elements mostly consisting of vesicles were seen at the peripheral region of the cell. Rough endoplastic reticulum was mostly vesiculated, but lamellar stacks were still recognized at the cell periphery. It is evident that fully differentiated cells of the anterior pituitary glands are capable of continuing the activity of secretion, i.e., formation and release of secretory granules during mitosis.