Development of the sheep conceptus during the first fourteen days.

Abstract

This paper describes the development of conceptuses obtained from animals in which the onset of estrus was known to within about 8 hours. Also the viability of conceptuses of similar age but variable sizes was studied after transfer to recipient ewes. There were 177 donor ewes superovulated with 800-1000 I.U. pregnant mares serum on Day 12 of the cycle. Conceptuses were removed by operation between Days 5 and 9 in 86 and between Days 12 and 14 in 91 with Day 0 the onset of estrus although ovulation normally takes place 24 hours later. 521 developing conceptuses and 81 unfertilized eggs were recovered. (47 of the unfertilized eggs were from ewes showing an exaggerated response to gonadotrophin with more than 20 ovulations per animal. ) 97 morulae were recovered 95 between the ages of 112 to 128 hours postestrum 2 at 160 hours. There were 30 in the 16-32 cell stage and 67 in the 32-64 cell stage. The development of a central cavity determined the blastocyst stage ovserved in 190 recovered from 128 to 224 hours after estrus. Some were still within the intact zona pellucida. Others were larger with no zona pellucida. By 208 hours postestrum the blastocysts showed a prominent embryonic disc. In the third phase of development many were damaged by recovery methods but 123 were obtained undamaged. At about Day 12 the conceptus begins to elongate and within 24 hours changes to an elongated chorionic sac with the embryo clearly discernable. The enlongating process continues rapidly but the embryonic disc enlarges only slightly. On Day 12 some degree of damage to embryonic membranes was tolerated when conceptuses were transferred to suitable recipients. When of the same age transferred conceptuses did equally well irrespective of different sizes but had all remained within the donor uterus many would have degenerated and died due to the competition. The marked growth of the conceptus from Day 12 to Day 14 postertrus commences when the corpus luteum becomes dependent on the presence of the conceptus for its continued maintenance. Other experiments have shown that the sheep conceptus can develop until Day 11 even in rabbit oviducts but does not survive longer there.