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The aims of the present series of experiments were to establish a possible relationship between bovine oocyte diameter and follicle size, investigate the developmental ability of oocytes of different diameter groups, and investigate the relationship between oocyte diameter and RNA transcriptional activity of the oocyte. Follicles were recovered from slaughterhouse ovaries by mechanical dissection, measured, and assigned to one of the following size categories: > or = 4 mm, < 3-4 mm, < 2-3 mm, < 1-2 mm, and < 1 mm. Oocytes were liberated from the follicles and their diameters recorded. The correlation coefficient between oocyte diameter and follicle size was 0.32 (P < 0.0001). Oocytes were recovered by aspiration and/or slicing of slaughterhouse ovaries and divided into four groups ( < 100 microns, 100- < 110 microns, 110- < 120 microns, and > 120 microns) based on diameter. Oocytes were processed through standard procedures for in vitro maturation and stained in order to assess nuclear development. Rates of in vitro development to metaphase II were 21.2%, 42.3%, 75.9%, and 80.7%, respectively, for the four groups. On a separate occasion immature oocytes from the above diameter groups were cultured in the presence of 3H-uridine for 45 min and scored for degree of RNA synthesis as indicated by the presence of autoradiographic labeling. Oocytes < 110 microns showed a greater degree of 3H-uridine incorporation than those > or = 110 microns, suggesting that they were involved in RNA synthesis and therefore still in the growth phase.

The aim of the present study was to describe in detail the changes occurring in the cytoplasmic ultrastructure of the bovine oocyte from the onset of growth in the primordial follicle until the completion of growth in the tertiary follicle. Bovine oocytes from primordial, primary, secondary and early to mid-antral follicles were processed and analysed by light and transmission electron microscopy. The primordial follicular oocyte was characterized by numerous coated pits on the oolemma and the accumulation of free and organelle-related smooth (SER) and rough (RER) endoplasmic reticulum, round mitochondria and Golgi complexes around the nucleus, which was located slightly off centre. Up to the secondary follicular stage the oocyte displayed an increase in the number of microvilli, elongated mitochondria and Golgi complexes. During the secondary follicular stage, formation of the zona pellucida, development 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 follicular oocyte up to 100 microm 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 >120 microm, the organelles were dislocated further to the peripheral region, the extent of the free SER and RER compartments were reduced, the number of individual cortical granules increased, hooded mitochondria became abundant and the perivitelline space developed. In conclusion, the growth of the bovine oocyte is associated with the relocation and modulation of a number of cytoplasmic organelles as well as the development of oocyte specific structures such as the zona pellucida and cortical granules.