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Analysis of Mitochondrial Structure and Distribution in Graded Bovine Oocytes

Emily C. Lynch, Lauri Willingham-Rocky, Helga Sittertz-Bharkar, Rola Barhoumi, and Robert C. Burghardt.

Department of Veterinary Anatomy and Public Health, Texas A&M University, College Station, TX


Objective: The long-term goal is to establish identifiable characteristics of high quality mammalian eggs that have full developmental potential. This objective of the study was to examine graded bovine oocytes by electron microscopy and attempt to identify correlates between oocyte ultrastructure and physiology derived from multiphoton imaging.

Sample Population: Bovine oocytes

Procedure: Mature bovine oocytes were assigned a quality rating of G1, G2 or G3, with G1 being the highest quality and G3 the lowest quality with respect to shape, color and size.  The oocytes were further  and then divided into 3 groups of graded oocytes for analysis.  Group one"A" oocytes were matured further and fertilized in vitro, as the control.  Group was oocytes were imaged for to assess mitochondrial distribution and activity using rhodamine 123 and multiphoton microscopy, and then matured and fertilized. The resulting embryos were allowed to develop in vitro.  Group three "C" oocytes were fixed and prepared for TEM.

Results:  The ultrastructural distribution of mitochondria in G1 oocytes appeared to be homogeneous, whereas the mitochondria of G2 and G3 oocytes were concentrated in the cortical cytoplasm and interspersed with lipid droplets.  All three grades of oocytes displayed some variation in mitochondrial distribution ultrastructurally and physiologically.

Conclusion: Quantitative conclusions on the relationship between mitochondrial distribution and oocyte competence were not established, nor were distinct correlates identified between the ultrastructure and the physiology due to the small sample size evaluated. The examination of structural and physiological parameters has shown that it is inadequate to assess oocyte quality based solely on morphological characteristics.

Impact for Human Medicine: Using physiological parameters to determine oocyte quality may ameliorate some of the negative consequences to offspring derived from IVF and IVM oocytes.