In vitro-matured rat oocytes have low mitochondrial deoxyribonucleic acid and adenosine triphosphate contents and have abnormal mitochondrial redistribution.

Objective: To study the development and function of mitochondria in in vitro-matured rat oocytes derived from follicles of different sizes.

Design: Experimental animal study.

Setting: Department of Anatomy at the University of Hong Kong.

Animal(s): Immature female Sprague-Dawley rats that were 25 days of age.

Intervention(s): Immature oocytes were collected from rat ovarian follicles of different sizes and were induced to mature in vitro.

Main Outcome Measure(s): The number of copies of mitochondrial DNA, mitochondrial activity, adenosine triphosphate content of matured oocytes, and rates of fertilization and blastulation were determined.

Result(s): The mitochondrial DNA copy number of oocytes increased linearly with the diameter of antral follicles. The mitochondrial DNA copy number, adenosine triphosphate content, and proportion of oocytes with peripheral distribution of mitochondria in in vitro-matured oocytes from small antral follicles were significantly lower than those from preovulatory follicles and in vivo-matured oocytes. Compared with in vitro-matured oocytes from small antral follicles, those from preovulatory follicles and in vivo-matured oocytes also had significantly better fertilization potential and higher blastulation rate.

Conclusion(s): The inferior developmental potential of in vitro-matured oocytes may be attributed partly to a reduced number of mitochondria, resulting in insufficient production of adenosine triphosphate for required developmental events.