Different intervals of ovum pick-up affect the competence of oocytes to support the preimplantation development of cloned bovine embryos

The objective of this study was to determine the effect of different frequencies of transvaginal ovum pick-up (OPU) on the quantity of recovered cumulus oocyte complexes (COCs) and subsequently the competence of matured oocytes to support the preimplantation development of cloned bovine embryos. The COCs were aspirated from the ovaries of 6 Chinese Holstein cows by transvaginal follicle aspiration twice a week (every 3 or 4 days) (Group I), every 5 days (Group II), once a week (every 7 days) (Group III), every 10 days (Group IV), and once every 2 weeks (every 14 days) (Group V). The developmental stages of the follicles were confirmed by the diameter of the dominant follicle (DF) and harvested COCs, and the dynamics of the follicular wave were clarified. In addition, extrusions of the first polar body (PB I) from the oocytes were observed at different time intervals after the initiation of in vitro maturation (IVM) to identify the appropriate culture time window for somatic cell nuclear transfer. Matured oocytes were used to produce cloned bovine embryos that were subsequently cultured in the goat oviduct. After 7 days, the embryos were flushed out, and the developmental rates of the blastocysts were compared among the five groups. The results showed that the aspirations of all follicles >or=3 mm in diameter (D1) induced and synchronized the dynamics of the follicular wave, and the subordinate follicles became atretic after 4 days (D5). Another follicular wave started between D7 and D10, and atresia in the subordinate follicles in the second follicular wave began on D14. The timing of meiotic progression (from the initiation of IVM to the extrusion of PB I) in the oocytes obtained by OPU was later than that of the oocytes obtained from the abattoir. Between 20 and 24 hr after the initiation of IVM, 20% of the oocytes extruded their PB I. Further, 80% (520/650) of the harvested COCs were arrested at metaphase II (MII) by 22 hr of the initiation of IVM and were used as cytoplast donors. The rates of development of the reconstituted embryos to the blastocyst stage were 23.1% (Group I), 15.0% (Group II), 10.9% (Group III), 4.9% (Group IV), and 29.0% (Group V). The results indicate that the developmental potential of follicles from the same living donors were different when different intervals of OPU were adopted and early atretic follicles from the second follicular wave had higher competence to support the early development of cloned bovine embryos.