Ultrastructural changes in mitochondria during oogenesis in two phylogenetically close fish species.

The ultrastructure of oocyte mitochondria and their contribution to the endogenous autosynthesis of the yolk was investigated in two clupeoid species, the Mediterranean sardine, Sardina pilchardus, and the European anchovy, Engraulis encrasicolus. The structure and abundance of mitochondria differ in secondary growth oocytes of the two species, whereas they are similar in chromatin nucleolus and primary growth oocytes. Sardine oocytes show a higher percentage of mitochondria in the cytoplasm as they develop. However, the individual size of each mitochondrion decreases, becoming smaller than those observed in anchovy oocytes. The volume fraction of cristae in mitochondria of sardine oocytes gradually increased throughout the oocyte developmental phases up to the early secondary growth phase and then slightly decreased during the mid-secondary growth phase. In the cytoplasm of early secondary growth oocytes of anchovy, the percentage of mitochondria is larger than in mid-secondary growth oocytes. As oocytes develop, the size of mitochondria diminishes as well. In contrast to the volume fraction of cristae in mitochondria of sardine oocytes, the volume fraction of cristae in anchovy was decreased in early secondary growth oocytes and then it was increased during the mid-secondary growth phase. As a result, based on both cytoplasmic dynamics of each species and mitochondrial alterations, it was assumed that mitochondria in sardine play a role in the formation of yolk granules, whereas mitochondria in anchovy play a role in the lipid synthesis pathway. Both species showed exogenous heterosynthesis of yolk, through the process of pinocytosis in the zona radiata of oocytes.