Centrosome and microtubule dynamics during early stages of meiosis in mouse oocytes

Centrosomes, major regulatory sites for the microtubule (MT) nucleation, are regulated in a dynamic manner throughout the process of meiotic maturation. Recently, centrosome orientation in mouse oocytes has been demonstrated in metaphase I through metaphase II. However, centrosomal protein expression in concordance with MT polymerization in earlier stages of oocyte maturation from germinal vesicle stage (GV) to prometaphase I still remains unclear. The present study aims to assess the centrosome-microtubule remodelling during the onset of meiosis based on strict criteria of nuclear maturation. Six consecutive stages were determined for scoring the oocytes as unrimmed nucleolus (UR), partially rimmed nucleolus (PR), fully rimmed nucleolus (FR), nuclear lamina dissolution (NLD), disappearance of nucleolus (DON), and chromatin condensation (CC). A centrosomal protein, pericentrin, was found tightly localized adjacent to nuclear lamina in UR, lacking any NIT nucleation activity. In concordance with the competency to resume meiosis, an increase in the amount and nucleation capacity of pericentrin is noted. In FR, cytoplasmic NIT almost disappeared while de-novo microtubule polymerization was found in small aggregates of pericentrin localized around the nucleus. Towards the end of DON and CC, a sudden burst of pericentrin was noted with an extreme NIT nucleation activity in an organized fashion that is essential for the rapid formation of first meiotic spindle. The results show that centrosomes display precisely controlled spatio-temporal changes during the onset of meiotic maturation. Accumulation of centrosomal proteins to a single locus followed by a sequestration to several spots might be evidence of a mechanism by which the proper distribution of centrosomal material during nuclear breakdown and subsequently formation of spindle are regulated in concordance with the nuclear maturation.