Fate of the germ cells in mammalian ovary: A review

Ovary has a fix number of germ cells during fetal life in mammals. The germ cells are depleted rapidly and a large number of germ cells (≥99%) are eliminated from the cohort of ovary through follicular atresia during prepubertal life. The various cell death pathways including apoptosis, autophagy, necrosis, and necroptosis are involved in follicular atresia. Hence, <1% of germ cells are culminated into oocytes that are available for meiotic maturation and ovulation during entire reproductive life. These oocytes are arrested at diplotene stage of meiotic prophase-I and remain arrested for few months to several years during entire reproductive life. Resumption from diplotene arrest in follicular oocytes starts in response to gonadotropins surge and progresses through metaphase-I to metaphase-II stage that extrudes first polar body at the time of ovulation. Surprisingly, oocytes do not wait for fertilizing spermatozoa and quickly undergo abortive spontaneous oocyte activation (SOA) in few mammalian species including humans. The abortive SOA makes oocyte unfit for fertilization and limits assisted reproductive technologies outcome. Indeed, majority of germ cells and oocytes are eliminated from the cohort of ovary and only few oocyte that are of good quality get selectively recruited to become right gamete after ovulation during entire reproductive life span in mammals.

Endocrine disruption and female reproductive health: Implications on cross-talk between endocrine and autocrine/paracrine axes in the ovary

Female reproduction is a blend of neuroendocrine, endocrine, and autocrine/paracrine factors that maintain the appropriate ovarian micro-environment. The growing urbanization prompted exposure to a myriad of environmental toxins carrying the ability to interfere with reproductive processes governed by endogenous hormones, making reproductive health a major global concern. These environmental anthropogenic contaminants, popularly termed as endocrine-disrupting chemicals (EDCs), can disrupt the ovarian homeostasis leading to serious perturbations, namely, anovulation, infertility, estrogen deficiency, and premature ovarian failure. Although gonadotropin action, biosynthesis of gonadal steroids vis-à-vis growth factors comprise the essential modulators within the ovary, the redox balance along with inflammatory and cell death response can dramatically influence the framework of ovarian dynamics; however, details of which remain relatively less understood. The present overview provides an update on candidates (endocrines and autocrine/paracrine) of oogenesis, and the potential impact of EDCs on diverse intra-ovarian entities including but not limited to gonadotropin action, steroidogenic potential, expression of growth factors, and modulation of maturational competence. Moreover, the relative importance of free radical-induced stress, inflammation, and elevated cell death (follicular atresia), in the regulation of ovarian functions and how these intricate yet conjoined mechanisms may alter the reproductive performance of a female will be an issue of discussion.

Cyclic nucleotides regulate oocyte meiotic maturation and quality in mammals

Oocyte meiosis is a prolong series of events that are comprised several intermittent channels in mammals. Oocyte meiosis starts during fetal life and then gets arrested at diplotene stage of first meiotic prophase in follicular oocyte. The continuous transfer of cyclic adenosine 3’, 5’-monophosphate (cAMP) and cyclic guanosine 3’, 5’-monophosphate (cGMP) from encircling granulosa cells to the oocyte through gap junctions helps in the maintenance of their high level required to achieve the long-lasting diplotene arrest so-called germinal vesicle stage. Phosphodiesterase inhibitors have been used to elevate intracellular level of both cyclic nucleotides and prevent spontaneous resumption of meiosis in oocytes under in vitro culture conditions. On the other hand, disruption of gap junction either by pituitary gonadotropin or by physical removal of encircling granulosa cells interrupts transfer of these nucleotides to the oocyte. As a result, intraoocyte cAMP as well as cGMP levels are decreased drastically that initiate downstream pathways to destabilize maturation-promoting factor (MPF). The destabilized MPF initiates meiotic resumption from diplotene arrest in mammalian oocytes. Oocyte meiosis further progresses from metaphase I to metaphase II stage and extrudes first polar body to get converted into haploid female gamete at the time of ovulation. Indeed, high level of cAMP as well as cGMP levels maintains diplotene arrest for a long time in follicular oocytes. On the other hand, transient decrease of their levels drives resumption from diplotene arrest, thereby meiotic maturation process, which enables oocyte to achieve developmental competency. Any defect in this process directly affects oocyte quality and thereby reproductive outcome in mammals including human.