Evidence for maternal control of blastocyst growth after asynchronous transfer of embryos to the uterus of the ewe

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Abstract
The fate of embryos transfered asynchronously in the ewe was investigated when the functional life of the corpus luteum was prolonged by both hemihysterectomy and by the presence of a 2nd synchronously transferred embryo. Development of asynchronously transferred embryos was assessed at progressively later stages after transfer. Prolongation of luteal function did not enable asynchronously transferred embryos to persist. Embryos from day-4 donors were retarded in their rate of development when placed in younger day-1 or -2 uteri and appeared unable to develop beyond the early blastocyst stage. Embryos from day-4 donors placed in older day-6 or -7 uteri showed accelerated growth and development which was maintained until the uterus rached day 12. Thereafter, further growth of the asynchronously transfered embryos was retarded, although synchronously transfered embryos then entered the phase of rapid blastodermic vesicle elongation. Asynchronously transfered embryos disappeared from the uterus when the ewe entered proestrus. There is an active relationship between the embryo and the maternal environment during mid-cycle and an apparent lack of association between embryo size, growth rate and physiological maturation.