Molecular and Cellular Endocrinology 282 (2008) 150–152 Do chilling injury and heat stress share the same mechanism of injury in oocytes? Arav Amir a,∗ , Roth Zvi b a Institute of Animal Science, Agricultural Research Organization, the Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel b Department of Animal Science, Faculty of Agricultural, Food and Environmental Quality Sciences, the Hebrew University of Jerusalem, Rehovot 76100, Israel Abstract Chilling injury is the major limiting factor for successful cryopreservation of both human and animal oocytes. Maternal hyperthermia is the main reason for summer infertility in dairy cows. In this paper, we will present evidence for the involvement of membrane lipid composition and its thermotrophic behavior and the mechanism by which chilling injury and/or heat stress disrupt oocytes’ developmental competence. Although oocyte and single zygote are similar in shape and size, the oocyte is known to be very sensitive to cryopreservation, whereas, the zygote is successfully cryopreserved. Recently, we have shown that the lipid-phase transition (LPT) in human MII oocytes occurs at about 20 ◦ C, while the LPT temperature of zygotes is lower by 10 ◦ C. Similarly, the LPT in oocytes collected from dairy cows was found to be elevated by 10 ◦ C in the summer vs. the winter. This feature was associated with alterations in membrane lipid composition. In particular, during the winter, the oocyte membrane is composed mainly of mono- and polyunsaturated fatty acids while in the hot summer, it is composed of more saturated fatty acids. In another study, we showed that exposure of bovine oocytes to physiologically relevant heat shock increases the proportion of oocytes that undergo apoptosis, presumably via sphingomyelin hydrolysis and ceramide formation (i.e. the sphingomyelin apoptosis pathway). Using a mouse model, we have recently shown that hyperthermia of 1.5 ◦ C affects the follicle enclosed oocyte as determined by lower developmental competence. Given the importance of the membrane’s composition and integrity, it appears that alterations in the oocyte-membrane underlie the disruption of developmental competence in mammalian oocytes exposed to thermal stress (i.e. chilling injury or heat shock). © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Chilling injury; Heat stress; Membrane composition; Oocyte competence 1. Chilling injury and oocyte-membrane composition Chilling injury is defined as the damage incurred by liv- ing cells during exposure to sub-physiological temperatures (Watson and Morris, 1987). Chilling susceptibility has been shown to be correlated with the membrane’s saturated-to- unsaturated fatty-acid ratio (Arav et al., 2000) and to be associated with the lipid-phase transition (LPT), as described for both sperm (Drobins et al., 1993) and oocytes (Arav et al., 1996). In a classical paper (Quinn, 1985), Quinn hypoth- esized that during chilling the membrane goes through a LPT with solidification of lamellar lipids, such as saturated fatty acids. The low-temperature-fluid lipids, such as unsat- ∗ Corresponding author. E-mail address: arav@agri.huji.ac.il (A. Arav). urated fatty acids, which in this case are nonlamellar, then separate (lipid-phase separation; LPS) and form a new non- lamellar structure, such a II hexagonal structure which interferes with membrane function and leads to ion leakage and cell death. There are several options for reducing chilling injury in gamete cryopreservation. The first and most common in sperm cryopreservation is the use of lipid stabilizers such as sugars or lipids (Strauss et al., 1986). Normally, sperm cells are cooled slowly (1 ◦ C/min) from RT to 4 ◦ C in the presence of an egg yolk, which interacts with the plasma membranes and reduces chilling injury. In this respect, we have shown that increasing the unsaturated-to-saturated lipid ratio in the membranes by direct exposure to liposomes (Zeron et al., 2002a) or by dietary sup- plementation of unsaturated fatty acids (Zeron et al., 2002b) reduces oocyte chilling susceptibility. Since chilling injury is a kinetic process (Zeron et al., 1999), another option for reduc- 0303-7207/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.mce.2007.11.026