Qualitative and quantitative structural changes during pig oocyte maturation.

Pig oocytes were examined at hourly intervals after stimulation with hCG. Meiosis was resumed between 20 and 30 h after hCG. This coincided with a decline in the number of mitochondria and evidence is presented which indicates that this was due to fusion. The number of lipid droplets increased and the volume fraction of large vesicles decreased. Both these organelles maintained a close spatial relationship with the endoplasmic reticulum (ER). Mitochondria were clustered at the periphery of the cell before hCG injection but dispersed with maturation. The volume occupied by large vesicles, 'protein bodies' and Golgi also decreased at the edge of the oocyte with the progression of maturation.