Effect of age and of follicular aging on the preovulatory oocyte

Abstract

The ultrastructure and ability of oocytes to mature in vitro were examined in oocytes collected from preovulatory follicles of mature normal-cycling proestrous rats (Day 3 of the cycle) and of aged proestrous rats (Day 3) and during a spontaneous period of ovulatory delay (Days 3-5). These studies demonstrated that aging resulted in: 1) the germinal vesicle (nuclear) membrane becoming undulated and the nucleoplasm becoming denser; 2) a deterioration of the stacked arrangement of the cytoplasmic rays with the formation of single stranded rays; 3) a reduced ability to form a polar body; 4) an increased tendency for the nucleolus to remain intact after germinal vesicle breakdown was completed; and 5) an increased percentage of oocytes that fragment and/or degenerate in vitro. These combined ultrastructural and physiological studies indicate that aging alters the oocyte such that it is unable to complete meiosis. Thus, fertilization of these aged oocytes could result in chromosomal and developmental errors within the developing embryos. Additional structural alterations were observed in the aged oocyte. These changes include a decrease in the number of cortical granules and microvilli, and a separation of the zona pellucida from the oocyte. The fibrils which compose the zona pellucida also become organized into continuous bands. These changes are similar to changes which render the zona pellucida impenetrable to spermatozoa. Thus, the aged oocytes would have a reduced fertilizability and this could partially account for the lower fertility of older irregularly cycling rats.