Disruption of nuclear maturation and rearrangement of cytoskeletal elements in bovine oocytes exposed to heat shock during maturation

Abstract

Meiotic maturation in mammalian oocytes is a complex process which involves extensive rearrangement of microtubules, actin filaments and chromosomes. Since cytoskeletal elements are sensitive to disruption by heat shock, a series of experiments were performed to determine whether physiologically relevant heat shock disrupts the progression of the oocyte through meiosis, fertilization and zygote formation. Cumulus–oocyte complexes were cultured at 38.5, 40.0 or 41.0 °C for the first 12 h of maturation. Incubation during the last 10 h of maturation and 18 h after fertilization was at 38.5 °C and in 5% (v/v) CO2for both treatments. Examination of the cytoskeleton and the chromosome organization in matured oocytes revealed that oocytes matured at 38.5°C were mostly at metaphase II (MII) stage, while the majority of heat-shocked oocytes were blocked at the first metaphase (MI), first anaphase or first telophase stages. A subset of heat-shocked oocytes possessed misshapen MI spindles with disorganized microtubules and unaligned chromosomes. A higher percentage of TUNEL-positive oocytes was noted for oocytes matured at 41.0 °C. Addition of 50 nmol/l sphingosine 1-phosphate to maturation medium blocked the effect of heat shock on progression through meiosis and apoptosis and increased the proportion of oocytes matured at 41.0 °C that were at MII. Following insemination, a high percentage of heat-shocked oocytes were unfertilized, while the majority of the control zygotes were fertilized and had two visible pronuclei. In conclusion, heat shock disrupts nuclear maturation and induces apoptosis. These alterations are likely to be involved in the mechanism underlying heat-shock-induced disruption of oocyte capacity for fertilization and subsequent development.