Abstract.A large proportion of follicles are lost during the initial ischemia that occurs after transplantation of ovarian tissues. Thus, the effect of hyperbaric oxygen therapy (HBO) on the follicular loss of ovarian tissues after transplantation was examined in mice. Ovarian slices from ICR mice were transplanted under the kidney capsule in ovariectomized ICR. Hyperbaric oxygen with 100% oxygen was initiated for 30 min at 2.5 atmospheres absolute immediately after transplantation, and this treatment was repeated at 48-h intervals for 2 weeks. The number of follicles was dramatically reduced at 2 weeks post transplantation. However, HBO was significantly effective in enhancing the survival of transplanted ovarian follicles. The survival rates of primordial and primary follicles in ovarian tissues of mice with HBO were significantly higher than those without HBO. These results indicate HBO can be effectively used for the enhancement of survival of transplanted ovarian tissues. Key words: Hyperbaric oxygen, Mice, Ovary, Transplantation (J. Reprod. Dev. 58: [260][261][262][263] 2012) S everal options such as cryopreservation of embryos, oocytes or ovarian tissue are currently available to preserve fertility in cancer patients. However, cryopreservation of ovarian tissue is the only option available for prepubertal girls and women in need of immediate chemotherapy [1,2]. Additionally, the cryopreservation of ovarian tissues is a potentially significant technology for the preservation of the genetic resources of working dogs as well as other laboratory and domestic animals [3,4]. Several reports have indicated that follicular loss in the mammalian ovary is not drastically extended by the cryopreservation procedure itself [3][4][5]. It is believed that the reason the primordial follicle is observably resistant to cryoinjury is because the oocyte it contains has a relatively inactive metabolism and lacks a meiotic spindle, zona pellucida and cortical granules [6]. In fact, a high percentage of oocytes as well as granulosa cells survive the cryopreservation and thawing procedures [3,[7][8][9]. Despite being able to survive freezing, it is well established that a large proportion of follicles are lost during the initial ischemia that occurs after transplantation of ovaries of mouse [10,11], sheep [12,13], dog [4,5] and human [14,15]. Most follicles that survive cryopreservation undergo ischemic loss during neovascularisation [16]. Several attempts have been made to prevent the follicular loss of cryopreserved ovarian tissues after transplantation. Treatment of ovarian tissues with a water soluble antioxidant (ascorbic acid) is known to reduce apoptosis in the ovarian cortex in vitro [17]. However, local application of sphingosine-1-phosphate, an apoptosis inhibitor, does not prevent follicular loss after transplantation in sheep [18].It has been reported that treatment with vitamin E, a lipid soluble antioxidant, improves the survival of follicles in ovarian grafts by reducing ischemic injury [19]. More recently, we ...