Development of one-cell fertilized sheep ova following microinjection into pronuclei.
Three experiments were conducted to identify, sources of loss of fertilized single-cell sheep eggs microinjected with DNA. In the first experiment, immediate transfer of eggs into synchronous recipients resulted in 86% of embryos developing (>32 cells) at Day 7. Incubating eggs in microdrops of Ham's F-10 medium+10% fetal calf serum for 5 h at 37 degrees C in an atmosphere of 95% air: 5% CO2 before transfer reduced development (65%>32 cells). Removing eggs from drops for 30 min of microscopic inspection, simulating manipulation during microinjection, caused no additional reduction in development (63%>32 cells). However, injection of eggs with buffer was detrimental to subsequent development (42%>32 cells). In Experiment 2, injection of buffer or injection of DNA in buffer into the pronuclei before transfer of eggs into recipient ewes resulted in 29 and 19%, respectively, of embryos developing to >32 cells at Day 7. In Experiment 3, more eggs developed when held in 5 ml of medium than in microdrops (P=0.07). No difference in development was found between eggs held in bicarbonate-buffered BMOC or in phosphate-buffered saline with added fetal bovine serum. The development of sheep eggs appears to be greatly reduced after microinjection, but until alternate procedures are found, a high rate of loss of injected eggs may be an unavoidable cost of inserting foreign genes into sheep.