Distributional arrangement of mitochondria in the granulosa cells surrounding stage III zebrafish (Danio rerio) oocytes

Abstract

Mitochondria play a vital role during oocyte maturation, fertilization, and embryo development. In this study, confocal microscopy with the mitochondrial membrane potential-sensitive dye JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide) was used to investigate mitochondria distribution and activity of stage III zebrafish ovarian follicles. To support the mitochondrial origin of the fluorescence obtained by JC-1, a second mitochondrial probe, MitoTracker Green FM, was used. Cryo-scanning and transmission electron microscopy were also used to validate the distribution and localization of mitochondria obtained by mitochondrial staining. The mitochondrial probes were unable to penetrate the oocyte, and as a result it was not possible to observe stained mitochondria in the oocyte cytoplasm. However, mitochondrial staining of the granulosa cell layer surrounding the stage III zebrafish oocyte exhibited a contiguous aggregation pattern of mitochondria. Cryo-scanning electron microscopy studies also showed the oocyte surface to be covered by polygonal patterns of ridges of the same dimensions as the distributional arrangement of mitochondria in the granulosa cells. Though the results suggested the need for defolliculation to assess mitochondrial distribution and activity in the stage III zebrafish oocyte cytoplasm, the findings of this study will contribute to our understanding of oogenesis and folliculogenesis processes in fish.