A series of experiments was conducted to evaluate plasminogen activator production and effects of supplementing culture medium with plasminogen or plasmin on development of rabbit embryos in vitro. In Expt 1, 495 one- to two-cell embryos were cultured in Ham's F-12 with 15 mg BSA ml−1 containing 0, 30, 60 or 120 pg porcine plasminogen ml−1 or 0, 75, 150 or 300 μg rabbit plasminogen ml−1. Percentages of embryos developing to the expanded blastocyst, hatching blastocyst and hatched blastocyst stages were greater (P < 0.05) in medium with 120 μg porcine plasminogen ml−1 than in the absence of plasminogen. More (P < 0.05) embryos developed to the blastocyst, expanded blastocyst and hatching blastocyst stages in medium with 300 μg rabbit plasminogen ml−1 than in the absence of plasminogen. In Expt 2, 216 one- to two-cell embryos were cultured in medium with 0, 30, 60 or 120 μg porcine plasminogen ml−1 for 96 h, fixed and stained with haematoxylin and eosin, and the number of cells determined. No differences (P> 0.05) were observed in number of cells of morulae but blastocysts developing in medium with 120 μg porcine plasminogen ml−1 had more (P < 0.05) cells (109.9 ± 10.4) than did blastocysts in medium with either 0 (69.4 ± 14.6) or 30 μg porcine plasminogen ml−1 (73.3 ± 12.2). In Expt 3, 144 one- to two-cell embryos were cultured in medium with 0, 13 or 45 μg porcine plasmin ml−1 or 120 pg porcine plasminogen ml−1. Development to the hatched blastocyst stage was greater (P < 0.05) in medium with 45 μg porcine plasmin ml−1 or 120 μg porcine plasminogen ml−1 than in medium with 0 or 13 μg porcine plasmin ml−1. In Expt 4, 101 embryos at days 3, 4 and 5 after mating were cultured to equivalent gestational age day 7, and medium was recovered at intervals of 24 h and assayed for plasminogen activator. Production of plasminogen activator was greater (P < 0.05) on days 6 and 7 than on days 4 and 5 for embryos collected on day 3, and was greater (P < 0.05) on day 7 than on day 6 for embryos collected on day 5. Although production of plasminogen activator progressively increased, no differences (P> 0.05) were observed by day in culture for embryos collected on day 4. These results suggest that rabbit embryos produce plasminogen activator during the peri-implantation period, and respond to plasmin and plasminogen in the culture medium with improved development.
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