Age-associated events in bovine oocytes and possible countermeasures
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REVIEW ARTICLE
Age-associated events in bovine oocytes and possible countermeasures Hisataka Iwata1
Received: 22 October 2015 / Accepted: 19 December 2015 Ó Japan Society for Reproductive Medicine 2016
Abstract Maternal aging profoundly affects oocyte quality. This has become common knowledge in industrialized countries and extensive studies addressing the causes and possible countermeasures against age-associated deterioration of oocytes suggest that mitochondrial dysfunction is a causal factor in infertility. However, almost all studies addressing age-associated events in oocytes have used mice as an animal model, and the reproductive life of mice is very short, making it difficult to study the gradual decline in fertility observed in humans. In the present review, age-associated changes in the quality and quantity of bovine oocytes and possible countermeasures related to mitochondrial quality control are introduced. Keywords SIRT1
Aging Cows Mitochondria Oocyte
Introduction Age-associated decline in fertility is common in mammals. In industrialized countries, the age of women’s first birth has increased to approximately 30 years; in women, fertility declines after 35 years of age [1, 2]. The causal factors of age-associated infertility and possible countermeasures against infertility have become social concerns. Clinical reports from in vitro fertilization have shown that the transfer percentage of women’s embryos that result in live birth declines after 35 years, while it & Hisataka Iwata [email protected] 1
Tokyo University of Agriculture, Funako 1737, Atsugi, Kanagawa 243-034, Japan
remains constant in the case of donor embryos [3], which indicates that the main cause of age-associated decline in fertility is low number or quality of oocytes and/or embryos. However, ethical and physical restrictions hamper studies of age-associated events in human oocytes and embryos, and there is, therefore, a need for appropriate animal models. Cows have a longer reproductive life (approximately 13 years) compared with other model animals, including rodents [4], and they show similar follicular wave, follicle selection, ovulation patterns as well as age-associated endocrinal changes to humans. [5–9]. In addition, many oocytes can easily be collected from slaughterhousederived ovaries or in vivo by using ovum pick-up (OPU) technology. Thus, cows have been suggested to be a good animal model for reproductive aging in humans. Moreover, in Japan, cows are tracked at a high degree of detail, allowing the breeds, age (in months) and farms of origin to be identified at the slaughterhouse. Thus, we have studied age-associated events in bovine oocytes and embryos. This review introduces age-associated changes in quality and quantity of bovine oocytes and suggests possible counter measures against age-associated events in bovine oocytes using mitochondrial quality control. Follicle number in bovine ovaries In general, age-associated infertility is believed to be related to a decline in the oocyte pool; the decline in
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