[Changes in the microenvironment of the maturing bovine oocyte in vivo].

The development of pre-ovulatory bovine follicles from the onset of oestrus until ovulation may be divided into four phases on the basis of the patterns of the steroid levels in the follicular fluid. Changes in the morphology of the follicular wall and the oocyte coincide with these patterns. Prior to the pre-ovulatory LH surge steroid synthesis is directed towards the production of oestradiol-17 beta, and androstenedione is the major substrate of aromatase. After the LH peak, steroid synthesis switches to the production of progesterone shortly prior to ovulation. The membrana granulosa cell-layer shows morphological luteinization at the time. The elevated levels of LH and FSH observed in the follicular fluid suggest that changes in the micro-environment of the maturing bovine oocyte are regulated by these gonadotrophins.