Activation of a new physiological state at the onset of vitellogenesis in Hyalophora follicles.

Ovarian follicles of Hyalophora cecropia are shown here to undergo a comprehensive transformation about 2 days after they are first formed and several hours before the first yolk spheres are visible. (1) Electrical coupling initiates between the follicle cells and the oocyte-nurse cell complex, as well as between adjacent follicles. (2) Oocyte and nurse cell membranes begin to hyperpolarize, adding an azide- and vanadate-sensitive component to a basal potential that is unaffected by these inhibitors. (3) The cytoplasmic pH of the oocyte rises from 6.7 to 7.4. (4) The nurse cells hyperpolarize more strongly than the oocyte, so that the charge-dependent restrictions on protein movement across the cytoplasmic bridges that connect these cells arise at this time. (5) The follicle swells and becomes more turgid. (6) Uridine incorporation shuts down in the germinal vesicle and accelerates in the other nuclei of the follicle. The changes are sufficiently synchronous to suggest that they may be responses to a single, branching cascade of activation. The altered cell potentials, cytoplasmic pH, and turgidity implicate the cell membrane in an early stage of activation.