Effects of cumulus cell density during in vitro maturation of the developmental competence of bovine oocytes.

To determine the role of cumulus cells in oocyte maturation, we carried out an investigation on the effects of addition of cumulus cells to the maturation medium on the developmental competence of corona-enclosed oocytes and oocytes denuded from their somatic cells. The addition of cumulus cell (1.6 x 10(6) cells/mL) improved the development of bovine corona-enclosed oocytes, however, addition of a similar number of cumulus cells as cumulus-oocyte-complexes (COCs, cumulus cell density: 4.2 x 10(6) cells/mL) had no effect on the development of oocytes denuded from their somatic cells. To determine if corona-enclosed oocytes can obtain developmental competence without the addition of extra cumulus cells, the effects of cell density during in vitro maturation on the developmental competence were studied. A density of 1.6 to 3.2 x 10(6) cumulus cells/mL was the most effective for in vitro maturation of oocytes with intact gap junctions. The effects of the medium conditioned by COCs on the developmental competence of oocytes was also examined. It was demonstrated that COC-conditioned medium improved the development of bovine oocytes to the blastocyst stage. These data suggest that the developmental competence of bovine oocytes surrounded with corona cells is supported in a cell density-dependent manner in the maturation medium. In addition, the data indicate that cumulus cells benefit bovine oocyte development either by secreting soluble factors which induce developmental competence or by removing an embryo development-suppressive component from the medium.