Oogenesis: Prospects and challenges for the future

Abstract

Oogenesis serves a singular role in the reproductive success of plants and animals. Of their remarkable differentiation pathway what stands out is the ability of oocytes to transform from a single cell into the totipotent lineages that seed the early embryo. As our understanding that commonalities between diverse organisms at the genetic, cellular and molecular levels are conserved to achieve successful reproduction, the notion that embryogenesis presupposes oogenesis has entered the day-to-day parlance of regenerative medicine and stem cell biology. With emphasis on the mammalian oocyte, this review will cover (1) current concepts regarding the birth, survival and growth of oocytes that depends on complex patterns of cell communication between germ line and soma, (2) the notion of ''maternal inheritance'' from a genetic and epigenetic perspective, and (3) Oogenesis is a protracted process that encompasses the birth, growth, and maturation of a cell unique in its ability to propagate another generation of organisms. In some sense, it is not a very efficient process when measured in terms of viable offspring. In fact, the relative fecundity of a particular organism varies widely according to the kind of reproductive strategy employed. For example, broadcast spawners like fish and many invertebrates are efficient at oogenesis but the fate of ovulated eggs is left up to the whims of the environment the resultant embryos find themselves in. These kinds of animals make a significant metabolic investment in oogenesis. On the other hand, primates expand oocyte numbers prior to birth and engage in a dramatic course of prolonged attrition with a small fraction of the egg endowment surviving to ovulation throughout the reproductive lifespan. As extreme as organisms may be in both the efficiency of egg production and the size of their ''spawn'' (hundreds to thousands for invertebrates; one for humans), the goals of oogenesis remain the same: producing a developmentally competent egg capable of generating live offspring. While developmental biologists have long been fascinated by the mechanisms by which an oocyte acquires and executes its totipotentiality during embryogenesis, a new generation of experimentalists have joined the campaign bringing with them research goals of great global and clinical importance. The emergence of the field of assisted reproductive technologies (ARTs) is dominant amongst these new areas of oocyte biology because of the increasing usage of ARTs to treat human infertility. Since the advent of this technology in the late 1970s, more than 3 million children have been born using ARTs and the prospects for innovations and other modifications in current applications are clear. Oocyte and ovary cryopreservation efforts are ongoing as a means to preserve or restore reproductive function to women who have undergone medical treatments that cause partial or complete sterility. Technology for preservation of oocytes, also known as egg banking, is also being viewed as a way to maintain the diversity of liv...

HDAC1 and HDAC2 in mouse oocytes and preimplantation embryos: Specificity versus compensation

“…40,41 Detailed temporal and spatial patterns of HDAC1 and HDAC2 expression have been established by immunocytochemical analysis. 35,42,43 HDAC1 and HDAC2 are concentrated in the nucleus throughout oocyte growth.…”

Compensatory functions of histone deacetylase 1 (HDAC1) and HDAC2 regulate transcription and apoptosis during mouse oocyte development

“…O ogenesis is a protracted process that encompasses meiosis and oocyte growth, and results in the only cell that, following fertilization, can develop into an organism (1). In mice, at approximately day 13.5 of gestation, oogonia undergo a final round of DNA replication and enter the first meiotic prophase, at which point they are called oocytes.…”

The DNA Damage Response in Mammalian Oocytes.

“…Before the end of gestation oocytes become arrested at the dictyate stage of the meiotic prophase (Rodrigues et al, 2008). During prophase arrest in oocytes, cyclin B-CDK1 remains inactive due to the maintenance of high levels of cAMP within the oocyte and the subsequent sustained activation of protein kinase A (PKA; Figure 3; Mehlmann et al, 2002;Schmitt and Nebreda, 2002).…”

“…From these follicles only a few reach the pre-ovulatory stage (Rodrigues et al, 2008). In the mouse, sustained unrepaired recombination-induced DSBs trigger oocyte apoptosis following the pachytene stage of meiotic prophase during embryonic life.…”