ORIGINAL PAPER Apoptosis in mammalian oocytes: a review Meenakshi Tiwari 1 • Shilpa Prasad 1 • Anima Tripathi 1 • Ashutosh N. Pandey 1 • Irfan Ali 1 • Arvind K. Singh 2 • Tulsidas G. Shrivastav 3 • Shail K. Chaube 1 Ó Springer Science+Business Media New York 2015 Abstract Apoptosis causes elimination of more than 99 % of germ cells from cohort of ovary through follicular atresia. Less than 1 % of germ cells, which are culminated in oocytes further undergo apoptosis during last phases of oogenesis and depletes ovarian reserve in most of the mammalian species including human. There are several players that induce apoptosis directly or indirectly in oocytes at various stages of meiotic cell cycle. Premature removal of encircling granulosa cells from immature oocytes, reduced levels of adenosine 3 0 ,5 0 -cyclic monophosphate and guanosine 3 0 ,5 0 -cyclic monophosphate, increased levels of calcium (Ca 2? ) and oxidants, sustained reduced level of maturation promoting factor, depletion of survival factors, nutrients and cell cycle proteins, reduced meiotic competency, increased levels of proapoptotic as well as apoptotic factors lead to oocyte apoptosis. The BH3-only proteins also act as key regulators of apoptosis in oocyte within the ovary. Both intrinsic (mitochondria-me- diated) as well as extrinsic (cell surface death receptor- mediated) pathways are involved in oocyte apoptosis. BID, a BH3-only protein act as a bridge between both apoptotic pathways and its cleavage activates cell death machinery of both the pathways inside the follicular microenvironment. Oocyte apoptosis leads to the depletion of ovarian reserve that directly affects reproductive outcome of various mammals including human. In this review article, we highlight some of the important players and describe the pathways involved during oocyte apoptosis in mammals. Keywords Ovary Á Oocyte Á Granulosa cells Á Signal molecules Á Apoptotic pathways Introduction Mammalian ovary is responsible for generating competent oocytes required for the successful fertilization and early embryonic development. Apoptosis, a programmed cell death, plays a major role in the elimination of germ cells at all the stages of oogenesis and even after ovulation [1, 2]. More than 99 % of germ cells are eliminated from ovary via apoptosis through follicular atresia, while less than 1 % are culminated into oogonia [1, 3]. These oogonia enter into meiosis to give rise to primary oocytes [4, 5]. Primary oocytes are arrested at diplotene stage for several months to several years depending upon the mammalian species [6, 7]. These diplotene-arrested oocytes are encircled by sev- eral layers of granulosa cells inside the follicular microenvironment. A cross-talk between encircling granulosa cells and diplotene-arrested oocytes is important for the survival of both cell types [8, 9]. The granulosa cell apoptosis and/or premature removal of encircling granulosa cells deprive oocyte from growth factors, nutrients and survival factors that may lead to apoptosis in diplotene-arrested oocytes cultured in vitro [10–14]. Our studies suggest that granu- losa cell apoptosis inside the follicular microenvironment leads to oocyte apoptosis in rat [14–16]. The granulosa cell intactness protects oocytes from oxidative stress damage & Shail K. Chaube shailchaubey@gmail.com 1 Cell Physiology Laboratory, Department of Zoology, Banaras Hindu University, Varanasi- 221005, U.P., India 2 Genetics Laboratory, Department of Zoology, Banaras Hindu University, Varanasi 221005, U.P., India 3 Department of Reproductive Biomedicine, National Institute of Health and Family Welfare, Baba Gang Nath Marg, Munirka, New Delhi 110067, India 123 Apoptosis DOI 10.1007/s10495-015-1136-y