The developmental capacity of blastomeres from 4 - and 8 - cell sheep embryos

The developmental capacity of (a) single blastomeres of 4-cell embryos, (Jb) pairs of blastomeres of 8-cell embryos and (c) single blastomeres of 8-cell embryos was studied. The blastomeres were inserted into evacuated foreign zonae pellucidae, embedded in agar, transferred to ligated oviducts of dioestrous ewes, and recovered when their total age was 5-Jto 6̂ days. At lecovery the majority of embryos belonging to categories (a) and (b) had developed into small blastocysts, whereas most embryos of category (c) were vesicular forms with no clearly observable inner cell mass. The viability of embryos which had continued their development during culture in the sheep oviduct was tested by their transfer to the uterine horns of recipient ewes which had come into oestrus on the same day as the respective embryo donors. Three lambs were produced after transfer of eight embryos of category (a) deriving from two parent embryos. These three lambs derived from a single 4-cell embryo. Nine lambs were produced after transfer of 16 embryos of category (b) deriving from four parent embryos. Four of these lambs derived from a single 8-cell embryo. When ewes which had come into oestrus one or two days after the respective embryo donors were used as recipients for 16 embryos of category (a) and 16 embryos of category (b) one and five lambs, respectively, were produced. Finally, two lambs were produced after transfer of 31 embryos of category (e). These lambs derived from two different parent embryos. The principal conclusions drawn from the study are that at least some individual blastomeres from 4and 8-cell sheep embryos can give rise to an entire conceptus, and that the viability of embryos produced by blastomere separation depends more on the degree of reduction in cell number than on the stage of development at which separation is performed.