The aim of the present study was to examine the effect of embryo source (in vivo v. in vitro) and the progesterone environment into which it was transferred on Day 7 on embryo survival and size on Day 13. Day 7 blastocysts were produced either in vivo using superovulation, artificial insemination and non-surgical embryo recovery or in vitro using in vitro maturation, fertilisation and culture. In order to produce animals with divergent progesterone concentrations, following synchronisation recipients were either superovulated (High progesterone; n = 10) or not (Control progesterone; n = 10). Ten blastocysts, produced either in vivo or in vitro, were transferred to each recipient on Day 7. Both groups were killed on Day 13. The mean progesterone concentration from Day 7 to Day 13 (the period when the embryos were in the uterus) in the High and Control progesterone recipients was 36.32 +/- 1.28 and 10.30 +/- 0.51 ng mL(-1), respectively. Of the in vivo embryos transferred, the overall recovery rate at Day 13 was 64%, which was higher (P < 0.001) than that of 20% for the in vitro embryos transferred. The mean area of embryos recovered from High progesterone recipients was 3.86 +/- 0.45 mm(2) (n = 28) compared with 1.66 +/- 0.38 mm(2) (n = 24) for embryos recovered from Control progesterone recipients (P < 0.001). Similarly, the origin of the embryo used for transfer affected embryo size on Day 13. In summary, the recovery rate of blastocysts was higher for in vivo- than in vitro-derived embryos. Blastocyst size was approximately 2.3-fold greater in recipients with high compared with normal progesterone. The present study lends strong support to the hypothesis that an earlier rise in progesterone after conception stimulates blastocyst growth and the development of competent embryos.
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