OOCYTE ULTRASTRUCTURE IN BOVINE PRIMORDIAL TO EARLY TERTIARY FOLLICLES

The aim of the present study was to describe in detail the changes occ urring in the cytoplasmic ultrastructure of the bovine oocyte from the onset of growth in the primordial follicle until the completion of gr owth in the tertiary follicle. Bovine oocytes from primordial, primary , secondary and early to mid-antral follicles were processed and analy sed by light and transmission electron microscopy. The primordial foll icular oocyte was characterized by numerous coated pits on the oolemma and the accumulation of free and organelle-related smooth (SER) and r ough (RER) endoplasmic reticulum, round mitochondria and Golgi complex es around the nucleus, which was located slightly off centre. Up to th e secondary follicular stage the oocyte displayed an increase in the n umber of microvilli, elongated mitochondria and Golgi complexes. Durin g the secondary follicular stage, formation of the zona pellucida, dev elopment of gap junctions between the oocyte and the granulosa cells, formation of the cortical granules in the oocyte and reduction in the number of coated pits on the oolemma were seen. In the tertiary follic ular oocyte up to 100 mu m in diameter, the number of Golgi complexes and lipid droplets increased and the organelles were dislocated to the deep cortical region. During the final growth of the oocyte up to >12 0 mu m, the organelles were dislocated further to the peripheral regio n, the extent of the free SER and RER compartments were reduced, the n umber of individual cortical granules increased, hooded mitochondria b ecame abundant and the perivitelline space developed. In conclusion, t he growth of the bovine oocyte is associated with the relocation and m odulation of a number of cytoplasmic organelles as well as the develop ment of oocyte specific structures such as the zona pellucida and cort ical granules.