Introduction

The mammalian oocyte is a remarkable cell, providing for a diverse range of essential functions to initiate each new life. Unique demands at the start of embryogenesis are met by equally unique capacities in the oocyte. These include the ability to undergo early oocyte activation and blocks to polyspermy upon fertilization, suppressing cell death after successful fertilization, supporting early nuclear reprogramming and remodeling events that enable production of a totipotent genome (a capacity that underlies cloning by somatic cell nuclear transfer), temporal regulation of the cell cycle, continuous provision of macromolecules required for cellular physiology and organization, and eventually transcriptional activation of the embryonic genome with the correct array of genes being activated. The reservoir of stored maternal mRNAs deposited in the oocyte provides the driving force behind these processes, as temporally regulated recruitment and translation of stored mRNAs provides for the dynamic production of different proteins at the correct times when they are needed. This chapter reviews the dynamic regulation of gene transcription, genome silencing, and maternal mRNA storage during oogenesis, and the subsequent mechanisms that enable regulated use of these mRNAs during oocyte maturation and after fertilization.

Gene transcription

The transition of primordial follicles to antral follicles and subsequent ovulation of high quality oocytes that are capable of undergoing successful fertilization and development to term are accompanied by stage-specific changes in mRNA expression [1]. Oocytes obtained from mouse primordial follicles display a pattern distinct from oocytes obtained from other stages. Of the 11660 mRNAs detected in these oocytes, 5020 genes display a twofold change in relative abundance, with 50% of genes up- or down-regulated in their level of expression at the transition from primordial to primary follicles. This phenomenal change in the oocyte transcriptome coincides with a dramatic reorganization of follicle structure and initiation of development of growth in primary follicles. The second biggest change in the oocyte transcriptome is observed between the secondary and small antral follicle stages, and noticeably affects genes involved in microtubule-based processes. The overall transition from secondary to large antral follicles coincides with acquisition of oocyte meiotic and developmental competence, including the ability to form a meiotic spindle [2, 3].