Polarization of mitochondria in the unfertilized mouse oocyte

Abstract

Maturation of an immature oocyte into one capable of being fertilized involves tightly choreographed movements of chromosomes and organelles. The localization of mitochondria during maturation was studied in live mouse oocytes by confocal laser scanning microscopy (CLSM). Mitochondria were labeled with rhodamine 123 or Mitotracker (Molecular Probes, Eugene, OR) both of which are cell permeant and accumulate in mitochondria; acridine orange was used to mark chromatin. Prior to maturation, oocytes appeared to be radially symmetrical with no evident polarity; fully mature oocytes exhibited obvious polarity marked by the position of the metaphase II spindle in the cortex. CLSM revealed several interesting features of mitochondrial distribution: 1) A cortical clump of mitochondria was seen approximately 30-45 degrees to one side of the metaphase II spindle and marked the region of polar body I extrusion. 2) Large foci of mitochondria (7-14 microM) were frequently found around the central region of the mature oocyte, while the central region often exhibited markedly fewer mitochondria. 3) Small mitochondrial foci (3 microM) in the cortex and near the GV characterized several oocytes which failed to mature. 4) Non-spindle-associated mitochondria were not uniformly distributed in the mature oocyte but were concentrated in the hemisphere containing the metaphase II spindle. 5) The distal margins of this mitochondrial hemisphere were sharply demarcated at the cortex. These findings should help us understand organelle localization during mammalian oocyte maturation, and may give insights into possible causes of infertility and into early events of preimplantation development.

Correlation of Abnormal Mitochondrial Distribution in Mouse Oocytes with Reduced Developmental Competence

“…All embryos arrested at the 4-cell stage after vitrification and thawing, showing that the embryonic developmental competence in this group was significantly inferior to the control group. In mice, previous studies of the correlation of mitochondrial distribution in oocytes with oocyte maturation have shown that GV oocytes with an abnormal mitochondrial distribution fail to progress to MI (Van Blerkom and Runner 1984), and that oocytes with small mitochondrial foci in the cortex have arrested maturation (Calarco 1995). Furthermore, mitochondria in a so-called blocking out strain have been shown to be dispersed in the cytoplasm, whereas in a non-blocking strain significant accumulation of mitochondria around the nuclei was observed (Tokura et al 1993;Bavister and Squirrell 2000).…”

“…In mice, it has been shown that mitochondria are generally translocated to the nuclear region during germinal vesicle (GV) breakdown and metaphase I stage (MI) spindle formation in oocytes. Small mitochondrial foci in the cortex and proximity to the GV are characteristics of oocytes that fail to mature (Van Blerkom and Runner 1984;Calarco 1995).…”

Distribution of mitochondria in reconstructed mouse oocytes

“…Another problem could be the amount of mitochondria transferred. In immature oocytes the mitochondria are preferentially accumulated around GVs and in the cortical region (Calarco 1995, Van Blerkom et al 1998. As it is accepted that for the manifestation of certain diseases the amount of mutated mtDNA can be as low as 10%, this may represent a crucial problem (Wallace 1999).…”

Calcium wave pacemakers in eggs

“…In the mature mouse egg, ER-rich domains are larger in the vegetal cortex (Kline et al, 1999), whereas mitochondria are more abundant in the animal hemisphere (Calarco, 1995;Van Blerkom et al, 2002). The MII pacemaker of the mouse egg resides in the ER-enriched vegetal cortex, which is probably a site of enhanced sensitivity to Ins(1,4,5)P3.…”