Infrequent structures in cattle oocytes.

Uncommon and controversial structures in cattle oocytes were studied in dominant and subordinate follicles of unstimulated Bos taurus and zebu (Bos indicus), and in FSH-stimulated B. taurus cattle, before or after administration of cloprostenol. Growing oocytes were very rare in follicles more than or equal to 5 mm in diameter. For the first time, special vesicles that may be involved in cortical granule synthesis were observed. Classical rough endoplasmic reticulum was very rare in unstimulated oocytes, but was seen at superovulation. Vacuolation of the nucleolus was a common feature in dominant oocytes that were collected after cloprostenol injection, except in superovulated cattle. Annulate lamellae were very rare or absent in oocytes of B. taurus, while they were more frequent in zebu oocytes.