Development of the mitochondrial mass and accumulation of mtDNA in previtellogenic stages of Xenopus laevis oocytes.

The development of the mitochondrial mass of the previtellogenic oocytes of Xenopus laevis has been analysed by a morphometric method and it has been correlated with the growth of the occyte itself. The study which was performed with oocytes from females of very different ages shows that the relative size of the mitochondrial mass in the smallest oocytes decreases gradually with the age of the females. Cytophotometric analysis of the Feulgen-stained mitochondrial mass enabled the mtDNA content of that structure to be quantified during its development. This development involves about 12 rounds of replication out of the total 16--17 rounds of replication during complete differentiation of large oocytes. It is possible to estimate the mean replication rate of the mtDNA during the course of the previtellogenic period. The calculated rate of replication is very high in the mitochondrial mass of all the growing oocytes of very young females(about 1--2 months old), then it decreases rapidly when the oocytes reach 120 microns in diameter. These results suggest that the mitochondrial masses found in small oocytes of old females do not correspond to sites of active mitochondrial biogenesis but rather they represent a physiologically arrested collection of organelles. Thus the aggregation of mitochondria is the cytological expression of rapid and strictly localized mitochondriogenesis only in very young females.