Variation of cholesterol contents in porcine cumulus-oocyte complexes is a key factor in regulation of fertilizing capacity

The aims were to explore substances affecting maturation of porcine oocytes and effects of cholesterol efflux by methyl-β-cyclodextrin (MBCD). Cumulus-oocyte complexes (COCs) were collected from ovaries with or without corpora lutea (CL). Ovarian cholesterol content was determined and histologic sections were prepared for immunostaining of lanosterol 14α-demethylase, a catalytic enzyme during cholesterogenesis. In addition, COCs collected from ovaries without a CL (prepubertal gilts) were subjected to in vitro maturation with MBCD for 22 hours, followed by maturation without MBCD for 22 hours. Fertilizability and developmental competence of matured oocytes were monitored. The cholesterol content in COCs from the ovaries with CL (2.73 μg/μg protein) was higher (P < 0.05) than that from the ovaries without CL (1.88 μg/μg protein). Immunoreactive lanosterol 14α-demethylase was localized mainly in cells within a CL and in proximity to the CL. In COCs from ovaries without a CL, the cholesterol content just before in vitro maturation was 1.29 μg/μg protein, but it was decreased (P < 0.05; 0.51 μg/μg protein) by culturing in MBCD-containing medium for 22 hours, and subsequently increased (1.55 μg/μg protein) by culturing in MBCD-free medium for 22 hours. When oocytes were matured with MBCD for 22 hours and then matured without MBCD for the next 22 hours, the fertilization rate improved (P < 0.05) to 76.9%, and the blastocyst rate (9.5%) decreased (P < 0.05; fertilization and blastocyst rates were 69.6% and 26.3%, respectively, in the control group). We concluded that ovarian cholesterogenesis depended on sexual maturity of the donor and that variation in cholesterol content in COCs during in vitro maturation of porcine oocytes affected their ability to be fertilized.