Effects of low O-2 and ageing on spindles and chromosomes in mouse oocytesfrom pre-antral follicle culture

To assess their quality, spindles were analysed in mouse oocytes from pre-a ntral follicle culture. High or low oxygen tension was present during the l ast 16 or 20 h post human chorionic gonadotrophin (HCG)/epidermal growth fa ctor (EGF) addition. Most oocytes from pre-antral follicle culture possesse d typical anastral spindles with flat poles resembling those of ovulated, i n-vivo-matured oocytes of sexually mature mice, while denuded oocytes in-vi tro matured to metaphase II (MII) formed significantly longer, slender spin dles with pointed, narrow poles. Spindles in oocytes from follicle culture were only slightly shorter and less compact at the equator as compared with those of oocytes matured in vivo. Chromosomes were well aligned at the equ ator in MII oocytes obtained from follicle culture with high oxygen. Matura tion rate was significantly reduced by lowering oxygen tension to 5% O-2. P rolonged culture and the presence of only 5% O-2 dramatically increased the percentage of MII oocytes with unaligned chromosomes. These observations i ndicate that sufficient oxygen supply and time of retrieval after initiatio n of resumption of maturation by HCG as well as the microenvironment and ce ll-cell interactions between oocytes and their somatic compartment are crit ical in affecting the oocyte's capacity to mature to MII, to form a functio nal spindle, and to align chromosomes correctly.