Accumulation of secretory granules in pituitary clonal cells derived from the epithelium of Rathke's pouch.

2A8 clonal cells derived from the epithelium of Rahtke's pouch of the fetal rat are essentially agranular when grown in vitro, in spite of their active secretion of hormones, i.e., ACTH, prolactin and growth hormone. These cells do not produce detectable amounts of thyrotrophic or gonadotrophic hormones in vitro. When the growth medium (Ham's F10) was supplemented with rat median eminence extract (MEE), L-thyroxin and fresh serum from hypophysectomized rats, some of the 2A8 cells accumulated and stored secretory granules which were characteristic of the cells of the intact pituitary gland. Typical thyrotrophic and gonadotrophic cells also became recognizable and all six anterior pituitary hormones were released into the culture medium. Growth of 2A8 cells in this modified culture medium resulted in an increased production of all hormones with increasing time in culture. These results indicate that the processes that lead to the accumulation of typical mature secretory granules which characterize the individual pituitary cell types are initiated or promoted by some unidentified factor or factors which are present in fresh rat serum. It is also apparent that fresh rat serum can promote the differentiation of gonadotrophs and thyrotrophs in vitro.