Influence of hyaluronic acid synthesis and cumulus mucification on bovine oocyte in vitro maturation, fertilisation and embryo development

During cumulus-oocyte complex (COC) maturation, cumulus expansion involves the deposition of mucoelastic compounds, especially hyaluronic acid, synthesised from glucose via the hexosamine biosynthesis pathway. The aim of the present study was to determine the effects of uridine monophosphate (UMP) and 6-diazo-5-oxo-L-norleucine (DON), inhibitors of hyaluronic acid synthesis, during bovine oocyte in vitro maturation (IVM) on cumulus expansion, glucose uptake, protein synthesis, cumulus cell number, meiotic maturation, cleavage rate and subsequent embryo development. A further aim of the study was to examine the effect of hyaluronic acid on sperm capacitation and acrosome reaction in relation to the capacity of COCs to be fertilised in vitro. A low correlation between glucose uptake and degree of cumulus expansion was observed. Total and partial inhibition of cumulus expansion was observed with DON and UMP, respectively, and was accompanied by a decrease in glucose uptake with DON. Total protein content and cumulus cell number per COC increased during IVM, but was unaffected by the presence of DON or UMP, as was oocyte meiotic maturation. Rates of cleavage and blastocyst development decreased in oocytes matured with DON and UMP, although this inhibition was reversed when the in vitro fertilisation (IVF) medium contained heparin. Hyaluronic acid induced capacitation and the acrosome reaction, and in IVF medium prevented the inhibition of cleavage and blastocyst development by DON in a similar fashion to heparin. Hyaluronic acid synthesis during cumulus mucification contributes to the penetration and fertilisation of bovine oocytes, most likely by facilitating the processes of capacitation and acrosome reaction. Mucification during IVM is independent of cumulus cell proliferation, COC protein content, oocyte meiotic maturation and subsequent developmental competence once fertilised.