The process of cumulus expansion is a current topic of interest for in vitro production of embryos. In the present study, we examined the components of cumulus expansion, molecular mechanisms of cumulus expansion, and role of cumulus expansion for porcine oocyte maturation. The degree of cumulus expansion in the porcine cumulus-oocyte complexes (COCs) increased gradually until 48 h in culture in TCM-199. On the other hand, when the COCs were cultured in TCM-199 with a hyaluronan synthesis inhibitor and hyaluronidase, they showed no evidence of cumulus expansion during the culture period. Furthermore, the expression of hyaluronan synthase 2 (has2) in cumulus cells is accompanied by cumulus expansion. Hyaluronan receptor CD44 mRNA expressed in the cumulus cell, but not in the oocyte extracts. CD44 protein also expressed in/on the membrane of cumulus cells and its expression increased in a manner dependent on the degree of cumulus expansion. Moreover, we found that hyaluronan-CD44 system during cumulus expansion induces the activation of maturation promoting factor, resulting in germinal vesicle breakdown of the oocytes, and the tyrosine-phosphorylation of Cx43 in the COCs. The present results showed that the main component of cumulus expansion in the COCs is hyaluronan; the hyaluronan-CD44 system during cumulus expansion regulates the disruption of gap junctions in the COCs, and concurrently controls the incidence of meiotic resumption in the porcine oocytes.