OVULATED OOCYTES COLLECTED FROM PMSG HCG-TREATED AND CYCLING DJUNGARIAN OR SIBERIAN HAMSTERS (PHODOPUS-SUNGORUS) ARE STRUCTURALLY SIMILAR WITH NO EVIDENCE OF POLAR BODY FORMATION, INDICATING ARREST IN MEIOSIS .1./

This study was undertaken (1) to devise a method of inducing multiple follicular development and subsequent ovulation in the Djungarian or S iberian hamster (Phodopus sungorus) and (2) to assess the quality of o vulated oocytes collected from PMSG/hCG treated animals in comparison to naturally ovulating animals. Hamsters (4-5 weeks; n = 70) received 5 IU PMSG followed 50-52 hr later by 10 IU hCG. Ovulated oocytes were collected 14-20 hr after hCG injection. Ovulated oocytes were flushed from oviducts of cycling animals (7-12 weeks; n = 30) exhibiting two c onsecutive estrous cycles. Oocytes were fixed and subjected to triple fluorescence immunostaining using anti-tubulin antibodies, fluorescein phalloidin, and Hoechst 33258. The mean number of ovulated oocytes co llected from cycling animals was 4.8 +/- 0.4 (range 1-7). Ovulation oc curred in 73% of the PMSG/hCG-stimulated animals. The mean number of o ocytes ovulated from stimulated animals was 9.2 +/- 0.8 (range 0-22). The ovaries of animals that did not ovulate or that ovulated few oocyt es did respond to PMSG, as indicated by the presence of multiple folli cular development and pre-ovulatory stigmata. There was no evidence of a polar body in ovulated oocytes collected from PMSG/hCG-treated or c ycling animals, indicating that oocytes were arrested in meiosis I. In the majority (80%) of ovulated oocytes from PMSG/hCG-treated and cycl ing animals, cortically placed chromosomes were aligned on a metaphase plate equidistant from a bipolar spindle. Sparse f-actin staining was observed in the region of the ooplasm surrounding the chromosomes. As the interval between hCG injection and the time of collection increas ed, chromosomes lost their proper alignment and migrated away from the cortex of the oocyte concomitant with a disruption of spindle integri ty. This collapse of proper chromosome alignment and disruption of spi ndle architecture also characterized aging oocytes collected from cycl ing animals. These data show that in the Djungarian or Siberian hamste r (Phodopus sungorus), (1) there is individual animal variation in res ponsiveness to hCG following PMSG treatment, (2) there are no cytologi cal differences in the quality of oocytes collected from hormonally tr eated animals when compared to cycling animals, and (3) oocytes are ov ulated in meiosis I. (C) 1995 Wiley-Liss, Inc.