Introduction
During oocyte growth, maturation and zygote development, the morphology and structure of the nucleus undergoes dynamic changes to adjust to nuclear activities Oocyte growth is accompanied by active transcription activity and chromatin displays a loose configuration. The oocyte chromatin undergoes condensation at a later GV stage and becomes further condensed during meiotic maturation. Before full oocyte growth, oocyte chromatin is less condensed and not confined around the nucleolus, which is termed the non-surrounded nucleolus (NSN) configuration. However, when it comes to fully grown oocytes, the transcription of the oocytes is terminated, and the chromatin shows a condensed peri-nucleolar ring, which is termed surrounded nucleolus (SN) configuration (Liu et al., 2009).
After fertilization, the maternal chromatin undergoes the reverse process. The chromatin structure starts to loosen to form the pronucleus, in order to allow the whole genome to re-initiate DNA replication and thereafter transcription activity (Yu et al., 2016), which is essential for further embryo development and successful zygotic genome activation (ZGA) (Li et al., 2013).
Condensins play essential roles in chromatin condensation and chromosome formation. In prophase, condensins mediate loop array formations. In prometaphase, the combined action of condensin I and II forms hectically arranged nested loop arrays (Wood et al., 2010). The knockdown of condensing subunits, causes disordered chromosome structure and incorrect segregation during cell division, resulting in abnormal chromosome alignment, aneuploidy, and even apoptosis (Piazza et al., 2013).
Structural Maintenance of Chromosomes 2 (SMC2) is the core component of the condensin complex (Hirano, 2006a). It is essential for building higher order chromosome structure. A loss of SMC2 in neural stem cells results in loose chromosomes, nuclear accumulation of p53 and cell death (Nishide & Hirano, 2014). Apart from the function in the organization of the genetic material, SMC2 protein also plays an important role in gene activity regulation. The depletion of SMC2 in chicken DT40 cells caused a misregulation of gene expression during interphase (Zhang et al., 2016).
The functions of SMC2 in meiosis is little understood. Injection of anti-SMC2 antibody in vitro into meiotic mouse oocytes caused severe defects in kinetochore orientation and chromosome segregation (Lee et al., 2011). However, its participation in oocyte growth, maturation and embryo development has never been studied in vivo , due to the early mortality of SMC2 knockout embryos (Nishide & Hirano, 2014); thus we used a loxp-Cre mouse model to specifically delete SMC2 in order to clarify its function in female reproduction. We demonstrated that maternal SMC2 is absolutely required for oocyte chromosome condensation and subsequent zygote development.