The effect of oocyte quality on development

ABSTRACT Oocyte quality affects early embryonic survival, the establishment and maintenance of pregnancy, fetal development, and even adult disease. Quality, or developmental competence, is acquired during folliculogenesis as the oocyte grows, and during the period of oocyte maturation. Assisted reproductive technologies involving ovarian hyperstimulation, or collection of immature oocytes followed by maturation in vitro, perturb this process and result in oocytes with reduced quality. In domestic livestock species, offspring have been produced using in vitro oocyte maturation, although only a small percentage of the original pool of immature oocytes is capable of developing to the blastocyst stage and subsequently resulting in pregnancy. In vitro maturation, as it is currently undertaken, does not support the correct development of oocyte competence. Follicle size affects oocyte quality, potentially implicating messenger RNA or protein stores as factors involved in oocyte competence. Oocytes from preantral follicles grown in vitro are competent to resume meiosis, although development to the blastocyst stage is decreased. An offspring from oocytes produced using this technique was normal at birth but experienced delayed onset health issues, highlighting the importance of oocyte quality long after embryogenesis. Metabolism may play a critical role in oocyte quality because glycolytic activity in mature oocytes is correlated with increased embryonic development. Communication between the oocyte and its surrounding cumulus cells is also important for the development of a competent oocyte. Ovarian stimulation causes delayed embryonic development, increased abnormal blastocyst formation, fetal growth retardation, and increased fetal loss. Thus, although meiosis and even early development may be completed successfully, there are a variety of other processes occurring within the cytoplasm of the oocyte that are required for complete developmental competence. However, the cellular mechanisms that impart oocyte quality are unclear. Until the mechanisms involved in oocyte quality are elucidated, any effort to use assisted reproductive technologies in animals for production or biomedical purposes will be inefficient at best.

Key Words: Oocyte, Maturation, Meiosis, Metabolism

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E14-E23

Implications

Ultimately, we hope to identify biochemical markers in oocytes that are correlated with developmental competence. With this information, we can design appropriate ovarian stimulation regimens and maturation conditions, as well as assess developmental competence of oocytes matured in vitro. We can apply this information to both alleviate infertility and increase efficiency of in vitro embryo production. Research investigating the components of oocyte quality is also critical because it will enable the development of assays to test for oocyte developmental competence, improved media formulation for in vitro oocyte maturation, and perhaps even the identification and treatment of specific types of infertility by RNA or protein injection into oocytes. In addition, the production of competent oocytes will positively affect the application of nuclear transfer and transgenic technologies to animals for agricultural applications, as well as the production of human pharmaceutical proteins.