Effect of follicle cells and steroidogenesis on maturation and fertilization in vitro of pig oocytes.

Follicle cell numbers, steroidogenesis, and culture conditions were manipulated in a series of experiments designed to identify their effect during in vitro maturation of pig oocytes upon meiotic cycle progression, sperm penetration and male pronuclear formation. In Experiment 1 a static culture system was used to determine the effect of follicle cell numbers on maturation. When the follicle cell number was 3.5 x 10(3) cells per complex or less, cell cycle progression to metaphase II (M-II) and male pronuclear formation were both low (< 30%). Increasing follicle cell numbers to 15 x 10(3) per complex significantly enhanced both meiotic maturation (93%) and male pronuclear formation (83%). A further 10-fold increase in follicle cell numbers (150 x 10(3)), however, reduced subsequent male pronuclear formation (33%). The purpose of Experiment 2 was to determine whether the observed follicle cell effects were influenced by the type of culture system used for oocyte maturation. Comparisons were made between both static and non-static forms of culture. Meiotic progression was significantly depressed when non-static culture systems were used for the maturation of oocyte-cumulus complexes containing low numbers of somatic cells (< 15 x 10(3) cells). However, the addition of somatic cells (follicle shells) to non-static, but not static, culture systems significantly enhanced male pronuclear formation in all groups irrespective of the number of cumulus cells in the oocyte complex. Experiment 3 investigated the effects of the steroid environment on maturation and male pronuclear formation.(ABSTRACT TRUNCATED AT 250 WORDS)