Homology between mitochondriogenesis in the avian and amphibian oocyte.

Cytochrome oxidase cytochemistry was used to unequivocally identify the spread of mitochondria during oogenesis in the adult Japanese quail. This enabled us to compare their distribution with the distribution in the Xenopus laevis oocyte (Tourte et al, 1984). In the quail the paranuclear mitochondrial cloud initially disperses homogeneously but afterwards segregates into 2 populations: (i) a population localized in the basophilic cortical layer (surrounding the vegetal pole); and (ii) clusters of mitochondria distributed geometrically around the germinal vesicle in the animal pole. The mitochondria in these clusters have a high cytochrome oxidase activity, which reflects their functionality. This perinuclear crown of mitochondrial clusters actively replicates mtDNA in both animal species and builds up most of the stock of the mitochondria in the full-grown oocyte. Our study suggests that the perinuclear group of mitochondria will segregate in the somatic cells of the future embryo, whilst the original subcortical group will become localized in the primordial germ cells.