THE EFFECT OF PROLACTIN ON THE QUALITY OF HEIFER OOCYTES RETRIEVED BY TRANSVAGINAL PUNCTURE OF FOLLICLES
In both dairy and beef cattle breeding, producing the larger number of the offspring from the best mothers to increase the degree of genetic progress through the generations is of particular interests. One of the attractive ways to resolve this problem is the development and implementation of the technology for obtaining embryos in vitro (in vitro embryo production, IVP) using oocytes derived from live animals by transvaginal puncture of follicles - Ovum-Pick-Up (OPU). The extracorporeal maturation of oocytes is an important element of this technology that may significantly affect its efficiency. In this paper, for the first time, we evaluated the advantage of the pituitary hormone prolactin (PRL) - a potential regulator of the quality of mammalian oocytes, during the maturation of OPU-oocytes. The effect of this hormone on the completion of nuclear maturation of the OPU-oocytes, as well as on the development and quality of IVP embryos, was studied. The mature Simmental heifers at the age from 19 to 25 months ( n = 4) with a natural sexual cycle were used as the oocyte donors. Transvaginal aspiration of the follicles was performed every 4 days using the OPU system for cattle (Minitube, Germany). A total of 28 OPU sessions were carried out. The derived cumulus-oocyte complexes (COCs) were cultured in TS-199 medium supplemented with 10 % bovine fetal serum, 10 mg/ml of follicle-stimulating (FSH) and 10 mg/ml of luteinizing (LH) hormones in the absence (control) or presence of PRL. After 24 hours, the mature oocytes were subjected to fertilization to assess their developmental competence. Morphological analysis did not reveal the effect of culture conditions on the completion of nuclear maturation. The rate of mature oocytes was similar in both groups and was 82.8 and 88.9 %, respectively, in the control and the PRL groups. However, the oocyte cleavage rate after in vitro fertilization in the control group was lower comparing to the PRL group (69.7±2.4 % vs. 81.7±4.9 %, p < 0.05). A positive effect of the PRL on the development of mature oocytes to the blastocyst stage was observed. When COCs were cultured in the control medium, the yield of blastocysts was 11.0±1.8 %, while the adding of PRL into the IVM medium increased this indicator to 17.2±2.0 % (p < 0.05). However, we did not find significant differences among compared groups in the relation to the total number of nuclei in blastocysts. Thus, prolactin hormone in the maturation environment has a stimulating effect on the developmental competence of donor oocytes. The positive effect is observed at the stage of the first cleavage and maintained during the development of embryos to the blastocyst stage. Our data indicate the positive effect of prolactin on the quality of OPU-oocytes, that makes it reasonable the using this hormone at the maturation stage to increase the effectiveness of IVP technology.