Introduction

Early embryonic development of the human oocyte, as for the oocytes of all mammalian species, is under the control of the maternally inherited genome. It is not until the 4- to 8-cell stage of development in the human that the maternal genome is fully replaced by a transcriptionally active embryonic genome [1]. The maternal transcripts that control the events of mammalian oocyte growth, meiotic maturation, fertilization, and early embryonic development are transcribed and accumulated during oogenesis.

Transcription in primary human oocytes begins at a relatively high level during fetal life with levels falling in leptotene and zygotene and falling further, to almost undetectable levels, by early pachytene [2]. Transcription levels then rise again through mid-pachytene returning to levels similar to those observed in oogonia by the early diplotene stage [2]. Studies in the mouse suggest that transcription levels remain active, but low, in the mammalian oocytes in the resting pool within the ovary but increase significantly and dramatically when the oocyte enters the growth phase, peaking at the time that maximal oocyte diameter is attained and then falling from this point to very low levels a few hours before ovulation (Figure 24.1) [3–5]. During the growth phase there is no significant difference in the rates of accumulation of rRNA, tRNA, polyadenylated RNA, or specific mRNAs [6–8].