Effect of hormone pre-treatment of prepubertal sheep on the production and developmental capacity of oocytes in vitro and in vivo

Abstract

Twenty 36-week-old Merino lambs were given either 3, 1 or 0 treatments of 50 µg oestradiol benzoate and (48 h later) a 1·5 mg Norgestamet implant left in situ for 9 days (3-, 1- and 0CYCLE+G). On Day 7 after the last implant insertion, and on the same day for 0CYCLE+G, each lamb received 400 I.U. pregnant mare serum gonadotrophin and 6 mg follicle-stimulating hormone (FSH). The reproductive tracts were removed for oocyte collection 24 h after FSH. Reproductive tracts were also collected from 16-24-week-old lambs (n = 31) (0CYCLEG). The number of antral follicles per ovary was similar for the 3-, 1- and 0CYCLE+G treatments. Similar rates of in vitro maturation and monospermic fertilization were obtained for all groups. The proportion of blastocysts per cleaved oocyte was higher for 1CYCLE+G (50·5%) than for 3CYCLE+G (32·9%), 0CYCLE+G (24·3%), and 0CYCLEG (11·8%) (P < 0·05). Viable fetuses were obtained at Day 93 of pregnancy after transfer of embryos from all treatments. These results indicate that a single treatment with oestrogen and progesterone, prior to gonadotrophin stimulation, will increase the yield and developmental capacity of oocytes from prepubertal sheep.