Effects of Prolactin in Different Culture Systems on the Maturation of Bovine Oocytes and Their Capacity for Subsequent Development

Abstract

We studied the influence of bovine prolactin on the maturation of cumulus-enclosed bovine oocytes in different culture systems as well as on their capacity for subsequent development after in vitrofertilization. The prolactin effect on chromosome transformations in oocytes depended on the hormone concentration in the medium with fetal calf serum. Prolactin at 50 ng/ml proved to stimulate nuclear maturation of the oocytes. This concentration was used to compare various systems of oocytes cultivation. The prolactin effect on bovine oocytes maturation and their capacity for subsequent development depended on the composition of the cultivation medium. The introduction of prolactin into the medium with fetal calf serum, estradiol, and a follicle-stimulating hormone had no effect on the reinitiation of meiosis in the oocytes but stimulated its completion, which increased the proportion of the oocytes at telophase I and metaphase II stages as well as the proportion of the eggs cleft after fertilization. Prolactin affected neither the nuclear maturation nor the capacity for further development of the oocytes cultivated in the medium with the serum of estrous cows. The addition of prolactin to the medium with calf serum where the oocytes and granulosa cells were cocultured increased the subsequent yield of the embryos developed to the morula and blastocyst stages. In this culture system, the hormone did not affect the rate of oocytes that reached the final stages of meiosis but inhibited their transition from telophase I to metaphase II. We propose that prolactin may favor the completion of cytoplasmic modifications going into the maturing oocyte.