Elimination of the nucleus in preovulatory oocytes of the rainbow trout, Salmo gairdneri Richardson (Teleostei)

A process of elimination of the nucleus in oocytes of the rainbow trout displaying eight different stages is described. The phenomenon was observed only in stage III of sexual maturity. Initially, there is a slight shrinkage of the oocyte accompanied by the loss of yolk granules. This is followed by a cytoplasmatic protrusion into the ovarian lumen, which is covered by the follicular wall. Subsequently, the latter is ruptured and the nucleus migrates toward the opening. Finally, the nucleus leaves the follicle together with a portion of cytoplasm and then occasionally undergoes a breakdown into fragments in the ovarian lumen. The relevance of this mechanism in the process of preovulatory follicular atresia is discussed.