Maintenance of the corpus luteum after uterine transfer of trophoblastic vesicles to cyclic cows and ewes

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Abstract
One or 2 trophoblastic vesicles (0.4-2 mm diameter) from cow (day 14) or ewe (day 11-13) embryos without their disc were transferred, after culture for 24 h, into recipients. Each vesicle was transferred into the uterine horn ipsilateral to the CL [corpus luteum] by the cervical route in heifers and surgically in ewes on day 12 of the estrous cycle. In cows, daily measurements of plasma progesterone concentrations and checks for return to estrus showed that the CL was maintained in 8 of 12 recipients. These 8 cows had 25- to 37-day cycles while 4 recipient heifers returned to estrus normally. Three recipients with an extended cycle were slaughtered. The dissected uterus showed that trophoblastic vesicles had developed in the uterine horns. In ewes, the serum progesterone curve, determined in each recipient, showed that the CL was maintained in 7 of 12 recipients. These 7 ewes had 20- to 54-day cycles and the other 5 ewes had a normal cycle of 15-19 days comparable to that of 17.0 .+-. 0.5 days for the 6 control ewes. Whenever the CL was maintained, high blood progesterone levels were followed by rapid luteolysis. In cattle and sheep, therefore, a trophoblastic vesicle transferred into the uterus can develop in vivo, secreting the embryonic signals when there is no embryonic disc control and transforming the cyclic CL into a CL of pregnancy in .apprx. 60% of the cases. Evidently, the early signals inhibiting luteolysis may be of trophoblastic origin and confirm that after normal embryo transfer some of the late returns to estrus may be due to the development of trophoblastic tissue only in utero.