Role of hyaluronic acid in maturation and further early embryo development of bovine oocytes

Hyaluronic acid (HA), an important component of the extracellular matrix, plays a crucial role for cumulus cell expansion. Genes and proteins involved in HA synthesis and its receptor CD44 are expressed in cumulus oocyte complexes (COCs) in different animal species and increase during maturation. Hyaluronidase enzymes (Hyal) degrade HA into smaller biologically active HA fragments. To investigate the effects of the molecular size and concentration of HA on oocyte maturation and further embryo development, bovine oocytes were matured in vitro in the presence or absence of HA, Hyal-2 or 4-methylumbelliferone (4-MU); an HA synthesis inhibitor. The rates of oocyte nuclear maturation to metaphase II stage and development of embryos to blastocyst stage and blastocyst quality were recorded. Hyal-2 inhibited cumulus cell expansion without affecting oocyte maturation and further embryo development. Whereas, 4-MU at 1 mm reduced cumulus cell expansion, oocyte maturation rate and further embryo development; an effect which was partially abrogated by exogenous HA supplementation. These data suggest that HA production by cumulus cells during maturation is essential not only for cumulus cell expansion, but also for oocyte maturation and further embryo development. This effect is not affected by HA-degradation by Hyal-2.