Nucleolar size and the activity of rRNA transport in the course of morphogenetic reduction of cell dimensions.

In antheridial filaments of Chara vulgaris during the first period of spermatogenesis which consists of 6 synchronous cell division cycles there occurs a gradual decrease in sizes of cells entering successive mitoses. Present studies indicate that this process is correlated with a considerable reduction of total nucleolar volume in late G2 phase which, in turn, brings about decrease in sizes of nucleoli reappearing in telophase of the subsequent cell cycle. A consequence of the above phenomenon evidenced using 3H uridine autoradiography is a gradual increase--from one generation to the next--in an amount of rRNA transported into cytoplasm due to an increase in number of small nucleoli which were found to be more active in transport than larger nucleoli. This process leads to a lowering of an increase in nucleolar volumes during consecutive interphase periods owing to a progressively limited accumulation of rRNA for the sake of daughter cells, i.e. to a spontaneously magnifying reduction of nucleolar sizes in the forthcoming cell generations. Thus, diminution of nucleoli observed during the development of antheridial filaments seems to be due to a chain-series of processes connected with mechanisms which possibly regulate rRNA transport and accompany morphogenetic events.